Update version

Closes #25

.github/FUNDING.yml

@@ -1,4 +1,3 @@
 github: Tyrrrz
 patreon: Tyrrrz
-open_collective: Tyrrrz
+custom: ['buymeacoffee.com/Tyrrrz', 'tyrrrz.me/donate']
-custom: ['buymeacoffee.com/Tyrrrz']

.github/workflows/CD.yml

@@ -0,0 +1,25 @@
+name: CD
+
+on:
+  push:
+    tags:
+    - '*'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+
+    - name: Install .NET Core
+      uses: actions/setup-dotnet@v1.4.0
+      with:
+        dotnet-version: 3.1.100
+
+    - name: Pack
+      run: dotnet pack CliFx --configuration Release
+
+    - name: Deploy
+      run: dotnet nuget push CliFx/bin/Release/*.nupkg -s nuget.org -k ${{ secrets.NUGET_TOKEN }}

.github/workflows/CI.yml

@@ -0,0 +1,35 @@
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+
+    - name: Install .NET Core
+      uses: actions/setup-dotnet@v1.4.0
+      with:
+        dotnet-version: 3.1.100
+
+    - name: Build & test
+      run: dotnet test --configuration Release --logger GitHubActions
+
+    - name: Upload coverage
+      uses: codecov/codecov-action@v1.0.5
+      with:
+        token: ${{ secrets.CODECOV_TOKEN }}
+        file: CliFx.Tests/bin/Release/Coverage.xml
+
+    - name: Upload coverage (analyzers)
+      uses: codecov/codecov-action@v1.0.5
+      with:
+        token: ${{ secrets.CODECOV_TOKEN }}
+        file: CliFx.Analyzers.Tests/bin/Release/Coverage.xml

.gitignore

@@ -143,6 +143,7 @@ _TeamCity*
 _NCrunch_*
 .*crunch*.local.xml
 nCrunchTemp_*
+.ncrunchsolution
 
 # MightyMoose
 *.mm.*

Changelog.md

@@ -0,0 +1,16 @@
+### v1.2 (11-May-2020)
+
+- Added built-in Roslyn analyzers that help catch incorrect usage of the library. Currently, all analyzers report issues as warnings so as to not prevent the project from building. In the future that may change.
+- Added an optional parameter to `new CommandException(...)` called `showHelp` which can be used to instruct CliFx to show help for the current command after printing the error. (Thanks [@Domn Werner](https://github.com/domn1995))
+- Improved help text shown for enum options and parameters by providing the list of valid values that the enum can accept. (Thanks [@Domn Werner](https://github.com/domn1995))
+- Fixed an issue where it was possible to set an option without providing a value, while the option was marked as required.
+- Fixed an issue where it was possible to configure an option with an empty name or a name consisting of a single character. If you want to use a single character as a name, you should set the option's short name instead.
+- Added `CursorLeft` and `CursorTop` properties to `IConsole` and its implementations. In `VirtualConsole`, these are just auto-properties.
+- Improved exception messages.
+- Improved exceptions related to user input by also showing help text after the error message. (Thanks [@Domn Werner](https://github.com/domn1995))
+
+### v1.1 (16-Mar-2020)
+
+- Changed `IConsole` interface (and as a result, `SystemConsole` and `VirtualConsole`) to support writing binary data. Instead of `TextReader`/`TextWriter` instances, the streams are now exposed as `StreamReader`/`StreamWriter` which provide the `BaseStream` property that allows raw access. Existing usages inside commands should remain the same because `StreamReader`/`StreamWriter` are compatible with their base classes `TextReader`/`TextWriter`, but if you were using `VirtualConsole` in tests, you may have to update it to the new API. Refer to the readme for more info.
+- Changed argument binding behavior so that an error is produced if the user provides an argument that doesn't match with any parameter or option. This is done in order to improve user experience, as otherwise the user may make a typo without knowing that their input wasn't taken into account.
+- Changed argument binding behavior so that options can be set to multiple argument values while specifying them with mixed naming. For example, `--option value1 -o value2 --option value3` would result in the option being set to corresponding three values, assuming `--option` and `-o` match with the same option.

CliFx.Analyzers.Tests/AnalyzerTestCase.cs

@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+
+namespace CliFx.Analyzers.Tests
+{
+    public class AnalyzerTestCase
+    {
+        public string Name { get; }
+
+        public IReadOnlyList<DiagnosticDescriptor> TestedDiagnostics { get; }
+
+        public IReadOnlyList<string> SourceCodes { get; }
+
+        public AnalyzerTestCase(
+            string name,
+            IReadOnlyList<DiagnosticDescriptor> testedDiagnostics,
+            IReadOnlyList<string> sourceCodes)
+        {
+            Name = name;
+            TestedDiagnostics = testedDiagnostics;
+            SourceCodes = sourceCodes;
+        }
+
+        public AnalyzerTestCase(
+            string name,
+            IReadOnlyList<DiagnosticDescriptor> testedDiagnostics,
+            string sourceCode)
+            : this(name, testedDiagnostics, new[] {sourceCode})
+        {
+        }
+
+        public AnalyzerTestCase(
+            string name,
+            DiagnosticDescriptor testedDiagnostic,
+            string sourceCode)
+            : this(name, new[] {testedDiagnostic}, sourceCode)
+        {
+        }
+
+        public override string ToString() => $"{Name} [{string.Join(", ", TestedDiagnostics.Select(d => d.Id))}]";
+    }
+}

CliFx.Analyzers.Tests/CliFx.Analyzers.Tests.csproj

@@ -0,0 +1,29 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="../CliFx.props" />
+
+  <PropertyGroup>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+    <CollectCoverage>true</CollectCoverage>
+    <CoverletOutputFormat>opencover</CoverletOutputFormat>
+    <CoverletOutput>bin/$(Configuration)/Coverage.xml</CoverletOutput>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Gu.Roslyn.Asserts" Version="3.3.0" />
+    <PackageReference Include="GitHubActionsTestLogger" Version="1.0.0" />
+    <PackageReference Include="FluentAssertions" Version="5.10.2" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
+    <PackageReference Include="xunit" Version="2.4.0" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0" />
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.4.0" />
+    <PackageReference Include="coverlet.msbuild" Version="2.8.0" PrivateAssets="all" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\CliFx.Analyzers\CliFx.Analyzers.csproj" />
+    <ProjectReference Include="..\CliFx\CliFx.csproj" />
+  </ItemGroup>
+
+</Project>

CliFx.Analyzers.Tests/CommandSchemaAnalyzerTests.cs

@@ -0,0 +1,489 @@
+using System.Collections.Generic;
+using CliFx.Analyzers.Tests.Internal;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Xunit;
+
+namespace CliFx.Analyzers.Tests
+{
+    public class CommandSchemaAnalyzerTests
+    {
+        private static DiagnosticAnalyzer Analyzer { get; } = new CommandSchemaAnalyzer();
+
+        public static IEnumerable<object[]> GetValidCases()
+        {
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Non-command type",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+public class Foo
+{
+    public int Bar { get; set; } = 5;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Command implements interface and has attribute",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Command doesn't have an attribute but is an abstract type",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+public abstract class MyCommand : ICommand
+{
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Parameters with unique order",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(13)]
+    public string ParamA { get; set; }
+    
+    [CommandParameter(15)]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Parameters with unique names",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(13, Name = ""foo"")]
+    public string ParamA { get; set; }
+    
+    [CommandParameter(15, Name = ""bar"")]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Single non-scalar parameter",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(1)]
+    public string ParamA { get; set; }
+    
+    [CommandParameter(2)]
+    public HashSet<string> ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Non-scalar parameter is last in order",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(1)]
+    public string ParamA { get; set; }
+    
+    [CommandParameter(2)]
+    public IReadOnlyList<string> ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Option with a proper name",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption(""foo"")]
+    public string Param { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Option with a proper name and short name",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption(""foo"", 'f')]
+    public string Param { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Options with unique names",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption(""foo"")]
+    public string ParamA { get; set; }
+    
+    [CommandOption(""bar"")]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Options with unique short names",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption('f')]
+    public string ParamA { get; set; }
+    
+    [CommandOption('x')]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Options with unique environment variable names",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption('a', EnvironmentVariableName = ""env_var_a"")]
+    public string ParamA { get; set; }
+    
+    [CommandOption('b', EnvironmentVariableName = ""env_var_b"")]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+        }
+
+        public static IEnumerable<object[]> GetInvalidCases()
+        {
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Command is missing the attribute",
+                    DiagnosticDescriptors.CliFx0002,
+
+                    // language=cs
+                    @"
+public class MyCommand : ICommand
+{
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Command doesn't implement the interface",
+                    DiagnosticDescriptors.CliFx0001,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand
+{
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Parameters with duplicate order",
+                    DiagnosticDescriptors.CliFx0021,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(13)]
+    public string ParamA { get; set; }
+    
+    [CommandParameter(13)]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Parameters with duplicate names",
+                    DiagnosticDescriptors.CliFx0022,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(13, Name = ""foo"")]
+    public string ParamA { get; set; }
+    
+    [CommandParameter(15, Name = ""foo"")]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Multiple non-scalar parameters",
+                    DiagnosticDescriptors.CliFx0023,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(1)]
+    public IReadOnlyList<string> ParamA { get; set; }
+    
+    [CommandParameter(2)]
+    public HashSet<string> ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Non-last non-scalar parameter",
+                    DiagnosticDescriptors.CliFx0024,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(1)]
+    public IReadOnlyList<string> ParamA { get; set; }
+    
+    [CommandParameter(2)]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Option with an empty name",
+                    DiagnosticDescriptors.CliFx0041,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption("""")]
+    public string Param { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Option with a name which is too short",
+                    DiagnosticDescriptors.CliFx0042,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption(""a"")]
+    public string Param { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Options with duplicate names",
+                    DiagnosticDescriptors.CliFx0043,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption(""foo"")]
+    public string ParamA { get; set; }
+    
+    [CommandOption(""foo"")]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Options with duplicate short names",
+                    DiagnosticDescriptors.CliFx0044,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption('f')]
+    public string ParamA { get; set; }
+    
+    [CommandOption('f')]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Options with duplicate environment variable names",
+                    DiagnosticDescriptors.CliFx0045,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandOption('a', EnvironmentVariableName = ""env_var"")]
+    public string ParamA { get; set; }
+    
+    [CommandOption('b', EnvironmentVariableName = ""env_var"")]
+    public string ParamB { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+        }
+
+        [Theory]
+        [MemberData(nameof(GetValidCases))]
+        public void Valid(AnalyzerTestCase testCase) =>
+            Analyzer.Should().NotProduceDiagnostics(testCase);
+
+        [Theory]
+        [MemberData(nameof(GetInvalidCases))]
+        public void Invalid(AnalyzerTestCase testCase) =>
+            Analyzer.Should().ProduceDiagnostics(testCase);
+    }
+}

CliFx.Analyzers.Tests/ConsoleUsageAnalyzerTests.cs

@@ -0,0 +1,144 @@
+using System.Collections.Generic;
+using CliFx.Analyzers.Tests.Internal;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Xunit;
+
+namespace CliFx.Analyzers.Tests
+{
+    public class ConsoleUsageAnalyzerTests
+    {
+        private static DiagnosticAnalyzer Analyzer { get; } = new ConsoleUsageAnalyzer();
+
+        public static IEnumerable<object[]> GetValidCases()
+        {
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Using console abstraction",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    public ValueTask ExecuteAsync(IConsole console)
+    {
+        console.Output.WriteLine(""Hello world"");
+        return default;
+    }
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Console abstraction is not available in scope",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    public void SomeOtherMethod() => Console.WriteLine(""Test"");
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+        }
+
+        public static IEnumerable<object[]> GetInvalidCases()
+        {
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Not using available console abstraction in the ExecuteAsync method",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    public ValueTask ExecuteAsync(IConsole console)
+    {
+        Console.WriteLine(""Hello world"");
+        return default;
+    }
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Not using available console abstraction in the ExecuteAsync method when writing stderr",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    public ValueTask ExecuteAsync(IConsole console)
+    {
+        Console.Error.WriteLine(""Hello world"");
+        return default;
+    }
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Not using available console abstraction while referencing System.Console by full name",
+                    Analyzer.SupportedDiagnostics,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    public ValueTask ExecuteAsync(IConsole console)
+    {
+        System.Console.Error.WriteLine(""Hello world"");
+        return default;
+    }
+}"
+                )
+            };
+
+            yield return new object[]
+            {
+                new AnalyzerTestCase(
+                    "Not using available console abstraction in another method",
+                    DiagnosticDescriptors.CliFx0100,
+
+                    // language=cs
+                    @"
+[Command]
+public class MyCommand : ICommand
+{
+    public void SomeOtherMethod(IConsole console) => Console.WriteLine(""Test"");
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}"
+                )
+            };
+        }
+
+        [Theory]
+        [MemberData(nameof(GetValidCases))]
+        public void Valid(AnalyzerTestCase testCase) =>
+            Analyzer.Should().NotProduceDiagnostics(testCase);
+
+        [Theory]
+        [MemberData(nameof(GetInvalidCases))]
+        public void Invalid(AnalyzerTestCase testCase) =>
+            Analyzer.Should().ProduceDiagnostics(testCase);
+    }
+}

CliFx.Analyzers.Tests/Internal/AnalyzerAssertions.cs

@@ -0,0 +1,107 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using FluentAssertions.Execution;
+using FluentAssertions.Primitives;
+using Gu.Roslyn.Asserts;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace CliFx.Analyzers.Tests.Internal
+{
+    internal partial class AnalyzerAssertions : ReferenceTypeAssertions<DiagnosticAnalyzer, AnalyzerAssertions>
+    {
+        protected override string Identifier { get; } = "analyzer";
+
+        public AnalyzerAssertions(DiagnosticAnalyzer analyzer)
+            : base(analyzer)
+        {
+        }
+
+        public void ProduceDiagnostics(
+            IReadOnlyList<DiagnosticDescriptor> diagnostics,
+            IReadOnlyList<string> sourceCodes)
+        {
+            var producedDiagnostics = GetProducedDiagnostics(Subject, sourceCodes);
+
+            var expectedIds = diagnostics.Select(d => d.Id).Distinct().OrderBy(d => d).ToArray();
+            var producedIds = producedDiagnostics.Select(d => d.Id).Distinct().OrderBy(d => d).ToArray();
+
+            var result = expectedIds.Intersect(producedIds).Count() == expectedIds.Length;
+
+            Execute.Assertion.ForCondition(result).FailWith($@"
+Expected and produced diagnostics do not match.
+
+Expected: {string.Join(", ", expectedIds)}
+Produced: {(producedIds.Any() ? string.Join(", ", producedIds) : "<none>")}
+".Trim());
+        }
+
+        public void ProduceDiagnostics(AnalyzerTestCase testCase) =>
+            ProduceDiagnostics(testCase.TestedDiagnostics, testCase.SourceCodes);
+
+        public void NotProduceDiagnostics(
+            IReadOnlyList<DiagnosticDescriptor> diagnostics,
+            IReadOnlyList<string> sourceCodes)
+        {
+            var producedDiagnostics = GetProducedDiagnostics(Subject, sourceCodes);
+
+            var expectedIds = diagnostics.Select(d => d.Id).Distinct().OrderBy(d => d).ToArray();
+            var producedIds = producedDiagnostics.Select(d => d.Id).Distinct().OrderBy(d => d).ToArray();
+
+            var result = !expectedIds.Intersect(producedIds).Any();
+
+            Execute.Assertion.ForCondition(result).FailWith($@"
+Expected no produced diagnostics.
+
+Produced: {string.Join(", ", producedIds)}
+".Trim());
+        }
+
+        public void NotProduceDiagnostics(AnalyzerTestCase testCase) =>
+            NotProduceDiagnostics(testCase.TestedDiagnostics, testCase.SourceCodes);
+    }
+
+    internal partial class AnalyzerAssertions
+    {
+        private static IReadOnlyList<MetadataReference> DefaultMetadataReferences { get; } =
+            MetadataReferences.Transitive(typeof(CliApplication).Assembly).ToArray();
+
+        private static string WrapCodeWithUsingDirectives(string code)
+        {
+            var usingDirectives = new[]
+            {
+                "using System;",
+                "using System.Collections.Generic;",
+                "using System.Threading.Tasks;",
+                "using CliFx;",
+                "using CliFx.Attributes;",
+                "using CliFx.Exceptions;",
+                "using CliFx.Utilities;"
+            };
+
+            return
+                string.Join(Environment.NewLine, usingDirectives) +
+                Environment.NewLine +
+                code;
+        }
+
+        private static IReadOnlyList<Diagnostic> GetProducedDiagnostics(
+            DiagnosticAnalyzer analyzer,
+            IReadOnlyList<string> sourceCodes)
+        {
+            var compilationOptions = new CSharpCompilationOptions(OutputKind.ConsoleApplication);
+            var wrappedSourceCodes = sourceCodes.Select(WrapCodeWithUsingDirectives).ToArray();
+
+            return Analyze.GetDiagnostics(analyzer, wrappedSourceCodes, compilationOptions, DefaultMetadataReferences)
+                .SelectMany(d => d)
+                .ToArray();
+        }
+    }
+
+    internal static class AnalyzerAssertionsExtensions
+    {
+        public static AnalyzerAssertions Should(this DiagnosticAnalyzer analyzer) => new AnalyzerAssertions(analyzer);
+    }
+}

CliFx.Analyzers/CliFx.Analyzers.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="../CliFx.props" />
+
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+    <Nullable>annotations</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.4.0" PrivateAssets="all" />
+  </ItemGroup>
+
+</Project>

CliFx.Analyzers/CommandSchemaAnalyzer.cs

@@ -0,0 +1,297 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace CliFx.Analyzers
+{
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public class CommandSchemaAnalyzer : DiagnosticAnalyzer
+    {
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(
+            DiagnosticDescriptors.CliFx0001,
+            DiagnosticDescriptors.CliFx0002,
+            DiagnosticDescriptors.CliFx0021,
+            DiagnosticDescriptors.CliFx0022,
+            DiagnosticDescriptors.CliFx0023,
+            DiagnosticDescriptors.CliFx0024,
+            DiagnosticDescriptors.CliFx0041,
+            DiagnosticDescriptors.CliFx0042,
+            DiagnosticDescriptors.CliFx0043,
+            DiagnosticDescriptors.CliFx0044,
+            DiagnosticDescriptors.CliFx0045
+        );
+
+        private static bool IsScalarType(ITypeSymbol typeSymbol) =>
+            KnownSymbols.IsSystemString(typeSymbol) ||
+            !typeSymbol.AllInterfaces.Select(i => i.ConstructedFrom).Any(KnownSymbols.IsSystemCollectionsGenericIEnumerable);
+
+        private static void CheckCommandParameterProperties(
+            SymbolAnalysisContext context,
+            IReadOnlyList<IPropertySymbol> properties)
+        {
+            var parameters = properties
+                .Select(p =>
+                {
+                    var attribute = p
+                        .GetAttributes()
+                        .First(a => KnownSymbols.IsCommandParameterAttribute(a.AttributeClass));
+
+                    var order = attribute
+                        .ConstructorArguments
+                        .Select(a => a.Value)
+                        .FirstOrDefault() as int?;
+
+                    var name = attribute
+                        .NamedArguments
+                        .Where(a => a.Key == "Name")
+                        .Select(a => a.Value.Value)
+                        .FirstOrDefault() as string;
+
+                    return new
+                    {
+                        Property = p,
+                        Order = order,
+                        Name = name
+                    };
+                })
+                .ToArray();
+
+            // Duplicate order
+            var duplicateOrderParameters = parameters
+                .Where(p => p.Order != null)
+                .GroupBy(p => p.Order)
+                .Where(g => g.Count() > 1)
+                .SelectMany(g => g.AsEnumerable())
+                .ToArray();
+
+            foreach (var parameter in duplicateOrderParameters)
+            {
+                context.ReportDiagnostic(
+                    Diagnostic.Create(DiagnosticDescriptors.CliFx0021, parameter.Property.Locations.First()));
+            }
+
+            // Duplicate name
+            var duplicateNameParameters = parameters
+                .Where(p => !string.IsNullOrWhiteSpace(p.Name))
+                .GroupBy(p => p.Name, StringComparer.OrdinalIgnoreCase)
+                .Where(g => g.Count() > 1)
+                .SelectMany(g => g.AsEnumerable())
+                .ToArray();
+
+            foreach (var parameter in duplicateNameParameters)
+            {
+                context.ReportDiagnostic(
+                    Diagnostic.Create(DiagnosticDescriptors.CliFx0022, parameter.Property.Locations.First()));
+            }
+
+            // Multiple non-scalar
+            var nonScalarParameters = parameters
+                .Where(p => !IsScalarType(p.Property.Type))
+                .ToArray();
+
+            if (nonScalarParameters.Length > 1)
+            {
+                foreach (var parameter in nonScalarParameters)
+                {
+                    context.ReportDiagnostic(
+                        Diagnostic.Create(DiagnosticDescriptors.CliFx0023, parameter.Property.Locations.First()));
+                }
+            }
+
+            // Non-last non-scalar
+            var nonLastNonScalarParameter = parameters
+                .OrderByDescending(a => a.Order)
+                .Skip(1)
+                .LastOrDefault(p => !IsScalarType(p.Property.Type));
+
+            if (nonLastNonScalarParameter != null)
+            {
+                context.ReportDiagnostic(
+                    Diagnostic.Create(DiagnosticDescriptors.CliFx0024, nonLastNonScalarParameter.Property.Locations.First()));
+            }
+        }
+
+        private static void CheckCommandOptionProperties(
+            SymbolAnalysisContext context,
+            IReadOnlyList<IPropertySymbol> properties)
+        {
+            var options = properties
+                .Select(p =>
+                {
+                    var attribute = p
+                        .GetAttributes()
+                        .First(a => KnownSymbols.IsCommandOptionAttribute(a.AttributeClass));
+
+                    var name = attribute
+                        .ConstructorArguments
+                        .Where(a => KnownSymbols.IsSystemString(a.Type))
+                        .Select(a => a.Value)
+                        .FirstOrDefault() as string;
+
+                    var shortName = attribute
+                        .ConstructorArguments
+                        .Where(a => KnownSymbols.IsSystemChar(a.Type))
+                        .Select(a => a.Value)
+                        .FirstOrDefault() as char?;
+
+                    var envVarName = attribute
+                        .NamedArguments
+                        .Where(a => a.Key == "EnvironmentVariableName")
+                        .Select(a => a.Value.Value)
+                        .FirstOrDefault() as string;
+
+                    return new
+                    {
+                        Property = p,
+                        Name = name,
+                        ShortName = shortName,
+                        EnvironmentVariableName = envVarName
+                    };
+                })
+                .ToArray();
+
+            // No name
+            var noNameOptions = options
+                .Where(o => string.IsNullOrWhiteSpace(o.Name) && o.ShortName == null)
+                .ToArray();
+
+            foreach (var option in noNameOptions)
+            {
+                context.ReportDiagnostic(
+                    Diagnostic.Create(DiagnosticDescriptors.CliFx0041, option.Property.Locations.First()));
+            }
+
+            // Too short name
+            var invalidNameLengthOptions = options
+                .Where(o => !string.IsNullOrWhiteSpace(o.Name) && o.Name.Length <= 1)
+                .ToArray();
+
+            foreach (var option in invalidNameLengthOptions)
+            {
+                context.ReportDiagnostic(
+                    Diagnostic.Create(DiagnosticDescriptors.CliFx0042, option.Property.Locations.First()));
+            }
+
+            // Duplicate name
+            var duplicateNameOptions = options
+                .Where(p => !string.IsNullOrWhiteSpace(p.Name))
+                .GroupBy(p => p.Name, StringComparer.OrdinalIgnoreCase)
+                .Where(g => g.Count() > 1)
+                .SelectMany(g => g.AsEnumerable())
+                .ToArray();
+
+            foreach (var option in duplicateNameOptions)
+            {
+                context.ReportDiagnostic(
+                    Diagnostic.Create(DiagnosticDescriptors.CliFx0043, option.Property.Locations.First()));
+            }
+
+            // Duplicate name
+            var duplicateShortNameOptions = options
+                .Where(p => p.ShortName != null)
+                .GroupBy(p => p.ShortName)
+                .Where(g => g.Count() > 1)
+                .SelectMany(g => g.AsEnumerable())
+                .ToArray();
+
+            foreach (var option in duplicateShortNameOptions)
+            {
+                context.ReportDiagnostic(
+                    Diagnostic.Create(DiagnosticDescriptors.CliFx0044, option.Property.Locations.First()));
+            }
+
+            // Duplicate environment variable name
+            var duplicateEnvironmentVariableNameOptions = options
+                .Where(p => !string.IsNullOrWhiteSpace(p.EnvironmentVariableName))
+                .GroupBy(p => p.EnvironmentVariableName, StringComparer.OrdinalIgnoreCase)
+                .Where(g => g.Count() > 1)
+                .SelectMany(g => g.AsEnumerable())
+                .ToArray();
+
+            foreach (var option in duplicateEnvironmentVariableNameOptions)
+            {
+                context.ReportDiagnostic(
+                    Diagnostic.Create(DiagnosticDescriptors.CliFx0045, option.Property.Locations.First()));
+            }
+        }
+
+        private static void CheckCommandType(SymbolAnalysisContext context)
+        {
+            // Named type: MyCommand
+            if (!(context.Symbol is INamedTypeSymbol namedTypeSymbol))
+                return;
+
+            // Only classes
+            if (namedTypeSymbol.TypeKind != TypeKind.Class)
+                return;
+
+            // Implements ICommand?
+            var implementsCommandInterface = namedTypeSymbol
+                .AllInterfaces
+                .Any(KnownSymbols.IsCommandInterface);
+
+            // Has CommandAttribute?
+            var hasCommandAttribute = namedTypeSymbol
+                .GetAttributes()
+                .Select(a => a.AttributeClass)
+                .Any(KnownSymbols.IsCommandAttribute);
+
+            var isValidCommandType =
+                // implements interface
+                implementsCommandInterface && (
+                    // and either abstract class or has attribute
+                    namedTypeSymbol.IsAbstract || hasCommandAttribute
+                );
+
+            if (!isValidCommandType)
+            {
+                // See if this was meant to be a command type (either interface or attribute present)
+                var isAlmostValidCommandType = implementsCommandInterface ^ hasCommandAttribute;
+
+                if (isAlmostValidCommandType && !implementsCommandInterface)
+                    context.ReportDiagnostic(Diagnostic.Create(DiagnosticDescriptors.CliFx0001, namedTypeSymbol.Locations.First()));
+
+                if (isAlmostValidCommandType && !hasCommandAttribute)
+                    context.ReportDiagnostic(Diagnostic.Create(DiagnosticDescriptors.CliFx0002, namedTypeSymbol.Locations.First()));
+
+                return;
+            }
+
+            var properties = namedTypeSymbol
+                .GetMembers()
+                .Where(m => m.Kind == SymbolKind.Property)
+                .OfType<IPropertySymbol>().ToArray();
+
+            // Check parameters
+            var parameterProperties = properties
+                .Where(p => p
+                    .GetAttributes()
+                    .Select(a => a.AttributeClass)
+                    .Any(KnownSymbols.IsCommandParameterAttribute))
+                .ToArray();
+
+            CheckCommandParameterProperties(context, parameterProperties);
+
+            // Check options
+            var optionsProperties = properties
+                .Where(p => p
+                    .GetAttributes()
+                    .Select(a => a.AttributeClass)
+                    .Any(KnownSymbols.IsCommandOptionAttribute))
+                .ToArray();
+
+            CheckCommandOptionProperties(context, optionsProperties);
+        }
+
+        public override void Initialize(AnalysisContext context)
+        {
+            context.EnableConcurrentExecution();
+            context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+
+            context.RegisterSymbolAction(CheckCommandType, SymbolKind.NamedType);
+        }
+    }
+}

CliFx.Analyzers/ConsoleUsageAnalyzer.cs

@@ -0,0 +1,80 @@
+using System.Collections.Immutable;
+using System.Linq;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace CliFx.Analyzers
+{
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public class ConsoleUsageAnalyzer : DiagnosticAnalyzer
+    {
+        public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(
+            DiagnosticDescriptors.CliFx0100
+        );
+
+        private static bool IsSystemConsoleInvocation(
+            SyntaxNodeAnalysisContext context,
+            InvocationExpressionSyntax invocationSyntax)
+        {
+            // Get the method member access (Console.WriteLine or Console.Error.WriteLine)
+            if (!(invocationSyntax.Expression is MemberAccessExpressionSyntax memberAccessSyntax))
+                return false;
+
+            // Get the semantic model for the invoked method
+            if (!(context.SemanticModel.GetSymbolInfo(memberAccessSyntax).Symbol is IMethodSymbol methodSymbol))
+                return false;
+
+            // Check if contained within System.Console
+            if (KnownSymbols.IsSystemConsole(methodSymbol.ContainingType))
+                return true;
+
+            // In case with Console.Error.WriteLine that wouldn't work, we need to check parent member access too
+            if (!(memberAccessSyntax.Expression is MemberAccessExpressionSyntax parentMemberAccessSyntax))
+                return false;
+
+            // Get the semantic model for the parent member
+            if (!(context.SemanticModel.GetSymbolInfo(parentMemberAccessSyntax).Symbol is IPropertySymbol propertySymbol))
+                return false;
+
+            // Check if contained within System.Console
+            if (KnownSymbols.IsSystemConsole(propertySymbol.ContainingType))
+                return true;
+
+            return false;
+        }
+
+        private static void CheckSystemConsoleUsage(SyntaxNodeAnalysisContext context)
+        {
+            if (!(context.Node is InvocationExpressionSyntax invocationSyntax))
+                return;
+
+            if (!IsSystemConsoleInvocation(context, invocationSyntax))
+                return;
+
+            // Check if IConsole is available in the scope as a viable alternative
+            var isConsoleInterfaceAvailable = invocationSyntax
+                .Ancestors()
+                .OfType<MethodDeclarationSyntax>()
+                .SelectMany(m => m.ParameterList.Parameters)
+                .Select(p => p.Type)
+                .Select(t => context.SemanticModel.GetSymbolInfo(t).Symbol)
+                .Where(s => s != null)
+                .Any(KnownSymbols.IsConsoleInterface!);
+
+            if (!isConsoleInterfaceAvailable)
+                return;
+
+            context.ReportDiagnostic(Diagnostic.Create(DiagnosticDescriptors.CliFx0100, invocationSyntax.GetLocation()));
+        }
+
+        public override void Initialize(AnalysisContext context)
+        {
+            context.EnableConcurrentExecution();
+            context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+
+            context.RegisterSyntaxNodeAction(CheckSystemConsoleUsage, SyntaxKind.InvocationExpression);
+        }
+    }
+}

CliFx.Analyzers/DiagnosticDescriptors.cs

@@ -0,0 +1,79 @@
+using Microsoft.CodeAnalysis;
+
+namespace CliFx.Analyzers
+{
+    public static class DiagnosticDescriptors
+    {
+        public static readonly DiagnosticDescriptor CliFx0001 =
+            new DiagnosticDescriptor(nameof(CliFx0001),
+                "Type must implement the 'CliFx.ICommand' interface in order to be a valid command",
+                "Type must implement the 'CliFx.ICommand' interface in order to be a valid command",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0002 =
+            new DiagnosticDescriptor(nameof(CliFx0002),
+                "Type must be annotated with the 'CliFx.Attributes.CommandAttribute' in order to be a valid command",
+                "Type must be annotated with the 'CliFx.Attributes.CommandAttribute' in order to be a valid command",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0021 =
+            new DiagnosticDescriptor(nameof(CliFx0021),
+                "Parameter order must be unique within its command",
+                "Parameter order must be unique within its command",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0022 =
+            new DiagnosticDescriptor(nameof(CliFx0022),
+                "Parameter order must have unique name within its command",
+                "Parameter order must have unique name within its command",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0023 =
+            new DiagnosticDescriptor(nameof(CliFx0023),
+                "Only one non-scalar parameter per command is allowed",
+                "Only one non-scalar parameter per command is allowed",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0024 =
+            new DiagnosticDescriptor(nameof(CliFx0024),
+                "Non-scalar parameter must be last in order",
+                "Non-scalar parameter must be last in order",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0041 =
+            new DiagnosticDescriptor(nameof(CliFx0041),
+                "Option must have a name or short name specified",
+                "Option must have a name or short name specified",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0042 =
+            new DiagnosticDescriptor(nameof(CliFx0042),
+                "Option name must be at least 2 characters long",
+                "Option name must be at least 2 characters long",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0043 =
+            new DiagnosticDescriptor(nameof(CliFx0043),
+                "Option name must be unique within its command",
+                "Option name must be unique within its command",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0044 =
+            new DiagnosticDescriptor(nameof(CliFx0044),
+                "Option short name must be unique within its command",
+                "Option short name must be unique within its command",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0045 =
+            new DiagnosticDescriptor(nameof(CliFx0045),
+                "Option environment variable name must be unique within its command",
+                "Option environment variable name must be unique within its command",
+                "Usage", DiagnosticSeverity.Warning, true);
+
+        public static readonly DiagnosticDescriptor CliFx0100 =
+            new DiagnosticDescriptor(nameof(CliFx0100),
+                "Use the provided IConsole abstraction instead of System.Console to ensure that the command can be tested in isolation",
+                "Use the provided IConsole abstraction instead of System.Console to ensure that the command can be tested in isolation",
+                "Usage", DiagnosticSeverity.Warning, true);
+    }
+}

CliFx.Analyzers/Internal/RoslynExtensions.cs

@@ -0,0 +1,11 @@
+using System;
+using Microsoft.CodeAnalysis;
+
+namespace CliFx.Analyzers.Internal
+{
+    internal static class RoslynExtensions
+    {
+        public static bool DisplayNameMatches(this ISymbol symbol, string name) =>
+            string.Equals(symbol.ToDisplayString(), name, StringComparison.Ordinal);
+    }
+}

CliFx.Analyzers/KnownSymbols.cs

@@ -0,0 +1,37 @@
+using CliFx.Analyzers.Internal;
+using Microsoft.CodeAnalysis;
+
+namespace CliFx.Analyzers
+{
+    public static class KnownSymbols
+    {
+        public static bool IsSystemString(ISymbol symbol) =>
+            symbol.DisplayNameMatches("string") ||
+            symbol.DisplayNameMatches("System.String");
+
+        public static bool IsSystemChar(ISymbol symbol) =>
+            symbol.DisplayNameMatches("char") ||
+            symbol.DisplayNameMatches("System.Char");
+
+        public static bool IsSystemCollectionsGenericIEnumerable(ISymbol symbol) =>
+            symbol.DisplayNameMatches("System.Collections.Generic.IEnumerable<T>");
+
+        public static bool IsSystemConsole(ISymbol symbol) =>
+            symbol.DisplayNameMatches("System.Console");
+
+        public static bool IsConsoleInterface(ISymbol symbol) =>
+            symbol.DisplayNameMatches("CliFx.IConsole");
+
+        public static bool IsCommandInterface(ISymbol symbol) =>
+            symbol.DisplayNameMatches("CliFx.ICommand");
+
+        public static bool IsCommandAttribute(ISymbol symbol) =>
+            symbol.DisplayNameMatches("CliFx.Attributes.CommandAttribute");
+
+        public static bool IsCommandParameterAttribute(ISymbol symbol) =>
+            symbol.DisplayNameMatches("CliFx.Attributes.CommandParameterAttribute");
+
+        public static bool IsCommandOptionAttribute(ISymbol symbol) =>
+            symbol.DisplayNameMatches("CliFx.Attributes.CommandOptionAttribute");
+    }
+}

CliFx.Benchmarks/Benchmark.cs

@@ -1,34 +0,0 @@
-using System.Threading.Tasks;
-using BenchmarkDotNet.Attributes;
-using CliFx.Benchmarks.Commands;
-
-namespace CliFx.Benchmarks
-{
-    [CoreJob]
-    [RankColumn]
-    public class Benchmark
-    {
-        private static readonly string[] Arguments = { "--str", "hello world", "-i", "13", "-b" };
-
-        [Benchmark(Description = "CliFx", Baseline = true)]
-        public Task<int> ExecuteWithCliFx() => new CliApplicationBuilder().AddCommand(typeof(CliFxCommand)).Build().RunAsync(Arguments);
-
-        [Benchmark(Description = "System.CommandLine")]
-        public Task<int> ExecuteWithSystemCommandLine() => new SystemCommandLineCommand().ExecuteAsync(Arguments);
-
-        [Benchmark(Description = "McMaster.Extensions.CommandLineUtils")]
-        public int ExecuteWithMcMaster() => McMaster.Extensions.CommandLineUtils.CommandLineApplication.Execute<McMasterCommand>(Arguments);
-
-        // Skipped because this benchmark freezes after a couple of iterations
-        // Probably wasn't designed to run multiple times in single process execution
-        //[Benchmark(Description = "CommandLineParser")]
-        public void ExecuteWithCommandLineParser()
-        {
-            var parsed = CommandLine.Parser.Default.ParseArguments(Arguments, typeof(CommandLineParserCommand));
-            CommandLine.ParserResultExtensions.WithParsed<CommandLineParserCommand>(parsed, c => c.Execute());
-        }
-
-        [Benchmark(Description = "PowerArgs")]
-        public void ExecuteWithPowerArgs() => PowerArgs.Args.InvokeMain<PowerArgsCommand>(Arguments);
-    }
-}

CliFx.Benchmarks/Benchmarks.cs

@@ -0,0 +1,52 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Configs;
+using BenchmarkDotNet.Order;
+using BenchmarkDotNet.Running;
+using CliFx.Benchmarks.Commands;
+using CommandLine;
+
+namespace CliFx.Benchmarks
+{
+    [SimpleJob]
+    [RankColumn]
+    [Orderer(SummaryOrderPolicy.FastestToSlowest)]
+    public class Benchmarks
+    {
+        private static readonly string[] Arguments = {"--str", "hello world", "-i", "13", "-b"};
+
+        [Benchmark(Description = "CliFx", Baseline = true)]
+        public async ValueTask<int> ExecuteWithCliFx() =>
+            await new CliApplicationBuilder().AddCommand(typeof(CliFxCommand)).Build().RunAsync(Arguments, new Dictionary<string, string>());
+
+        [Benchmark(Description = "System.CommandLine")]
+        public async Task<int> ExecuteWithSystemCommandLine() =>
+            await new SystemCommandLineCommand().ExecuteAsync(Arguments);
+
+        [Benchmark(Description = "McMaster.Extensions.CommandLineUtils")]
+        public int ExecuteWithMcMaster() =>
+            McMaster.Extensions.CommandLineUtils.CommandLineApplication.Execute<McMasterCommand>(Arguments);
+
+        [Benchmark(Description = "CommandLineParser")]
+        public void ExecuteWithCommandLineParser() =>
+            new Parser()
+                .ParseArguments(Arguments, typeof(CommandLineParserCommand))
+                .WithParsed<CommandLineParserCommand>(c => c.Execute());
+
+        [Benchmark(Description = "PowerArgs")]
+        public void ExecuteWithPowerArgs() =>
+            PowerArgs.Args.InvokeMain<PowerArgsCommand>(Arguments);
+
+        [Benchmark(Description = "Clipr")]
+        public void ExecuteWithClipr() =>
+            clipr.CliParser.Parse<CliprCommand>(Arguments).Execute();
+
+        [Benchmark(Description = "Cocona")]
+        public void ExecuteWithCocona() =>
+            Cocona.CoconaApp.Run<CoconaCommand>(Arguments);
+
+        public static void Main() =>
+            BenchmarkRunner.Run<Benchmarks>(DefaultConfig.Instance.With(ConfigOptions.DisableOptimizationsValidator));
+    }
+}

CliFx.Benchmarks/CliFx.Benchmarks.csproj

@@ -1,15 +1,17 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="../CliFx.props" />
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
-    <LangVersion>latest</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.11.5" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.12.0" />
-    <PackageReference Include="CommandLineParser" Version="2.6.0" />
+    <PackageReference Include="clipr" Version="1.6.1" />
-    <PackageReference Include="McMaster.Extensions.CommandLineUtils" Version="2.3.4" />
+    <PackageReference Include="Cocona" Version="1.3.0" />
+    <PackageReference Include="CommandLineParser" Version="2.7.82" />
+    <PackageReference Include="McMaster.Extensions.CommandLineUtils" Version="2.6.0" />
     <PackageReference Include="PowerArgs" Version="3.6.0" />
     <PackageReference Include="System.CommandLine.Experimental" Version="0.3.0-alpha.19317.1" />
   </ItemGroup>

CliFx.Benchmarks/Commands/CliFxCommand.cs

@@ -1,6 +1,5 @@
 using System.Threading.Tasks;
 using CliFx.Attributes;
-using CliFx.Services;
 
 namespace CliFx.Benchmarks.Commands
 {
@@ -8,7 +7,7 @@ namespace CliFx.Benchmarks.Commands
     public class CliFxCommand : ICommand
     {
         [CommandOption("str", 's')]
-        public string StrOption { get; set; }
+        public string? StrOption { get; set; }
 
         [CommandOption("int", 'i')]
         public int IntOption { get; set; }
@@ -16,6 +15,6 @@ namespace CliFx.Benchmarks.Commands
         [CommandOption("bool", 'b')]
         public bool BoolOption { get; set; }
 
-        public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+        public ValueTask ExecuteAsync(IConsole console) => default;
     }
 }

CliFx.Benchmarks/Commands/CliprCommand.cs

@@ -0,0 +1,20 @@
+using clipr;
+
+namespace CliFx.Benchmarks.Commands
+{
+    public class CliprCommand
+    {
+        [NamedArgument('s', "str")]
+        public string? StrOption { get; set; }
+
+        [NamedArgument('i', "int")]
+        public int IntOption { get; set; }
+
+        [NamedArgument('b', "bool", Constraint = NumArgsConstraint.Optional, Const = true)]
+        public bool BoolOption { get; set; }
+
+        public void Execute()
+        {
+        }
+    }
+}

CliFx.Benchmarks/Commands/CoconaCommand.cs

@@ -0,0 +1,17 @@
+using Cocona;
+
+namespace CliFx.Benchmarks.Commands
+{
+    public class CoconaCommand
+    {
+        public void Execute(
+            [Option("str", new []{'s'})]
+            string? strOption,
+            [Option("int", new []{'i'})]
+            int intOption,
+            [Option("bool", new []{'b'})]
+            bool boolOption)
+        {
+        }
+    }
+}

CliFx.Benchmarks/Commands/CommandLineParserCommand.cs

@@ -5,7 +5,7 @@ namespace CliFx.Benchmarks.Commands
     public class CommandLineParserCommand
     {
         [Option('s', "str")]
-        public string StrOption { get; set; }
+        public string? StrOption { get; set; }
 
         [Option('i', "int")]
         public int IntOption { get; set; }

CliFx.Benchmarks/Commands/McMasterCommand.cs

@@ -5,7 +5,7 @@ namespace CliFx.Benchmarks.Commands
     public class McMasterCommand
     {
         [Option("--str|-s")]
-        public string StrOption { get; set; }
+        public string? StrOption { get; set; }
 
         [Option("--int|-i")]
         public int IntOption { get; set; }

CliFx.Benchmarks/Commands/PowerArgsCommand.cs

@@ -5,7 +5,7 @@ namespace CliFx.Benchmarks.Commands
     public class PowerArgsCommand
     {
         [ArgShortcut("--str"), ArgShortcut("-s")]
-        public string StrOption { get; set; }
+        public string? StrOption { get; set; }
 
         [ArgShortcut("--int"), ArgShortcut("-i")]
         public int IntOption { get; set; }

CliFx.Benchmarks/Commands/SystemCommandLineCommand.cs

@@ -14,7 +14,7 @@ namespace CliFx.Benchmarks.Commands
             {
                 new Option(new[] {"--str", "-s"})
                 {
-                    Argument = new Argument<string>()
+                    Argument = new Argument<string?>()
                 },
                 new Option(new[] {"--int", "-i"})
                 {

CliFx.Benchmarks/Program.cs

@@ -1,12 +0,0 @@
-using BenchmarkDotNet.Configs;
-using BenchmarkDotNet.Running;
-
-namespace CliFx.Benchmarks
-{
-    public static class Program
-    {
-        public static void Main() =>
-            BenchmarkRunner.Run(typeof(Program).Assembly, DefaultConfig.Instance
-                .With(ConfigOptions.DisableOptimizationsValidator));
-    }
-}

CliFx.Demo/CliFx.Demo.csproj

@@ -1,18 +1,19 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="../CliFx.props" />
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
-    <LangVersion>latest</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="2.2.0" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="3.1.2" />
-    <PackageReference Include="Newtonsoft.Json" Version="12.0.2" />
+    <PackageReference Include="Newtonsoft.Json" Version="12.0.3" />
   </ItemGroup>
 
   <ItemGroup>
     <ProjectReference Include="..\CliFx\CliFx.csproj" />
+    <ProjectReference Include="..\CliFx.Analyzers\CliFx.Analyzers.csproj" ReferenceOutputAssembly="false" OutputItemType="Analyzer" />
   </ItemGroup>
 
 </Project>

CliFx.Demo/Commands/BookAddCommand.cs

@@ -5,7 +5,6 @@ using CliFx.Demo.Internal;
 using CliFx.Demo.Models;
 using CliFx.Demo.Services;
 using CliFx.Exceptions;
-using CliFx.Services;
 
 namespace CliFx.Demo.Commands
 {
@@ -14,31 +13,25 @@ namespace CliFx.Demo.Commands
     {
         private readonly LibraryService _libraryService;
 
-        [CommandOption("title", 't', IsRequired = true, Description = "Book title.")]
+        [CommandParameter(0, Name = "title", Description = "Book title.")]
-        public string Title { get; set; }
+        public string Title { get; set; } = "";
 
         [CommandOption("author", 'a', IsRequired = true, Description = "Book author.")]
-        public string Author { get; set; }
+        public string Author { get; set; } = "";
 
         [CommandOption("published", 'p', Description = "Book publish date.")]
-        public DateTimeOffset Published { get; set; }
+        public DateTimeOffset Published { get; set; } = CreateRandomDate();
 
         [CommandOption("isbn", 'n', Description = "Book ISBN.")]
-        public Isbn Isbn { get; set; }
+        public Isbn Isbn { get; set; } = CreateRandomIsbn();
 
         public BookAddCommand(LibraryService libraryService)
         {
             _libraryService = libraryService;
         }
 
-        public Task ExecuteAsync(IConsole console)
+        public ValueTask ExecuteAsync(IConsole console)
         {
-            // To make the demo simpler, we will just generate random publish date and ISBN if they were not set
-            if (Published == default)
-                Published = CreateRandomDate();
-            if (Isbn == default)
-                Isbn = CreateRandomIsbn();
-
             if (_libraryService.GetBook(Title) != null)
                 throw new CommandException("Book already exists.", 1);
 
@@ -48,7 +41,7 @@ namespace CliFx.Demo.Commands
             console.Output.WriteLine("Book added.");
             console.RenderBook(book);
 
-            return Task.CompletedTask;
+            return default;
         }
     }
 
@@ -65,7 +58,7 @@ namespace CliFx.Demo.Commands
             Random.Next(1, 59),
             TimeSpan.Zero);
 
-        public static Isbn CreateRandomIsbn() => new Isbn(
+        private static Isbn CreateRandomIsbn() => new Isbn(
             Random.Next(0, 999),
             Random.Next(0, 99),
             Random.Next(0, 99999),

CliFx.Demo/Commands/BookCommand.cs

@@ -3,7 +3,6 @@ using CliFx.Attributes;
 using CliFx.Demo.Internal;
 using CliFx.Demo.Services;
 using CliFx.Exceptions;
-using CliFx.Services;
 
 namespace CliFx.Demo.Commands
 {
@@ -12,15 +11,15 @@ namespace CliFx.Demo.Commands
     {
         private readonly LibraryService _libraryService;
 
-        [CommandOption("title", 't', IsRequired = true, Description = "Book title.")]
+        [CommandParameter(0, Name = "title", Description = "Book title.")]
-        public string Title { get; set; }
+        public string Title { get; set; } = "";
 
         public BookCommand(LibraryService libraryService)
         {
             _libraryService = libraryService;
         }
 
-        public Task ExecuteAsync(IConsole console)
+        public ValueTask ExecuteAsync(IConsole console)
         {
             var book = _libraryService.GetBook(Title);
 
@@ -29,7 +28,7 @@ namespace CliFx.Demo.Commands
 
             console.RenderBook(book);
 
-            return Task.CompletedTask;
+            return default;
         }
     }
 }

CliFx.Demo/Commands/BookListCommand.cs

@@ -2,7 +2,6 @@
 using CliFx.Attributes;
 using CliFx.Demo.Internal;
 using CliFx.Demo.Services;
-using CliFx.Services;
 
 namespace CliFx.Demo.Commands
 {
@@ -16,7 +15,7 @@ namespace CliFx.Demo.Commands
             _libraryService = libraryService;
         }
 
-        public Task ExecuteAsync(IConsole console)
+        public ValueTask ExecuteAsync(IConsole console)
         {
             var library = _libraryService.GetLibrary();
 
@@ -32,7 +31,7 @@ namespace CliFx.Demo.Commands
                 console.RenderBook(book);
             }
 
-            return Task.CompletedTask;
+            return default;
         }
     }
 }

CliFx.Demo/Commands/BookRemoveCommand.cs

@@ -2,7 +2,6 @@
 using CliFx.Attributes;
 using CliFx.Demo.Services;
 using CliFx.Exceptions;
-using CliFx.Services;
 
 namespace CliFx.Demo.Commands
 {
@@ -11,15 +10,15 @@ namespace CliFx.Demo.Commands
     {
         private readonly LibraryService _libraryService;
 
-        [CommandOption("title", 't', IsRequired = true, Description = "Book title.")]
+        [CommandParameter(0, Name = "title", Description = "Book title.")]
-        public string Title { get; set; }
+        public string Title { get; set; } = "";
 
         public BookRemoveCommand(LibraryService libraryService)
         {
             _libraryService = libraryService;
         }
 
-        public Task ExecuteAsync(IConsole console)
+        public ValueTask ExecuteAsync(IConsole console)
         {
             var book = _libraryService.GetBook(Title);
 
@@ -30,7 +29,7 @@ namespace CliFx.Demo.Commands
 
             console.Output.WriteLine($"Book {Title} removed.");
 
-            return Task.CompletedTask;
+            return default;
         }
     }
 }

CliFx.Demo/Internal/Extensions.cs

@@ -1,6 +1,5 @@
 using System;
 using CliFx.Demo.Models;
-using CliFx.Services;
 
 namespace CliFx.Demo.Internal
 {

CliFx.Demo/Models/Isbn.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Globalization;
 
 namespace CliFx.Demo.Models
 {
@@ -24,21 +23,23 @@ namespace CliFx.Demo.Models
             CheckDigit = checkDigit;
         }
 
-        public override string ToString() => $"{EanPrefix:000}-{RegistrationGroup:00}-{Registrant:00000}-{Publication:00}-{CheckDigit:0}";
+        public override string ToString() =>
+            $"{EanPrefix:000}-{RegistrationGroup:00}-{Registrant:00000}-{Publication:00}-{CheckDigit:0}";
     }
 
     public partial class Isbn
     {
-        public static Isbn Parse(string value)
+        public static Isbn Parse(string value, IFormatProvider formatProvider)
         {
             var components = value.Split('-', 5, StringSplitOptions.RemoveEmptyEntries);
 
             return new Isbn(
-                int.Parse(components[0], CultureInfo.InvariantCulture),
+                int.Parse(components[0], formatProvider),
-                int.Parse(components[1], CultureInfo.InvariantCulture),
+                int.Parse(components[1], formatProvider),
-                int.Parse(components[2], CultureInfo.InvariantCulture),
+                int.Parse(components[2], formatProvider),
-                int.Parse(components[3], CultureInfo.InvariantCulture),
+                int.Parse(components[3], formatProvider),
-                int.Parse(components[4], CultureInfo.InvariantCulture));
+                int.Parse(components[4], formatProvider)
+            );
         }
     }
 }

CliFx.Demo/Program.cs

@@ -1,4 +1,5 @@
-using System.Threading.Tasks;
+using System;
+using System.Threading.Tasks;
 using CliFx.Demo.Commands;
 using CliFx.Demo.Services;
 using Microsoft.Extensions.DependencyInjection;
@@ -7,7 +8,7 @@ namespace CliFx.Demo
 {
     public static class Program
     {
-        public static Task<int> Main(string[] args)
+        private static IServiceProvider GetServiceProvider()
         {
             // We use Microsoft.Extensions.DependencyInjection for injecting dependencies in commands
             var services = new ServiceCollection();
@@ -21,13 +22,14 @@ namespace CliFx.Demo
             services.AddTransient<BookRemoveCommand>();
             services.AddTransient<BookListCommand>();
 
-            var serviceProvider = services.BuildServiceProvider();
+            return services.BuildServiceProvider();
-
-            return new CliApplicationBuilder()
-                .AddCommandsFromThisAssembly()
-                .UseCommandFactory(schema => (ICommand) serviceProvider.GetRequiredService(schema.Type))
-                .Build()
-                .RunAsync(args);
         }
+
+        public static async Task<int> Main() =>
+            await new CliApplicationBuilder()
+                .AddCommandsFromThisAssembly()
+                .UseTypeActivator(GetServiceProvider().GetService)
+                .Build()
+                .RunAsync();
     }
 }

CliFx.Demo/Readme.md

@@ -2,6 +2,6 @@
 
 Sample command line interface for managing a library of books.
 
-This demo project shows basic CliFx functionality such as command routing, option parsing, autogenerated help text, and some other things.
+This demo project shows basic CliFx functionality such as command routing, argument parsing, autogenerated help text, and some other things.
 
 You can get a list of available commands by running `CliFx.Demo --help`.

CliFx.Demo/Services/LibraryService.cs

@@ -25,7 +25,7 @@ namespace CliFx.Demo.Services
             return JsonConvert.DeserializeObject<Library>(data);
         }
 
-        public Book GetBook(string title) => GetLibrary().Books.FirstOrDefault(b => b.Title == title);
+        public Book? GetBook(string title) => GetLibrary().Books.FirstOrDefault(b => b.Title == title);
 
         public void AddBook(Book book)
         {

CliFx.Tests.Dummy/CliFx.Tests.Dummy.csproj

@@ -1,14 +1,14 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="../CliFx.props" />
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net46</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
-    <Version>1.2.3.4</Version>
-    <LangVersion>latest</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
     <ProjectReference Include="..\CliFx\CliFx.csproj" />
+    <ProjectReference Include="..\CliFx.Analyzers\CliFx.Analyzers.csproj" ReferenceOutputAssembly="false" OutputItemType="Analyzer" />
   </ItemGroup>
 
 </Project>

CliFx.Tests.Dummy/Commands/ConsoleTestCommand.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests.Dummy.Commands
+{
+    [Command("console-test")]
+    public class ConsoleTestCommand : ICommand
+    {
+        public ValueTask ExecuteAsync(IConsole console)
+        {
+            var input = console.Input.ReadToEnd();
+
+            console.WithColors(ConsoleColor.Black, ConsoleColor.White, () =>
+            {
+                console.Output.WriteLine(input);
+                console.Error.WriteLine(input);
+            });
+
+            return default;
+        }
+    }
+}

CliFx.Tests.Dummy/Commands/GreeterCommand.cs

@@ -1,31 +0,0 @@
-using System.Text;
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Services;
-
-namespace CliFx.Tests.Dummy.Commands
-{
-    [Command]
-    public class GreeterCommand : ICommand
-    {
-        [CommandOption("target", 't', Description = "Greeting target.")]
-        public string Target { get; set; } = "world";
-
-        [CommandOption('e', Description = "Whether the greeting should be exclaimed.")]
-        public bool IsExclaimed { get; set; }
-
-        public Task ExecuteAsync(IConsole console)
-        {
-            var buffer = new StringBuilder();
-
-            buffer.Append("Hello").Append(' ').Append(Target);
-
-            if (IsExclaimed)
-                buffer.Append('!');
-
-            console.Output.WriteLine(buffer.ToString());
-
-            return Task.CompletedTask;
-        }
-    }
-}

CliFx.Tests.Dummy/Commands/HelloWorldCommand.cs

@@ -0,0 +1,19 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests.Dummy.Commands
+{
+    [Command]
+    public class HelloWorldCommand : ICommand
+    {
+        [CommandOption("target", EnvironmentVariableName = "ENV_TARGET")]
+        public string Target { get; set; } = "World";
+
+        public ValueTask ExecuteAsync(IConsole console)
+        {
+            console.Output.WriteLine($"Hello {Target}!");
+
+            return default;
+        }
+    }
+}

CliFx.Tests.Dummy/Commands/LogCommand.cs

@@ -1,25 +0,0 @@
-using System;
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Services;
-
-namespace CliFx.Tests.Dummy.Commands
-{
-    [Command("log", Description = "Calculate the logarithm of a value.")]
-    public class LogCommand : ICommand
-    {
-        [CommandOption("value", 'v', IsRequired = true, Description = "Value whose logarithm is to be found.")]
-        public double Value { get; set; }
-
-        [CommandOption("base", 'b', Description = "Logarithm base.")]
-        public double Base { get; set; } = 10;
-
-        public Task ExecuteAsync(IConsole console)
-        {
-            var result = Math.Log(Value, Base);
-            console.Output.WriteLine(result);
-
-            return Task.CompletedTask;
-        }
-    }
-}

CliFx.Tests.Dummy/Commands/ShowErrorMessageThenHelpTextOnErrorCommand.cs

@@ -0,0 +1,20 @@
+using CliFx.Attributes;
+using CliFx.Exceptions;
+using System.Threading.Tasks;
+
+namespace CliFx.Tests.Dummy.Commands
+{
+    /// <summary>
+    /// Demos how to show an error message then help text from an organizational command.
+    /// </summary>
+    [Command("cmd-err", Description = "This is an organizational command. " +
+        "I don't do anything except provide a route to my subcommands. " +
+        "If you use just me, I print an error message then the help text " +
+        "to remind you of my subcommands.")]
+    public class ShowErrorMessageThenHelpTextOnCommandExceptionCommand : ICommand
+    {
+        public ValueTask ExecuteAsync(IConsole console) =>
+            throw new CommandException("It is an error to use me without a subcommand. " +
+                "Please refer to the help text below for guidance.", showHelp: true);
+    }
+}

CliFx.Tests.Dummy/Commands/ShowHelpTextOnErrorCommand.cs

@@ -0,0 +1,18 @@
+using CliFx.Attributes;
+using CliFx.Exceptions;
+using System.Threading.Tasks;
+
+namespace CliFx.Tests.Dummy.Commands
+{
+    /// <summary>
+    /// Demos how to show help text from an organizational command.
+    /// </summary>
+    [Command("cmd", Description = "This is an organizational command. " +
+        "I don't do anything except provide a route to my subcommands. " +
+        "If you use just me, I print the help text to remind you of my subcommands.")]
+    public class ShowHelpTextOnCommandExceptionCommand : ICommand
+    {
+        public ValueTask ExecuteAsync(IConsole console) =>
+            throw new CommandException(null, showHelp: false);
+    }
+}

CliFx.Tests.Dummy/Commands/SumCommand.cs

@@ -1,23 +0,0 @@
-using System.Collections.Generic;
-using System.Linq;
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Services;
-
-namespace CliFx.Tests.Dummy.Commands
-{
-    [Command("sum", Description = "Calculate the sum of all input values.")]
-    public class SumCommand : ICommand
-    {
-        [CommandOption("values", 'v', IsRequired = true, Description = "Input values.")]
-        public IReadOnlyList<double> Values { get; set; }
-
-        public Task ExecuteAsync(IConsole console)
-        {
-            var result = Values.Sum();
-            console.Output.WriteLine(result);
-
-            return Task.CompletedTask;
-        }
-    }
-}

CliFx.Tests.Dummy/Program.cs

@@ -1,21 +1,21 @@
-using System.Globalization;
+using System.Reflection;
 using System.Threading.Tasks;
 
 namespace CliFx.Tests.Dummy
 {
-    public static class Program
+    public static partial class Program
     {
-        public static Task<int> Main(string[] args)
+        public static Assembly Assembly { get; } = typeof(Program).Assembly;
-        {
-            // Set culture to invariant to maintain consistent format because we rely on it in tests
-            CultureInfo.DefaultThreadCurrentCulture = CultureInfo.InvariantCulture;
-            CultureInfo.DefaultThreadCurrentUICulture = CultureInfo.InvariantCulture;
 
-            return new CliApplicationBuilder()
+        public static string Location { get; } = Assembly.Location;
+    }
+
+    public static partial class Program
+    {
+        public static async Task Main() =>
+            await new CliApplicationBuilder()
                 .AddCommandsFromThisAssembly()
-                .UseDescription("Dummy program used for E2E tests.")
                 .Build()
-                .RunAsync(args);
+                .RunAsync();
-        }
     }
 }

CliFx.Tests/ApplicationSpecs.Commands.cs

@@ -0,0 +1,155 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests
+{
+    public partial class ApplicationSpecs
+    {
+        [Command]
+        private class NonImplementedCommand
+        {
+        }
+
+        private class NonAnnotatedCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("dup")]
+        private class DuplicateNameCommandA : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("dup")]
+        private class DuplicateNameCommandB : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class DuplicateParameterOrderCommand : ICommand
+        {
+            [CommandParameter(13)]
+            public string? ParameterA { get; set; }
+
+            [CommandParameter(13)]
+            public string? ParameterB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class DuplicateParameterNameCommand : ICommand
+        {
+            [CommandParameter(0, Name = "param")]
+            public string? ParameterA { get; set; }
+
+            [CommandParameter(1, Name = "param")]
+            public string? ParameterB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class MultipleNonScalarParametersCommand : ICommand
+        {
+            [CommandParameter(0)]
+            public IReadOnlyList<string>? ParameterA { get; set; }
+
+            [CommandParameter(1)]
+            public IReadOnlyList<string>? ParameterB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class NonLastNonScalarParameterCommand : ICommand
+        {
+            [CommandParameter(0)]
+            public IReadOnlyList<string>? ParameterA { get; set; }
+
+            [CommandParameter(1)]
+            public string? ParameterB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class EmptyOptionNameCommand : ICommand
+        {
+            [CommandOption("")]
+            public string? Apples { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class SingleCharacterOptionNameCommand : ICommand
+        {
+            [CommandOption("a")]
+            public string? Apples { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class DuplicateOptionNamesCommand : ICommand
+        {
+            [CommandOption("fruits")]
+            public string? Apples { get; set; }
+
+            [CommandOption("fruits")]
+            public string? Oranges { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class DuplicateOptionShortNamesCommand : ICommand
+        {
+            [CommandOption('x')]
+            public string? OptionA { get; set; }
+
+            [CommandOption('x')]
+            public string? OptionB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class DuplicateOptionEnvironmentVariableNamesCommand : ICommand
+        {
+            [CommandOption("option-a", EnvironmentVariableName = "ENV_VAR")]
+            public string? OptionA { get; set; }
+
+            [CommandOption("option-b", EnvironmentVariableName = "ENV_VAR")]
+            public string? OptionB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class ValidCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("hidden", Description = "Description")]
+        private class HiddenPropertiesCommand : ICommand
+        {
+            [CommandParameter(13, Name = "param", Description = "Param description")]
+            public string? Parameter { get; set; }
+
+            [CommandOption("option", 'o', Description = "Option description", EnvironmentVariableName = "ENV")]
+            public string? Option { get; set; }
+
+            public string? HiddenA { get; set; }
+
+            public bool? HiddenB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+    }
+}

CliFx.Tests/ApplicationSpecs.cs

@@ -0,0 +1,235 @@
+using System;
+using System.IO;
+using CliFx.Domain;
+using CliFx.Exceptions;
+using FluentAssertions;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace CliFx.Tests
+{
+    public partial class ApplicationSpecs
+    {
+        private readonly ITestOutputHelper _output;
+
+        public ApplicationSpecs(ITestOutputHelper output) => _output = output;
+
+        [Fact]
+        public void Application_can_be_created_with_a_default_configuration()
+        {
+            // Act
+            var app = new CliApplicationBuilder()
+                .AddCommandsFromThisAssembly()
+                .Build();
+
+            // Assert
+            app.Should().NotBeNull();
+        }
+
+        [Fact]
+        public void Application_can_be_created_with_a_custom_configuration()
+        {
+            // Act
+            var app = new CliApplicationBuilder()
+                .AddCommand(typeof(ValidCommand))
+                .AddCommandsFrom(typeof(ValidCommand).Assembly)
+                .AddCommands(new[] {typeof(ValidCommand)})
+                .AddCommandsFrom(new[] {typeof(ValidCommand).Assembly})
+                .AddCommandsFromThisAssembly()
+                .AllowDebugMode()
+                .AllowPreviewMode()
+                .UseTitle("test")
+                .UseExecutableName("test")
+                .UseVersionText("test")
+                .UseDescription("test")
+                .UseConsole(new VirtualConsole(Stream.Null))
+                .UseTypeActivator(Activator.CreateInstance!)
+                .Build();
+
+            // Assert
+            app.Should().NotBeNull();
+        }
+
+        [Fact]
+        public void At_least_one_command_must_be_defined_in_an_application()
+        {
+            // Arrange
+            var commandTypes = Array.Empty<Type>();
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Commands_must_implement_the_corresponding_interface()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(NonImplementedCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Commands_must_be_annotated_by_an_attribute()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(NonAnnotatedCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Commands_must_have_unique_names()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(DuplicateNameCommandA), typeof(DuplicateNameCommandB)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_parameters_must_have_unique_order()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(DuplicateParameterOrderCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_parameters_must_have_unique_names()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(DuplicateParameterNameCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_parameter_can_be_non_scalar_only_if_no_other_such_parameter_is_present()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(MultipleNonScalarParametersCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_parameter_can_be_non_scalar_only_if_it_is_the_last_in_order()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(NonLastNonScalarParameterCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_options_must_have_names_that_are_not_empty()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(EmptyOptionNameCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_options_must_have_names_that_are_longer_than_one_character()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(SingleCharacterOptionNameCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_options_must_have_unique_names()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(DuplicateOptionNamesCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_options_must_have_unique_short_names()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(DuplicateOptionShortNamesCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_options_must_have_unique_environment_variable_names()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(DuplicateOptionEnvironmentVariableNamesCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_options_and_parameters_must_be_annotated_by_corresponding_attributes()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(HiddenPropertiesCommand)};
+
+            // Act
+            var schema = ApplicationSchema.Resolve(commandTypes);
+
+            // Assert
+            schema.Should().BeEquivalentTo(new ApplicationSchema(new[]
+            {
+                new CommandSchema(
+                    typeof(HiddenPropertiesCommand),
+                    "hidden",
+                    "Description",
+                    new[]
+                    {
+                        new CommandParameterSchema(
+                            typeof(HiddenPropertiesCommand).GetProperty(nameof(HiddenPropertiesCommand.Parameter))!,
+                            13,
+                            "param",
+                            "Param description")
+                    },
+                    new[]
+                    {
+                        new CommandOptionSchema(
+                            typeof(HiddenPropertiesCommand).GetProperty(nameof(HiddenPropertiesCommand.Option))!,
+                            "option",
+                            'o',
+                            "ENV",
+                            false,
+                            "Option description")
+                    })
+            }));
+
+            schema.ToString().Should().NotBeNullOrWhiteSpace(); // this is only for coverage, I'm sorry
+        }
+    }
+}

CliFx.Tests/ArgumentBindingSpecs.Commands.cs

@@ -0,0 +1,191 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests
+{
+    public partial class ArgumentBindingSpecs
+    {
+        [Command]
+        private class AllSupportedTypesCommand : ICommand
+        {
+            [CommandOption(nameof(Object))]
+            public object? Object { get; set; } = 42;
+
+            [CommandOption(nameof(String))]
+            public string? String { get; set; } = "foo bar";
+
+            [CommandOption(nameof(Bool))]
+            public bool Bool { get; set; }
+
+            [CommandOption(nameof(Char))]
+            public char Char { get; set; }
+
+            [CommandOption(nameof(Sbyte))]
+            public sbyte Sbyte { get; set; }
+
+            [CommandOption(nameof(Byte))]
+            public byte Byte { get; set; }
+
+            [CommandOption(nameof(Short))]
+            public short Short { get; set; }
+
+            [CommandOption(nameof(Ushort))]
+            public ushort Ushort { get; set; }
+
+            [CommandOption(nameof(Int))]
+            public int Int { get; set; }
+
+            [CommandOption(nameof(Uint))]
+            public uint Uint { get; set; }
+
+            [CommandOption(nameof(Long))]
+            public long Long { get; set; }
+
+            [CommandOption(nameof(Ulong))]
+            public ulong Ulong { get; set; }
+
+            [CommandOption(nameof(Float))]
+            public float Float { get; set; }
+
+            [CommandOption(nameof(Double))]
+            public double Double { get; set; }
+
+            [CommandOption(nameof(Decimal))]
+            public decimal Decimal { get; set; }
+
+            [CommandOption(nameof(DateTime))]
+            public DateTime DateTime { get; set; }
+
+            [CommandOption(nameof(DateTimeOffset))]
+            public DateTimeOffset DateTimeOffset { get; set; }
+
+            [CommandOption(nameof(TimeSpan))]
+            public TimeSpan TimeSpan { get; set; }
+
+            [CommandOption(nameof(CustomEnum))]
+            public CustomEnum CustomEnum { get; set; }
+
+            [CommandOption(nameof(IntNullable))]
+            public int? IntNullable { get; set; }
+
+            [CommandOption(nameof(CustomEnumNullable))]
+            public CustomEnum? CustomEnumNullable { get; set; }
+
+            [CommandOption(nameof(TimeSpanNullable))]
+            public TimeSpan? TimeSpanNullable { get; set; }
+
+            [CommandOption(nameof(TestStringConstructable))]
+            public StringConstructable? TestStringConstructable { get; set; }
+
+            [CommandOption(nameof(TestStringParseable))]
+            public StringParseable? TestStringParseable { get; set; }
+
+            [CommandOption(nameof(TestStringParseableWithFormatProvider))]
+            public StringParseableWithFormatProvider? TestStringParseableWithFormatProvider { get; set; }
+
+            [CommandOption(nameof(ObjectArray))]
+            public object[]? ObjectArray { get; set; }
+
+            [CommandOption(nameof(StringArray))]
+            public string[]? StringArray { get; set; }
+
+            [CommandOption(nameof(IntArray))]
+            public int[]? IntArray { get; set; }
+
+            [CommandOption(nameof(CustomEnumArray))]
+            public CustomEnum[]? CustomEnumArray { get; set; }
+
+            [CommandOption(nameof(IntNullableArray))]
+            public int?[]? IntNullableArray { get; set; }
+
+            [CommandOption(nameof(TestStringConstructableArray))]
+            public StringConstructable[]? TestStringConstructableArray { get; set; }
+
+            [CommandOption(nameof(Enumerable))]
+            public IEnumerable? Enumerable { get; set; }
+
+            [CommandOption(nameof(StringEnumerable))]
+            public IEnumerable<string>? StringEnumerable { get; set; }
+
+            [CommandOption(nameof(StringReadOnlyList))]
+            public IReadOnlyList<string>? StringReadOnlyList { get; set; }
+
+            [CommandOption(nameof(StringList))]
+            public List<string>? StringList { get; set; }
+
+            [CommandOption(nameof(StringHashSet))]
+            public HashSet<string>? StringHashSet { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class ArrayOptionCommand : ICommand
+        {
+            [CommandOption("option", 'o')]
+            public IReadOnlyList<string>? Option { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class RequiredOptionCommand : ICommand
+        {
+            [CommandOption(nameof(OptionA))]
+            public string? OptionA { get; set; }
+
+            [CommandOption(nameof(OptionB), IsRequired = true)]
+            public string? OptionB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class ParametersCommand : ICommand
+        {
+            [CommandParameter(0)]
+            public string? ParameterA { get; set; }
+
+            [CommandParameter(1)]
+            public string? ParameterB { get; set; }
+
+            [CommandParameter(2)]
+            public IReadOnlyList<string>? ParameterC { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class UnsupportedPropertyTypeCommand : ICommand
+        {
+            [CommandOption(nameof(Option))]
+            public DummyType? Option { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class UnsupportedEnumerablePropertyTypeCommand : ICommand
+        {
+            [CommandOption(nameof(Option))]
+            public CustomEnumerable<string>? Option { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class NoParameterCommand : ICommand
+        {
+            [CommandOption(nameof(OptionA))]
+            public string? OptionA { get; set; }
+
+            [CommandOption(nameof(OptionB))]
+            public string? OptionB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+    }
+}

CliFx.Tests/ArgumentBindingSpecs.Types.cs

@@ -0,0 +1,53 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+
+namespace CliFx.Tests
+{
+    public partial class ArgumentBindingSpecs
+    {
+        private enum CustomEnum
+        {
+            Value1 = 1,
+            Value2 = 2,
+            Value3 = 3
+        }
+
+        private class StringConstructable
+        {
+            public string Value { get; }
+
+            public StringConstructable(string value) => Value = value;
+        }
+
+        private class StringParseable
+        {
+            public string Value { get; }
+
+            private StringParseable(string value) => Value = value;
+
+            public static StringParseable Parse(string value) => new StringParseable(value);
+        }
+
+        private class StringParseableWithFormatProvider
+        {
+            public string Value { get; }
+
+            private StringParseableWithFormatProvider(string value) => Value = value;
+
+            public static StringParseableWithFormatProvider Parse(string value, IFormatProvider formatProvider) =>
+                new StringParseableWithFormatProvider(value + " " + formatProvider);
+        }
+
+        private class DummyType
+        {
+        }
+
+        public class CustomEnumerable<T> : IEnumerable<T>
+        {
+            public IEnumerator<T> GetEnumerator() => ((IEnumerable<T>) Array.Empty<T>()).GetEnumerator();
+
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        }
+    }
+}

CliFx.Tests/ArgumentSyntaxSpecs.cs

@@ -0,0 +1,315 @@
+using System;
+using CliFx.Domain;
+using FluentAssertions;
+using Xunit;
+
+namespace CliFx.Tests
+{
+    public class ArgumentSyntaxSpecs
+    {
+        [Fact]
+        public void Input_is_empty_if_no_arguments_are_provided()
+        {
+            // Arrange
+            var args = Array.Empty<string>();
+
+            // Act
+            var input = CommandLineInput.Parse(args);
+
+            // Assert
+            input.Should().BeEquivalentTo(CommandLineInput.Empty);
+        }
+
+        public static object[][] DirectivesTestData => new[]
+        {
+            new object[]
+            {
+                new[] {"[preview]"},
+                new CommandLineInputBuilder()
+                    .AddDirective("preview")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"[preview]", "[debug]"},
+                new CommandLineInputBuilder()
+                    .AddDirective("preview")
+                    .AddDirective("debug")
+                    .Build()
+            }
+        };
+
+        [Theory]
+        [MemberData(nameof(DirectivesTestData))]
+        internal void Directive_can_be_enabled_by_specifying_its_name_in_square_brackets(string[] arguments, CommandLineInput expectedInput)
+        {
+            // Act
+            var input = CommandLineInput.Parse(arguments);
+
+            // Assert
+            input.Should().BeEquivalentTo(expectedInput);
+        }
+
+        public static object[][] OptionsTestData => new[]
+        {
+            new object[]
+            {
+                new[] {"--option"},
+                new CommandLineInputBuilder()
+                    .AddOption("option")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"--option", "value"},
+                new CommandLineInputBuilder()
+                    .AddOption("option", "value")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"--option", "value1", "value2"},
+                new CommandLineInputBuilder()
+                    .AddOption("option", "value1", "value2")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"--option", "same value"},
+                new CommandLineInputBuilder()
+                    .AddOption("option", "same value")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"--option1", "--option2"},
+                new CommandLineInputBuilder()
+                    .AddOption("option1")
+                    .AddOption("option2")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"--option1", "value1", "--option2", "value2"},
+                new CommandLineInputBuilder()
+                    .AddOption("option1", "value1")
+                    .AddOption("option2", "value2")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"--option1", "value1", "value2", "--option2", "value3", "value4"},
+                new CommandLineInputBuilder()
+                    .AddOption("option1", "value1", "value2")
+                    .AddOption("option2", "value3", "value4")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"--option1", "value1", "value2", "--option2"},
+                new CommandLineInputBuilder()
+                    .AddOption("option1", "value1", "value2")
+                    .AddOption("option2")
+                    .Build()
+            }
+        };
+
+        [Theory]
+        [MemberData(nameof(OptionsTestData))]
+        internal void Option_can_be_set_by_specifying_its_name_after_two_dashes(string[] arguments, CommandLineInput expectedInput)
+        {
+            // Act
+            var input = CommandLineInput.Parse(arguments);
+
+            // Assert
+            input.Should().BeEquivalentTo(expectedInput);
+        }
+
+        public static object[][] ShortOptionsTestData => new[]
+        {
+            new object[]
+            {
+                new[] {"-o"},
+                new CommandLineInputBuilder()
+                    .AddOption("o")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-o", "value"},
+                new CommandLineInputBuilder()
+                    .AddOption("o", "value")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-o", "value1", "value2"},
+                new CommandLineInputBuilder()
+                    .AddOption("o", "value1", "value2")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-o", "same value"},
+                new CommandLineInputBuilder()
+                    .AddOption("o", "same value")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-a", "-b"},
+                new CommandLineInputBuilder()
+                    .AddOption("a")
+                    .AddOption("b")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-a", "value1", "-b", "value2"},
+                new CommandLineInputBuilder()
+                    .AddOption("a", "value1")
+                    .AddOption("b", "value2")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-a", "value1", "value2", "-b", "value3", "value4"},
+                new CommandLineInputBuilder()
+                    .AddOption("a", "value1", "value2")
+                    .AddOption("b", "value3", "value4")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-a", "value1", "value2", "-b"},
+                new CommandLineInputBuilder()
+                    .AddOption("a", "value1", "value2")
+                    .AddOption("b")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-abc"},
+                new CommandLineInputBuilder()
+                    .AddOption("a")
+                    .AddOption("b")
+                    .AddOption("c")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-abc", "value"},
+                new CommandLineInputBuilder()
+                    .AddOption("a")
+                    .AddOption("b")
+                    .AddOption("c", "value")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"-abc", "value1", "value2"},
+                new CommandLineInputBuilder()
+                    .AddOption("a")
+                    .AddOption("b")
+                    .AddOption("c", "value1", "value2")
+                    .Build()
+            }
+        };
+
+        [Theory]
+        [MemberData(nameof(ShortOptionsTestData))]
+        internal void Option_can_be_set_by_specifying_its_short_name_after_a_single_dash(string[] arguments, CommandLineInput expectedInput)
+        {
+            // Act
+            var input = CommandLineInput.Parse(arguments);
+
+            // Assert
+            input.Should().BeEquivalentTo(expectedInput);
+        }
+
+        public static object[][] UnboundArgumentsTestData => new[]
+        {
+            new object[]
+            {
+                new[] {"foo"},
+                new CommandLineInputBuilder()
+                    .AddUnboundArgument("foo")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"foo", "bar"},
+                new CommandLineInputBuilder()
+                    .AddUnboundArgument("foo")
+                    .AddUnboundArgument("bar")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"[preview]", "foo"},
+                new CommandLineInputBuilder()
+                    .AddDirective("preview")
+                    .AddUnboundArgument("foo")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"foo", "--option", "value", "-abc"},
+                new CommandLineInputBuilder()
+                    .AddUnboundArgument("foo")
+                    .AddOption("option", "value")
+                    .AddOption("a")
+                    .AddOption("b")
+                    .AddOption("c")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"[preview]", "[debug]", "foo", "bar", "--option", "value", "-abc"},
+                new CommandLineInputBuilder()
+                    .AddDirective("preview")
+                    .AddDirective("debug")
+                    .AddUnboundArgument("foo")
+                    .AddUnboundArgument("bar")
+                    .AddOption("option", "value")
+                    .AddOption("a")
+                    .AddOption("b")
+                    .AddOption("c")
+                    .Build()
+            }
+        };
+
+        [Theory]
+        [MemberData(nameof(UnboundArgumentsTestData))]
+        internal void Any_remaining_arguments_are_treated_as_unbound_arguments(string[] arguments, CommandLineInput expectedInput)
+        {
+            // Act
+            var input = CommandLineInput.Parse(arguments);
+
+            // Assert
+            input.Should().BeEquivalentTo(expectedInput);
+        }
+    }
+}

CliFx.Tests/CancellationSpecs.Commands.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests
+{
+    public partial class CancellationSpecs
+    {
+        [Command("cancel")]
+        private class CancellableCommand : ICommand
+        {
+            public async ValueTask ExecuteAsync(IConsole console)
+            {
+                try
+                {
+                    await Task.Delay(TimeSpan.FromSeconds(3), console.GetCancellationToken());
+                    console.Output.WriteLine("Never printed");
+                }
+                catch (OperationCanceledException)
+                {
+                    console.Output.WriteLine("Cancellation requested");
+                    throw;
+                }
+            }
+        }
+    }
+}

CliFx.Tests/CancellationSpecs.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Xunit;
+
+namespace CliFx.Tests
+{
+    public partial class CancellationSpecs
+    {
+        [Fact]
+        public async Task Command_can_perform_additional_cleanup_if_cancellation_is_requested()
+        {
+            // Arrange
+            using var cts = new CancellationTokenSource();
+
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut, cancellationToken: cts.Token);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(CancellableCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            cts.CancelAfter(TimeSpan.FromSeconds(0.2));
+
+            var exitCode = await application.RunAsync(
+                new[] {"cancel"},
+                new Dictionary<string, string>());
+
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().NotBe(0);
+            stdOutData.Should().Be("Cancellation requested");
+        }
+    }
+}

CliFx.Tests/CliApplicationTests.Commands.cs

@@ -1,53 +0,0 @@
-using System;
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Exceptions;
-using CliFx.Services;
-
-namespace CliFx.Tests
-{
-    public partial class CliApplicationTests
-    {
-        [Command]
-        private class DefaultCommand : ICommand
-        {
-            public Task ExecuteAsync(IConsole console)
-            {
-                console.Output.WriteLine("DefaultCommand executed.");
-                return Task.CompletedTask;
-            }
-        }
-
-        [Command("cmd")]
-        private class NamedCommand : ICommand
-        {
-            public Task ExecuteAsync(IConsole console)
-            {
-                console.Output.WriteLine("NamedCommand executed.");
-                return Task.CompletedTask;
-            }
-        }
-    }
-
-    // Negative
-    public partial class CliApplicationTests
-    {
-        [Command("faulty1")]
-        private class FaultyCommand1 : ICommand
-        {
-            public Task ExecuteAsync(IConsole console) => throw new CommandException(150);
-        }
-
-        [Command("faulty2")]
-        private class FaultyCommand2 : ICommand
-        {
-            public Task ExecuteAsync(IConsole console) => throw new CommandException("FaultyCommand2 error message.", 150);
-        }
-
-        [Command("faulty3")]
-        private class FaultyCommand3 : ICommand
-        {
-            public Task ExecuteAsync(IConsole console) => throw new Exception("FaultyCommand3 error message.");
-        }
-    }
-}

CliFx.Tests/CliApplicationTests.cs

@@ -1,174 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.IO;
-using System.Threading.Tasks;
-using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public partial class CliApplicationTests
-    {
-        private static IEnumerable<TestCaseData> GetTestCases_RunAsync()
-        {
-            yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new string[0],
-                "DefaultCommand executed."
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(NamedCommand)},
-                new[] {"cmd"},
-                "NamedCommand executed."
-            );
-        }
-
-        private static IEnumerable<TestCaseData> GetTestCases_HelpAndVersion_RunAsync()
-        {
-            yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new string[0]
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new[] {"-h"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new[] {"--help"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new[] {"--version"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(NamedCommand)},
-                new string[0]
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(NamedCommand)},
-                new[] {"cmd", "-h"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(FaultyCommand1)},
-                new[] {"faulty1", "-h"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(FaultyCommand2)},
-                new[] {"faulty2", "-h"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(FaultyCommand3)},
-                new[] {"faulty3", "-h"}
-            );
-        }
-
-        private static IEnumerable<TestCaseData> GetTestCases_RunAsync_Negative()
-        {
-            yield return new TestCaseData(
-                new Type[0],
-                new string[0]
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new[] {"non-existing"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(FaultyCommand1)},
-                new[] {"faulty1"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(FaultyCommand2)},
-                new[] {"faulty2"}
-            );
-
-            yield return new TestCaseData(
-                new[] {typeof(FaultyCommand3)},
-                new[] {"faulty3"}
-            );
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_RunAsync))]
-        public async Task RunAsync_Test(IReadOnlyList<Type> commandTypes, IReadOnlyList<string> commandLineArguments, string expectedStdOut)
-        {
-            // Arrange
-            using (var stdout = new StringWriter())
-            {
-                var console = new VirtualConsole(stdout);
-
-                var application = new CliApplicationBuilder()
-                    .AddCommands(commandTypes)
-                    .UseConsole(console)
-                    .Build();
-
-                // Act
-                var exitCode = await application.RunAsync(commandLineArguments);
-
-                // Assert
-                exitCode.Should().Be(0);
-                stdout.ToString().Trim().Should().Be(expectedStdOut);
-            }
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_HelpAndVersion_RunAsync))]
-        public async Task RunAsync_HelpAndVersion_Test(IReadOnlyList<Type> commandTypes, IReadOnlyList<string> commandLineArguments)
-        {
-            // Arrange
-            using (var stdout = new StringWriter())
-            {
-                var console = new VirtualConsole(stdout);
-
-                var application = new CliApplicationBuilder()
-                    .AddCommands(commandTypes)
-                    .UseConsole(console)
-                    .Build();
-
-                // Act
-                var exitCode = await application.RunAsync(commandLineArguments);
-
-                // Assert
-                exitCode.Should().Be(0);
-                stdout.ToString().Should().NotBeNullOrWhiteSpace();
-            }
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_RunAsync_Negative))]
-        public async Task RunAsync_Negative_Test(IReadOnlyList<Type> commandTypes, IReadOnlyList<string> commandLineArguments)
-        {
-            // Arrange
-            using (var stderr = new StringWriter())
-            {
-                var console = new VirtualConsole(TextWriter.Null, stderr);
-
-                var application = new CliApplicationBuilder()
-                    .AddCommands(commandTypes)
-                    .UseConsole(console)
-                    .Build();
-
-                // Act
-                var exitCode = await application.RunAsync(commandLineArguments);
-
-                // Assert
-                exitCode.Should().NotBe(0);
-                stderr.ToString().Should().NotBeNullOrWhiteSpace();
-            }
-        }
-    }
-}

CliFx.Tests/CliFx.Tests.csproj

@@ -1,25 +1,27 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="../CliFx.props" />
 
   <PropertyGroup>
-    <TargetFramework>net46</TargetFramework>
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
     <CollectCoverage>true</CollectCoverage>
     <CoverletOutputFormat>opencover</CoverletOutputFormat>
     <CoverletOutput>bin/$(Configuration)/Coverage.xml</CoverletOutput>
-    <LangVersion>latest</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="FluentAssertions" Version="5.8.0" />
+    <Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
+  </ItemGroup>
-    <PackageReference Include="NUnit" Version="3.12.0" />
+
-    <PackageReference Include="NUnit3TestAdapter" Version="3.14.0" />
+  <ItemGroup>
-    <PackageReference Include="CliWrap" Version="2.3.1" />
+    <PackageReference Include="CliWrap" Version="3.0.0" />
-    <PackageReference Include="coverlet.msbuild" Version="2.6.3">
+    <PackageReference Include="FluentAssertions" Version="5.10.2" />
-      <PrivateAssets>all</PrivateAssets>
+    <PackageReference Include="GitHubActionsTestLogger" Version="1.0.0" />
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.5.0" />
-    </PackageReference>
+    <PackageReference Include="xunit" Version="2.4.1" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.1" />
+    <PackageReference Include="coverlet.msbuild" Version="2.8.0" PrivateAssets="all" />
   </ItemGroup>
 
   <ItemGroup>
@@ -27,4 +29,12 @@
     <ProjectReference Include="..\CliFx\CliFx.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <None Include="../CliFx.Tests.Dummy/bin/$(Configuration)/$(TargetFramework)/CliFx.Tests.Dummy.runtimeconfig.json">
+      <Link>CliFx.Tests.Dummy.runtimeconfig.json</Link>
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+      <Visible>False</Visible>
+    </None>
+  </ItemGroup>
+
 </Project>

CliFx.Tests/CommandFactoryTests.Commands.cs

@@ -1,15 +0,0 @@
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Services;
-
-namespace CliFx.Tests
-{
-    public partial class CommandFactoryTests
-    {
-        [Command]
-        private class TestCommand : ICommand
-        {
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-    }
-}

CliFx.Tests/CommandFactoryTests.cs

@@ -1,36 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using CliFx.Models;
-using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public partial class CommandFactoryTests
-    {
-        private static CommandSchema GetCommandSchema(Type commandType) =>
-            new CommandSchemaResolver().GetCommandSchemas(new[] {commandType}).Single();
-
-        private static IEnumerable<TestCaseData> GetTestCases_CreateCommand()
-        {
-            yield return new TestCaseData(GetCommandSchema(typeof(TestCommand)));
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_CreateCommand))]
-        public void CreateCommand_Test(CommandSchema commandSchema)
-        {
-            // Arrange
-            var factory = new CommandFactory();
-
-            // Act
-            var command = factory.CreateCommand(commandSchema);
-
-            // Assert
-            command.Should().BeOfType(commandSchema.Type);
-        }
-    }
-}

CliFx.Tests/CommandInitializerTests.Commands.cs

@@ -1,21 +0,0 @@
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Services;
-
-namespace CliFx.Tests
-{
-    public partial class CommandInitializerTests
-    {
-        [Command]
-        private class TestCommand : ICommand
-        {
-            [CommandOption("int", 'i', IsRequired = true)]
-            public int IntOption { get; set; } = 24;
-
-            [CommandOption("str", 's')]
-            public string StringOption { get; set; } = "foo bar";
-
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-    }
-}

CliFx.Tests/CommandInitializerTests.cs

@@ -1,96 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using CliFx.Exceptions;
-using CliFx.Models;
-using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public partial class CommandInitializerTests
-    {
-        private static CommandSchema GetCommandSchema(Type commandType) =>
-            new CommandSchemaResolver().GetCommandSchemas(new[] {commandType}).Single();
-
-        private static IEnumerable<TestCaseData> GetTestCases_InitializeCommand()
-        {
-            yield return new TestCaseData(
-                new TestCommand(),
-                GetCommandSchema(typeof(TestCommand)),
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("int", "13")
-                }),
-                new TestCommand {IntOption = 13}
-            );
-
-            yield return new TestCaseData(
-                new TestCommand(),
-                GetCommandSchema(typeof(TestCommand)),
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("int", "13"),
-                    new CommandOptionInput("str", "hello world")
-                }),
-                new TestCommand {IntOption = 13, StringOption = "hello world"}
-            );
-
-            yield return new TestCaseData(
-                new TestCommand(),
-                GetCommandSchema(typeof(TestCommand)),
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("i", "13")
-                }),
-                new TestCommand {IntOption = 13}
-            );
-        }
-
-        private static IEnumerable<TestCaseData> GetTestCases_InitializeCommand_Negative()
-        {
-            yield return new TestCaseData(
-                new TestCommand(),
-                GetCommandSchema(typeof(TestCommand)),
-                CommandInput.Empty
-            );
-
-            yield return new TestCaseData(
-                new TestCommand(),
-                GetCommandSchema(typeof(TestCommand)),
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("str", "hello world")
-                })
-            );
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_InitializeCommand))]
-        public void InitializeCommand_Test(ICommand command, CommandSchema commandSchema, CommandInput commandInput, ICommand expectedCommand)
-        {
-            // Arrange
-            var initializer = new CommandInitializer();
-
-            // Act
-            initializer.InitializeCommand(command, commandSchema, commandInput);
-
-            // Assert
-            command.Should().BeEquivalentTo(expectedCommand, o => o.RespectingRuntimeTypes());
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_InitializeCommand_Negative))]
-        public void InitializeCommand_Negative_Test(ICommand command, CommandSchema commandSchema, CommandInput commandInput)
-        {
-            // Arrange
-            var initializer = new CommandInitializer();
-
-            // Act & Assert
-            initializer.Invoking(i => i.InitializeCommand(command, commandSchema, commandInput))
-                .Should().ThrowExactly<MissingCommandOptionInputException>();
-        }
-    }
-}

CliFx.Tests/CommandInputParserTests.cs

@@ -1,184 +0,0 @@
-using System.Collections.Generic;
-using CliFx.Models;
-using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public class CommandInputParserTests
-    {
-        private static IEnumerable<TestCaseData> GetTestCases_ParseCommandInput()
-        {
-            yield return new TestCaseData(new string[0], CommandInput.Empty);
-
-            yield return new TestCaseData(
-                new[] {"--option", "value"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("option", "value")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"--option1", "value1", "--option2", "value2"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("option1", "value1"),
-                    new CommandOptionInput("option2", "value2")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"--option", "value1", "value2"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("option", new[] {"value1", "value2"})
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"--option", "value1", "--option", "value2"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("option", new[] {"value1", "value2"})
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"-a", "value"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("a", "value")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"-a", "value1", "-b", "value2"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("a", "value1"),
-                    new CommandOptionInput("b", "value2")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"-a", "value1", "value2"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("a", new[] {"value1", "value2"})
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"-a", "value1", "-a", "value2"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("a", new[] {"value1", "value2"})
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"--option1", "value1", "-b", "value2"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("option1", "value1"),
-                    new CommandOptionInput("b", "value2")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"--switch"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("switch")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"--switch1", "--switch2"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("switch1"),
-                    new CommandOptionInput("switch2")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"-s"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("s")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"-a", "-b"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("a"),
-                    new CommandOptionInput("b")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"-ab"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("a"),
-                    new CommandOptionInput("b")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"-ab", "value"},
-                new CommandInput(new[]
-                {
-                    new CommandOptionInput("a"),
-                    new CommandOptionInput("b", "value")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"command"},
-                new CommandInput("command")
-            );
-
-            yield return new TestCaseData(
-                new[] {"command", "--option", "value"},
-                new CommandInput("command", new[]
-                {
-                    new CommandOptionInput("option", "value")
-                })
-            );
-
-            yield return new TestCaseData(
-                new[] {"long", "command", "name"},
-                new CommandInput("long command name")
-            );
-
-            yield return new TestCaseData(
-                new[] {"long", "command", "name", "--option", "value"},
-                new CommandInput("long command name", new[]
-                {
-                    new CommandOptionInput("option", "value")
-                })
-            );
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_ParseCommandInput))]
-        public void ParseCommandInput_Test(IReadOnlyList<string> commandLineArguments, CommandInput expectedCommandInput)
-        {
-            // Arrange
-            var parser = new CommandInputParser();
-
-            // Act
-            var commandInput = parser.ParseCommandInput(commandLineArguments);
-
-            // Assert
-            commandInput.Should().BeEquivalentTo(expectedCommandInput);
-        }
-    }
-}

CliFx.Tests/CommandOptionInputConverterTests.Types.cs

@@ -1,63 +0,0 @@
-using System;
-
-namespace CliFx.Tests
-{
-    public partial class CommandOptionInputConverterTests
-    {
-        private enum TestEnum
-        {
-            Value1,
-            Value2,
-            Value3
-        }
-
-        private class TestStringConstructable
-        {
-            public string Value { get; }
-
-            public TestStringConstructable(string value)
-            {
-                Value = value;
-            }
-        }
-
-        private class TestStringParseable
-        {
-            public string Value { get; }
-
-            private TestStringParseable(string value)
-            {
-                Value = value;
-            }
-
-            public static TestStringParseable Parse(string value) => new TestStringParseable(value);
-        }
-
-        private class TestStringParseableWithFormatProvider
-        {
-            public string Value { get; }
-
-            private TestStringParseableWithFormatProvider(string value)
-            {
-                Value = value;
-            }
-
-            public static TestStringParseableWithFormatProvider Parse(string value, IFormatProvider formatProvider) =>
-                new TestStringParseableWithFormatProvider(value + " " + formatProvider);
-        }
-    }
-
-    // Negative
-    public partial class CommandOptionInputConverterTests
-    {
-        private class NonStringParseable
-        {
-            public int Value { get; }
-
-            public NonStringParseable(int value)
-            {
-                Value = value;
-            }
-        }
-    }
-}

CliFx.Tests/CommandOptionInputConverterTests.cs

@@ -1,305 +0,0 @@
-using System;
-using System.Collections;
-using System.Collections.Generic;
-using System.Globalization;
-using CliFx.Exceptions;
-using CliFx.Models;
-using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public partial class CommandOptionInputConverterTests
-    {
-        private static IEnumerable<TestCaseData> GetTestCases_ConvertOptionInput()
-        {
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "value"),
-                typeof(string),
-                "value"
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "value"),
-                typeof(object),
-                "value"
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "true"),
-                typeof(bool),
-                true
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "false"),
-                typeof(bool),
-                false
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option"),
-                typeof(bool),
-                true
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "a"),
-                typeof(char),
-                'a'
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "15"),
-                typeof(sbyte),
-                (sbyte) 15
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "15"),
-                typeof(byte),
-                (byte) 15
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "15"),
-                typeof(short),
-                (short) 15
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "15"),
-                typeof(ushort),
-                (ushort) 15
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "123"),
-                typeof(int),
-                123
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "123"),
-                typeof(uint),
-                123u
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "123"),
-                typeof(long),
-                123L
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "123"),
-                typeof(ulong),
-                123UL
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "123.45"),
-                typeof(float),
-                123.45f
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "123.45"),
-                typeof(double),
-                123.45
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "123.45"),
-                typeof(decimal),
-                123.45m
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "28 Apr 1995"),
-                typeof(DateTime),
-                new DateTime(1995, 04, 28)
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "28 Apr 1995"),
-                typeof(DateTimeOffset),
-                new DateTimeOffset(new DateTime(1995, 04, 28))
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "00:14:59"),
-                typeof(TimeSpan),
-                new TimeSpan(00, 14, 59)
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "value2"),
-                typeof(TestEnum),
-                TestEnum.Value2
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "666"),
-                typeof(int?),
-                666
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option"),
-                typeof(int?),
-                null
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "value3"),
-                typeof(TestEnum?),
-                TestEnum.Value3
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option"),
-                typeof(TestEnum?),
-                null
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "01:00:00"),
-                typeof(TimeSpan?),
-                new TimeSpan(01, 00, 00)
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option"),
-                typeof(TimeSpan?),
-                null
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "value"),
-                typeof(TestStringConstructable),
-                new TestStringConstructable("value")
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "value"),
-                typeof(TestStringParseable),
-                TestStringParseable.Parse("value")
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "value"),
-                typeof(TestStringParseableWithFormatProvider),
-                TestStringParseableWithFormatProvider.Parse("value", CultureInfo.InvariantCulture)
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value2"}),
-                typeof(string[]),
-                new[] {"value1", "value2"}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value2"}),
-                typeof(object[]),
-                new[] {"value1", "value2"}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"47", "69"}),
-                typeof(int[]),
-                new[] {47, 69}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value3"}),
-                typeof(TestEnum[]),
-                new[] {TestEnum.Value1, TestEnum.Value3}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"1337", "2441"}),
-                typeof(int?[]),
-                new int?[] {1337, 2441}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value2"}),
-                typeof(TestStringConstructable[]),
-                new[] {new TestStringConstructable("value1"), new TestStringConstructable("value2")}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value2"}),
-                typeof(IEnumerable),
-                new[] {"value1", "value2"}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value2"}),
-                typeof(IEnumerable<string>),
-                new[] {"value1", "value2"}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value2"}),
-                typeof(IReadOnlyList<string>),
-                new[] {"value1", "value2"}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value2"}),
-                typeof(List<string>),
-                new List<string> {"value1", "value2"}
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", new[] {"value1", "value2"}),
-                typeof(HashSet<string>),
-                new HashSet<string> {"value1", "value2"}
-            );
-        }
-
-        private static IEnumerable<TestCaseData> GetTestCases_ConvertOptionInput_Negative()
-        {
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "1234.5"),
-                typeof(int)
-            );
-
-            yield return new TestCaseData(
-                new CommandOptionInput("option", "123"),
-                typeof(NonStringParseable)
-            );
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_ConvertOptionInput))]
-        public void ConvertOptionInput_Test(CommandOptionInput optionInput, Type targetType, object expectedConvertedValue)
-        {
-            // Arrange
-            var converter = new CommandOptionInputConverter();
-
-            // Act
-            var convertedValue = converter.ConvertOptionInput(optionInput, targetType);
-
-            // Assert
-            convertedValue.Should().BeEquivalentTo(expectedConvertedValue);
-            convertedValue?.Should().BeAssignableTo(targetType);
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_ConvertOptionInput_Negative))]
-        public void ConvertOptionInput_Negative_Test(CommandOptionInput optionInput, Type targetType)
-        {
-            // Arrange
-            var converter = new CommandOptionInputConverter();
-
-            // Act & Assert
-            converter.Invoking(c => c.ConvertOptionInput(optionInput, targetType))
-                .Should().ThrowExactly<InvalidCommandOptionInputException>();
-        }
-    }
-}

CliFx.Tests/CommandSchemaResolverTests.Commands.cs

@@ -1,81 +0,0 @@
-using System;
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Services;
-
-namespace CliFx.Tests
-{
-    public partial class CommandSchemaResolverTests
-    {
-        [Command("cmd", Description = "NormalCommand1 description.")]
-        private class NormalCommand1 : ICommand
-        {
-            [CommandOption("option-a", 'a')]
-            public int OptionA { get; set; }
-
-            [CommandOption("option-b", IsRequired = true)]
-            public string OptionB { get; set; }
-
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-
-        [Command(Description = "NormalCommand2 description.")]
-        private class NormalCommand2 : ICommand
-        {
-            [CommandOption("option-c", Description = "OptionC description.")]
-            public bool OptionC { get; set; }
-
-            [CommandOption("option-d", 'd')]
-            public DateTimeOffset OptionD { get; set; }
-
-            public string NotAnOption { get; set; }
-
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-    }
-
-    // Negative
-    public partial class CommandSchemaResolverTests
-    {
-        [Command("conflict")]
-        private class ConflictingCommand1 : ICommand
-        {
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-
-        [Command("conflict")]
-        private class ConflictingCommand2 : ICommand
-        {
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-
-        [Command]
-        private class InvalidCommand1
-        {
-        }
-
-        [Command]
-        private class InvalidCommand2 : ICommand
-        {
-            [CommandOption("conflict")]
-            public string ConflictingOption1 { get; set; }
-
-            [CommandOption("conflict")]
-            public string ConflictingOption2 { get; set; }
-
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-
-        [Command]
-        private class InvalidCommand3 : ICommand
-        {
-            [CommandOption('c')]
-            public string ConflictingOption1 { get; set; }
-
-            [CommandOption('c')]
-            public string ConflictingOption2 { get; set; }
-
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-    }
-}

CliFx.Tests/CommandSchemaResolverTests.cs

@@ -1,94 +0,0 @@
-using System;
-using System.Collections.Generic;
-using CliFx.Exceptions;
-using CliFx.Models;
-using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public partial class CommandSchemaResolverTests
-    {
-        private static IEnumerable<TestCaseData> GetTestCases_GetCommandSchemas()
-        {
-            yield return new TestCaseData(
-                new[] {typeof(NormalCommand1), typeof(NormalCommand2)},
-                new[]
-                {
-                    new CommandSchema(typeof(NormalCommand1), "cmd", "NormalCommand1 description.",
-                        new[]
-                        {
-                            new CommandOptionSchema(typeof(NormalCommand1).GetProperty(nameof(NormalCommand1.OptionA)),
-                                "option-a", 'a', false, null),
-                            new CommandOptionSchema(typeof(NormalCommand1).GetProperty(nameof(NormalCommand1.OptionB)),
-                                "option-b", null, true, null)
-                        }),
-                    new CommandSchema(typeof(NormalCommand2), null, "NormalCommand2 description.",
-                        new[]
-                        {
-                            new CommandOptionSchema(typeof(NormalCommand2).GetProperty(nameof(NormalCommand2.OptionC)),
-                                "option-c", null, false, "OptionC description."),
-                            new CommandOptionSchema(typeof(NormalCommand2).GetProperty(nameof(NormalCommand2.OptionD)),
-                                "option-d", 'd', false, null)
-                        })
-                }
-            );
-        }
-
-        private static IEnumerable<TestCaseData> GetTestCases_GetCommandSchemas_Negative()
-        {
-            yield return new TestCaseData(new object[]
-            {
-                new Type[0]
-            });
-
-            yield return new TestCaseData(new object[]
-            {
-                new[] {typeof(ConflictingCommand1), typeof(ConflictingCommand2)}
-            });
-
-            yield return new TestCaseData(new object[]
-            {
-                new[] {typeof(InvalidCommand1)}
-            });
-
-            yield return new TestCaseData(new object[]
-            {
-                new[] {typeof(InvalidCommand2)}
-            });
-
-            yield return new TestCaseData(new object[]
-            {
-                new[] {typeof(InvalidCommand3)}
-            });
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_GetCommandSchemas))]
-        public void GetCommandSchemas_Test(IReadOnlyList<Type> commandTypes, IReadOnlyList<CommandSchema> expectedCommandSchemas)
-        {
-            // Arrange
-            var commandSchemaResolver = new CommandSchemaResolver();
-
-            // Act
-            var commandSchemas = commandSchemaResolver.GetCommandSchemas(commandTypes);
-
-            // Assert
-            commandSchemas.Should().BeEquivalentTo(expectedCommandSchemas);
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_GetCommandSchemas_Negative))]
-        public void GetCommandSchemas_Negative_Test(IReadOnlyList<Type> commandTypes)
-        {
-            // Arrange
-            var resolver = new CommandSchemaResolver();
-
-            // Act & Assert
-            resolver.Invoking(r => r.GetCommandSchemas(commandTypes))
-                .Should().ThrowExactly<InvalidCommandSchemaException>();
-        }
-    }
-}

CliFx.Tests/ConsoleSpecs.cs

@@ -0,0 +1,72 @@
+using System;
+using System.IO;
+using System.Threading.Tasks;
+using CliWrap;
+using CliWrap.Buffered;
+using FluentAssertions;
+using Xunit;
+
+namespace CliFx.Tests
+{
+    public class ConsoleSpecs
+    {
+        [Fact]
+        public async Task Real_implementation_of_console_maps_directly_to_system_console()
+        {
+            // Arrange
+            var command = "Hello world" | Cli.Wrap("dotnet")
+                .WithArguments(a => a
+                    .Add(Dummy.Program.Location)
+                    .Add("console-test"));
+
+            // Act
+            var result = await command.ExecuteBufferedAsync();
+
+            // Assert
+            result.StandardOutput.TrimEnd().Should().Be("Hello world");
+            result.StandardError.TrimEnd().Should().Be("Hello world");
+        }
+
+        [Fact]
+        public void Fake_implementation_of_console_can_be_used_to_execute_commands_in_isolation()
+        {
+            // Arrange
+            using var stdIn = new MemoryStream(Console.InputEncoding.GetBytes("input"));
+            using var stdOut = new MemoryStream();
+            using var stdErr = new MemoryStream();
+
+            var console = new VirtualConsole(
+                input: stdIn,
+                output: stdOut,
+                error: stdErr);
+
+            // Act
+            console.Output.Write("output");
+            console.Error.Write("error");
+
+            var stdInData = console.Input.ReadToEnd();
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray());
+            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray());
+
+            console.ResetColor();
+            console.ForegroundColor = ConsoleColor.DarkMagenta;
+            console.BackgroundColor = ConsoleColor.DarkMagenta;
+
+            // Assert
+            stdInData.Should().Be("input");
+            stdOutData.Should().Be("output");
+            stdErrData.Should().Be("error");
+
+            console.Input.Should().NotBeSameAs(Console.In);
+            console.Output.Should().NotBeSameAs(Console.Out);
+            console.Error.Should().NotBeSameAs(Console.Error);
+
+            console.IsInputRedirected.Should().BeTrue();
+            console.IsOutputRedirected.Should().BeTrue();
+            console.IsErrorRedirected.Should().BeTrue();
+
+            console.ForegroundColor.Should().NotBe(Console.ForegroundColor);
+            console.BackgroundColor.Should().NotBe(Console.BackgroundColor);
+        }
+    }
+}

CliFx.Tests/DelegateCommandFactoryTests.Commands.cs

@@ -1,15 +0,0 @@
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Services;
-
-namespace CliFx.Tests
-{
-    public partial class DelegateCommandFactoryTests
-    {
-        [Command]
-        private class TestCommand : ICommand
-        {
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-    }
-}

CliFx.Tests/DelegateCommandFactoryTests.cs

@@ -1,39 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using CliFx.Models;
-using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public partial class DelegateCommandFactoryTests
-    {
-        private static CommandSchema GetCommandSchema(Type commandType) =>
-            new CommandSchemaResolver().GetCommandSchemas(new[] {commandType}).Single();
-
-        private static IEnumerable<TestCaseData> GetTestCases_CreateCommand()
-        {
-            yield return new TestCaseData(
-                new Func<CommandSchema, ICommand>(schema => (ICommand) Activator.CreateInstance(schema.Type)),
-                GetCommandSchema(typeof(TestCommand))
-            );
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_CreateCommand))]
-        public void CreateCommand_Test(Func<CommandSchema, ICommand> factoryMethod, CommandSchema commandSchema)
-        {
-            // Arrange
-            var factory = new DelegateCommandFactory(factoryMethod);
-
-            // Act
-            var command = factory.CreateCommand(commandSchema);
-
-            // Assert
-            command.Should().BeOfType(commandSchema.Type);
-        }
-    }
-}

CliFx.Tests/DependencyInjectionSpecs.Commands.cs

@@ -0,0 +1,37 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests
+{
+    public partial class DependencyInjectionSpecs
+    {
+        [Command]
+        private class WithoutDependenciesCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        private class DependencyA
+        {
+        }
+
+        private class DependencyB
+        {
+        }
+
+        [Command]
+        private class WithDependenciesCommand : ICommand
+        {
+            private readonly DependencyA _dependencyA;
+            private readonly DependencyB _dependencyB;
+
+            public WithDependenciesCommand(DependencyA dependencyA, DependencyB dependencyB)
+            {
+                _dependencyA = dependencyA;
+                _dependencyB = dependencyB;
+            }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+    }
+}

CliFx.Tests/DependencyInjectionSpecs.cs

@@ -0,0 +1,63 @@
+using CliFx.Exceptions;
+using FluentAssertions;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace CliFx.Tests
+{
+    public partial class DependencyInjectionSpecs
+    {
+        private readonly ITestOutputHelper _output;
+
+        public DependencyInjectionSpecs(ITestOutputHelper output) => _output = output;
+
+        [Fact]
+        public void Default_type_activator_can_initialize_a_command_if_it_has_a_parameterless_constructor()
+        {
+            // Arrange
+            var activator = new DefaultTypeActivator();
+
+            // Act
+            var obj = activator.CreateInstance(typeof(WithoutDependenciesCommand));
+
+            // Assert
+            obj.Should().BeOfType<WithoutDependenciesCommand>();
+        }
+
+        [Fact]
+        public void Default_type_activator_cannot_initialize_a_command_if_it_does_not_have_a_parameterless_constructor()
+        {
+            // Arrange
+            var activator = new DefaultTypeActivator();
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => activator.CreateInstance(typeof(WithDependenciesCommand)));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Delegate_type_activator_can_initialize_a_command_using_a_custom_function()
+        {
+            // Arrange
+            var activator = new DelegateTypeActivator(_ =>
+                new WithDependenciesCommand(new DependencyA(), new DependencyB()));
+
+            // Act
+            var obj = activator.CreateInstance(typeof(WithDependenciesCommand));
+
+            // Assert
+            obj.Should().BeOfType<WithDependenciesCommand>();
+        }
+
+        [Fact]
+        public void Delegate_type_activator_throws_if_the_underlying_function_returns_null()
+        {
+            // Arrange
+            var activator = new DelegateTypeActivator(_ => null!);
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => activator.CreateInstance(typeof(WithDependenciesCommand)));
+            _output.WriteLine(ex.Message);
+        }
+    }
+}

CliFx.Tests/DirectivesSpecs.Commands.cs

@@ -0,0 +1,14 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests
+{
+    public partial class DirectivesSpecs
+    {
+        [Command("cmd")]
+        private class NamedCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+    }
+}

CliFx.Tests/DirectivesSpecs.cs

@@ -0,0 +1,36 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Xunit;
+
+namespace CliFx.Tests
+{
+    public partial class DirectivesSpecs
+    {
+        [Fact]
+        public async Task Preview_directive_can_be_enabled_to_print_provided_arguments_as_they_were_parsed()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(NamedCommand))
+                .UseConsole(console)
+                .AllowPreviewMode()
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                new[] {"[preview]", "cmd", "param", "-abc", "--option", "foo"},
+                new Dictionary<string, string>());
+
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().Be(0);
+            stdOutData.Should().ContainAll("cmd", "<param>", "[-a]", "[-b]", "[-c]", "[--option foo]");
+        }
+    }
+}

CliFx.Tests/DummyTests.cs

@@ -1,73 +0,0 @@
-using System.Threading.Tasks;
-using CliWrap;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public class DummyTests
-    {
-        private static string DummyFilePath => typeof(Dummy.Program).Assembly.Location;
-
-        private static string DummyVersionText => typeof(Dummy.Program).Assembly.GetName().Version.ToString();
-
-        [Test]
-        [TestCase("", "Hello world")]
-        [TestCase("-t .NET", "Hello .NET")]
-        [TestCase("-e", "Hello world!")]
-        [TestCase("sum -v 1 2", "3")]
-        [TestCase("sum -v 2.75 3.6 4.18", "10.53")]
-        [TestCase("sum -v 4 -v 16", "20")]
-        [TestCase("sum --values 2 5 --values 3", "10")]
-        [TestCase("log -v 100", "2")]
-        [TestCase("log --value 256 --base 2", "8")]
-        public async Task CliApplication_RunAsync_Test(string arguments, string expectedOutput)
-        {
-            // Arrange & Act
-            var result = await Cli.Wrap(DummyFilePath)
-                .SetArguments(arguments)
-                .EnableExitCodeValidation()
-                .EnableStandardErrorValidation()
-                .ExecuteAsync();
-
-            // Assert
-            result.StandardOutput.Trim().Should().Be(expectedOutput);
-        }
-
-        [Test]
-        [TestCase("--version")]
-        public async Task CliApplication_RunAsync_ShowVersion_Test(string arguments)
-        {
-            // Arrange & Act
-            var result = await Cli.Wrap(DummyFilePath)
-                .SetArguments(arguments)
-                .EnableExitCodeValidation()
-                .EnableStandardErrorValidation()
-                .ExecuteAsync();
-
-            // Assert
-            result.StandardOutput.Trim().Should().Be(DummyVersionText);
-        }
-
-        [Test]
-        [TestCase("--help")]
-        [TestCase("-h")]
-        [TestCase("sum -h")]
-        [TestCase("sum --help")]
-        [TestCase("log -h")]
-        [TestCase("log --help")]
-        public async Task CliApplication_RunAsync_ShowHelp_Test(string arguments)
-        {
-            // Arrange & Act
-            var result = await Cli.Wrap(DummyFilePath)
-                .SetArguments(arguments)
-                .EnableExitCodeValidation()
-                .EnableStandardErrorValidation()
-                .ExecuteAsync();
-
-            // Assert
-            result.StandardOutput.Trim().Should().NotBeNullOrWhiteSpace();
-        }
-    }
-}

CliFx.Tests/EnvironmentVariablesSpecs.Commands.cs

@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests
+{
+    public partial class EnvironmentVariablesSpecs
+    {
+        [Command]
+        private class EnvironmentVariableCollectionCommand : ICommand
+        {
+            [CommandOption("opt", EnvironmentVariableName = "ENV_OPT")]
+            public IReadOnlyList<string>? Option { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class EnvironmentVariableCommand : ICommand
+        {
+            [CommandOption("opt", EnvironmentVariableName = "ENV_OPT")]
+            public string? Option { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+    }
+}

CliFx.Tests/EnvironmentVariablesSpecs.cs

@@ -0,0 +1,96 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using CliFx.Domain;
+using CliWrap;
+using CliWrap.Buffered;
+using FluentAssertions;
+using Xunit;
+
+namespace CliFx.Tests
+{
+    public partial class EnvironmentVariablesSpecs
+    {
+        // This test uses a real application to make sure environment variables are actually read correctly
+        [Fact]
+        public async Task Option_can_use_a_specific_environment_variable_as_fallback()
+        {
+            // Arrange
+            var command = Cli.Wrap("dotnet")
+                .WithArguments(a => a
+                    .Add(Dummy.Program.Location))
+                .WithEnvironmentVariables(e => e
+                    .Set("ENV_TARGET", "Mars"));
+
+            // Act
+            var stdOut = await command.ExecuteBufferedAsync().Select(r => r.StandardOutput);
+
+            // Assert
+            stdOut.TrimEnd().Should().Be("Hello Mars!");
+        }
+
+        // This test uses a real application to make sure environment variables are actually read correctly
+        [Fact]
+        public async Task Option_only_uses_environment_variable_as_fallback_if_the_value_was_not_directly_provided()
+        {
+            // Arrange
+            var command = Cli.Wrap("dotnet")
+                .WithArguments(a => a
+                    .Add(Dummy.Program.Location)
+                    .Add("--target")
+                    .Add("Jupiter"))
+                .WithEnvironmentVariables(e => e
+                    .Set("ENV_TARGET", "Mars"));
+
+            // Act
+            var stdOut = await command.ExecuteBufferedAsync().Select(r => r.StandardOutput);
+
+            // Assert
+            stdOut.TrimEnd().Should().Be("Hello Jupiter!");
+        }
+
+        [Fact]
+        public void Option_of_non_scalar_type_can_take_multiple_separated_values_from_an_environment_variable()
+        {
+            // Arrange
+            var schema = ApplicationSchema.Resolve(new[] {typeof(EnvironmentVariableCollectionCommand)});
+
+            var input = CommandLineInput.Empty;
+            var envVars = new Dictionary<string, string>
+            {
+                ["ENV_OPT"] = $"foo{Path.PathSeparator}bar"
+            };
+
+            // Act
+            var command = schema.InitializeEntryPoint(input, envVars);
+
+            // Assert
+            command.Should().BeEquivalentTo(new EnvironmentVariableCollectionCommand
+            {
+                Option = new[] {"foo", "bar"}
+            });
+        }
+
+        [Fact]
+        public void Option_of_scalar_type_can_only_take_a_single_value_from_an_environment_variable_even_if_it_contains_separators()
+        {
+            // Arrange
+            var schema = ApplicationSchema.Resolve(new[] {typeof(EnvironmentVariableCommand)});
+
+            var input = CommandLineInput.Empty;
+            var envVars = new Dictionary<string, string>
+            {
+                ["ENV_OPT"] = $"foo{Path.PathSeparator}bar"
+            };
+
+            // Act
+            var command = schema.InitializeEntryPoint(input, envVars);
+
+            // Assert
+            command.Should().BeEquivalentTo(new EnvironmentVariableCommand
+            {
+                Option = $"foo{Path.PathSeparator}bar"
+            });
+        }
+    }
+}

CliFx.Tests/ErrorReportingSpecs.Commands.cs

@@ -0,0 +1,77 @@
+using System;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Exceptions;
+
+namespace CliFx.Tests
+{
+    public partial class ErrorReportingSpecs
+    {
+        [Command("exc")]
+        private class GenericExceptionCommand : ICommand
+        {
+            [CommandOption("msg", 'm')]
+            public string? Message { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => throw new Exception(Message);
+        }
+
+        [Command("exc")]
+        private class CommandExceptionCommand : ICommand
+        {
+            [CommandOption("code", 'c')]
+            public int ExitCode { get; set; } = 1337;
+
+            [CommandOption("msg", 'm')]
+            public string? Message { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => throw new CommandException(Message, ExitCode);
+        }
+
+        [Command("exc")]
+        private class ShowHelpTextOnlyCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => throw new CommandException(null, showHelp: true);
+        }
+
+        [Command("exc sub")]
+        private class ShowHelpTextOnlySubCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("exc")]
+        private class ShowErrorMessageThenHelpTextCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) =>
+                throw new CommandException("Error message.", showHelp: true);
+        }
+
+        [Command("exc sub")]
+        private class ShowErrorMessageThenHelpTextSubCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("exc")]
+        private class StackTraceOnlyCommand : ICommand
+        {
+            [CommandOption("msg", 'm')]
+            public string? Message { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => throw new CommandException(null);
+        }
+
+        [Command("inv")]
+        private class InvalidUserInputCommand : ICommand
+        {
+            [CommandOption("required", 'r')]
+            public string? RequiredOption { get; }
+
+            public ValueTask ExecuteAsync(IConsole console)
+            {
+                throw new NotImplementedException();
+            }
+        }
+    }
+}

CliFx.Tests/ErrorReportingSpecs.cs

@@ -0,0 +1,238 @@
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace CliFx.Tests
+{
+    public partial class ErrorReportingSpecs
+    {
+        private readonly ITestOutputHelper _output;
+
+        public ErrorReportingSpecs(ITestOutputHelper output) => _output = output;
+
+        [Fact]
+        public async Task Command_may_throw_a_generic_exception_which_exits_and_prints_error_message_and_stack_trace()
+        {
+            // Arrange
+            await using var stdErr = new MemoryStream();
+            var console = new VirtualConsole(error: stdErr);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(GenericExceptionCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                new[] {"exc", "-m", "Kaput"},
+                new Dictionary<string, string>());
+
+            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().NotBe(0);
+            stdErrData.Should().ContainAll(
+                "System.Exception:",
+                "Kaput", "at",
+                "CliFx.Tests");
+
+            _output.WriteLine(stdErrData);
+        }
+
+        [Fact]
+        public async Task Command_may_throw_a_specialized_exception_which_exits_with_custom_code_and_prints_minimal_error_details()
+        {
+            // Arrange
+            await using var stdErr = new MemoryStream();
+            var console = new VirtualConsole(error: stdErr);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(CommandExceptionCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                new[] {"exc", "-m", "Kaput", "-c", "69"},
+                new Dictionary<string, string>());
+
+            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().Be(69);
+            stdErrData.Should().Be("Kaput");
+
+            _output.WriteLine(stdErrData);
+        }
+
+        [Fact]
+        public async Task Command_may_throw_a_specialized_exception_without_error_message_which_exits_and_prints_full_error_details()
+        {
+            // Arrange
+            await using var stdErr = new MemoryStream();
+            var console = new VirtualConsole(error: stdErr);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(CommandExceptionCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                new[] {"exc", "-m", "Kaput"},
+                new Dictionary<string, string>());
+
+            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().NotBe(0);
+            stdErrData.Should().NotBeEmpty();
+
+            _output.WriteLine(stdErrData);
+        }
+
+        [Fact]
+        public async Task Command_may_throw_a_specialized_exception_which_shows_only_the_help_text()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            await using var stdErr = new MemoryStream();
+
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(ShowHelpTextOnlyCommand))
+                .AddCommand(typeof(ShowHelpTextOnlySubCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"exc"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+            var stdErrData = console.Output.Encoding.GetString(stdErr.ToArray()).TrimEnd();
+
+            // Assert
+            stdErrData.Should().BeEmpty();
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "[command]",
+                "Options",
+                "-h|--help", "Shows help text.",
+                "Commands",
+                "sub",
+                "You can run", "to show help on a specific command."
+            );
+
+            _output.WriteLine(stdOutData);
+            _output.WriteLine(stdErrData);
+        }
+
+        [Fact]
+        public async Task Command_may_throw_specialized_exception_which_shows_the_error_message_then_the_help_text()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            await using var stdErr = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut, error: stdErr);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(ShowErrorMessageThenHelpTextCommand))
+                .AddCommand(typeof(ShowErrorMessageThenHelpTextSubCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"exc"});
+            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdErrData.Should().Be("Error message.");
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "[command]",
+                "Options",
+                "-h|--help", "Shows help text.",
+                "Commands",
+                "sub",
+                "You can run", "to show help on a specific command."
+            );
+
+            _output.WriteLine(stdOutData);
+            _output.WriteLine(stdErrData);
+        }
+
+        [Fact]
+        public async Task Command_may_throw_a_specialized_exception_which_shows_only_a_stack_trace_and_no_help_text()
+        {
+            // Arrange
+            await using var stdErr = new MemoryStream();
+            var console = new VirtualConsole(error: stdErr);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(GenericExceptionCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                new[] {"exc", "-m", "Kaput"},
+                new Dictionary<string, string>());
+
+            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().NotBe(0);
+            stdErrData.Should().ContainAll(
+                "System.Exception:",
+                "Kaput", "at",
+                "CliFx.Tests");
+
+            _output.WriteLine(stdErrData);
+        }
+
+        [Fact]
+        public async Task Command_shows_help_text_on_exceptions_related_to_invalid_user_input()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            await using var stdErr = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut, error: stdErr);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(InvalidUserInputCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                new[] {"not-a-valid-command", "-r", "foo"},
+                new Dictionary<string, string>());
+
+            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().NotBe(0);
+            stdErrData.Should().ContainAll(
+                "Can't find a command that matches the following arguments:",
+                "not-a-valid-command"
+            );
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "[command]",
+                "Options",
+                "-h|--help", "Shows help text.",
+                "Commands",
+                "inv",
+                "You can run", "to show help on a specific command."
+            );
+
+            _output.WriteLine(stdOutData);
+            _output.WriteLine(stdErrData);
+        }
+    }
+}

CliFx.Tests/HelpTextRendererTests.Commands.cs

@@ -1,42 +0,0 @@
-using System.Threading.Tasks;
-using CliFx.Attributes;
-using CliFx.Services;
-
-namespace CliFx.Tests
-{
-    public partial class HelpTextRendererTests
-    {
-        [Command(Description = "DefaultCommand description.")]
-        private class DefaultCommand : ICommand
-        {
-            [CommandOption("option-a", 'a', Description = "OptionA description.")]
-            public string OptionA { get; set; }
-
-            [CommandOption("option-b", 'b', Description = "OptionB description.")]
-            public string OptionB { get; set; }
-
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-
-        [Command("cmd", Description = "NamedCommand description.")]
-        private class NamedCommand : ICommand
-        {
-            [CommandOption("option-c", 'c', Description = "OptionC description.")]
-            public string OptionC { get; set; }
-
-            [CommandOption("option-d", 'd', Description = "OptionD description.")]
-            public string OptionD { get; set; }
-
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-
-        [Command("cmd sub", Description = "NamedSubCommand description.")]
-        private class NamedSubCommand : ICommand
-        {
-            [CommandOption("option-e", 'e', Description = "OptionE description.")]
-            public string OptionE { get; set; }
-
-            public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
-        }
-    }
-}

CliFx.Tests/HelpTextRendererTests.cs

@@ -1,105 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.IO;
-using System.Linq;
-using CliFx.Models;
-using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
-
-namespace CliFx.Tests
-{
-    [TestFixture]
-    public partial class HelpTextRendererTests
-    {
-        private static HelpTextSource CreateHelpTextSource(IReadOnlyList<Type> availableCommandTypes, Type targetCommandType)
-        {
-            var commandSchemaResolver = new CommandSchemaResolver();
-
-            var applicationMetadata = new ApplicationMetadata("TestApp", "testapp", "1.0", null);
-            var availableCommandSchemas = commandSchemaResolver.GetCommandSchemas(availableCommandTypes);
-            var targetCommandSchema = availableCommandSchemas.Single(s => s.Type == targetCommandType);
-
-            return new HelpTextSource(applicationMetadata, availableCommandSchemas, targetCommandSchema);
-        }
-
-        private static IEnumerable<TestCaseData> GetTestCases_RenderHelpText()
-        {
-            yield return new TestCaseData(
-                CreateHelpTextSource(
-                    new[] {typeof(DefaultCommand), typeof(NamedCommand), typeof(NamedSubCommand)},
-                    typeof(DefaultCommand)),
-
-                new[]
-                {
-                    "Usage",
-                    "[command]", "[options]",
-                    "Options",
-                    "-a|--option-a", "OptionA description.",
-                    "-b|--option-b", "OptionB description.",
-                    "-h|--help", "Shows help text.",
-                    "--version", "Shows version information.",
-                    "Commands",
-                    "cmd", "NamedCommand description.",
-                    "You can run", "to show help on a specific command."
-                }
-            );
-
-            yield return new TestCaseData(
-                CreateHelpTextSource(
-                    new[] {typeof(DefaultCommand), typeof(NamedCommand), typeof(NamedSubCommand)},
-                    typeof(NamedCommand)),
-
-                new[]
-                {
-                    "Description",
-                    "NamedCommand description.",
-                    "Usage",
-                    "cmd", "[command]", "[options]",
-                    "Options",
-                    "-c|--option-c", "OptionC description.",
-                    "-d|--option-d", "OptionD description.",
-                    "-h|--help", "Shows help text.",
-                    "Commands",
-                    "sub", "NamedSubCommand description.",
-                    "You can run", "to show help on a specific command."
-                }
-            );
-
-            yield return new TestCaseData(
-                CreateHelpTextSource(
-                    new[] {typeof(DefaultCommand), typeof(NamedCommand), typeof(NamedSubCommand)},
-                    typeof(NamedSubCommand)),
-
-                new[]
-                {
-                    "Description",
-                    "NamedSubCommand description.",
-                    "Usage",
-                    "cmd sub", "[options]",
-                    "Options",
-                    "-e|--option-e", "OptionE description.",
-                    "-h|--help", "Shows help text."
-                }
-            );
-        }
-
-        [Test]
-        [TestCaseSource(nameof(GetTestCases_RenderHelpText))]
-        public void RenderHelpText_Test(HelpTextSource source, IReadOnlyList<string> expectedSubstrings)
-        {
-            // Arrange
-            using (var stdout = new StringWriter())
-            {
-                var renderer = new HelpTextRenderer();
-                var console = new VirtualConsole(stdout);
-
-                // Act
-                renderer.RenderHelpText(console, source);
-
-                // Assert
-                stdout.ToString().Should().ContainAll(expectedSubstrings);
-            }
-        }
-    }
-}

CliFx.Tests/HelpTextSpecs.Commands.cs

@@ -0,0 +1,114 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Exceptions;
+
+namespace CliFx.Tests
+{
+    public partial class HelpTextSpecs
+    {
+        [Command(Description = "DefaultCommand description.")]
+        private class DefaultCommand : ICommand
+        {
+            [CommandOption("option-a", 'a', Description = "OptionA description.")]
+            public string? OptionA { get; set; }
+
+            [CommandOption("option-b", 'b', Description = "OptionB description.")]
+            public string? OptionB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("cmd", Description = "NamedCommand description.")]
+        private class NamedCommand : ICommand
+        {
+            [CommandParameter(0, Name = "param-a", Description = "ParameterA description.")]
+            public string? ParameterA { get; set; }
+
+            [CommandOption("option-c", 'c', Description = "OptionC description.")]
+            public string? OptionC { get; set; }
+
+            [CommandOption("option-d", 'd', Description = "OptionD description.")]
+            public string? OptionD { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("cmd sub", Description = "NamedSubCommand description.")]
+        private class NamedSubCommand : ICommand
+        {
+            [CommandParameter(0, Name = "param-b", Description = "ParameterB description.")]
+            public string? ParameterB { get; set; }
+
+            [CommandParameter(1, Name = "param-c", Description = "ParameterC description.")]
+            public string? ParameterC { get; set; }
+
+            [CommandOption("option-e", 'e', Description = "OptionE description.")]
+            public string? OptionE { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("cmd-with-params")]
+        private class ParametersCommand : ICommand
+        {
+            [CommandParameter(0, Name = "first")]
+            public string? ParameterA { get; set; }
+
+            [CommandParameter(10)]
+            public int? ParameterB { get; set; }
+
+            [CommandParameter(20, Description = "A list of numbers", Name = "third list")]
+            public IEnumerable<int>? ParameterC { get; set; }
+
+            [CommandOption("option", 'o')]
+            public string? Option { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("cmd-with-req-opts")]
+        private class RequiredOptionsCommand : ICommand
+        {
+            [CommandOption("option-f", 'f', IsRequired = true)]
+            public string? OptionF { get; set; }
+
+            [CommandOption("option-g", 'g', IsRequired = true)]
+            public IEnumerable<int>? OptionG { get; set; }
+
+            [CommandOption("option-h", 'h')]
+            public string? OptionH { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("cmd-with-enum-args")]
+        private class EnumArgumentsCommand : ICommand
+        {
+            public enum TestEnum { Value1, Value2, Value3 };
+
+            [CommandParameter(0, Name = "value", Description = "Enum parameter.")]
+            public TestEnum ParamA { get; set; }
+
+            [CommandOption("value", Description = "Enum option.", IsRequired = true)]
+            public TestEnum OptionA { get; set; } = TestEnum.Value1;
+
+            [CommandOption("nullable-value", Description = "Nullable enum option.")]
+            public TestEnum? OptionB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command("cmd-with-env-vars")]
+        private class EnvironmentVariableCommand : ICommand
+        {
+            [CommandOption("option-a", 'a', IsRequired = true, EnvironmentVariableName = "ENV_OPT_A")]
+            public string? OptionA { get; set; }
+
+            [CommandOption("option-b", 'b', EnvironmentVariableName = "ENV_OPT_B")]
+            public string? OptionB { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+    }
+}

CliFx.Tests/HelpTextSpecs.cs

@@ -0,0 +1,297 @@
+using System.IO;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace CliFx.Tests
+{
+    public partial class HelpTextSpecs
+    {
+        private readonly ITestOutputHelper _output;
+
+        public HelpTextSpecs(ITestOutputHelper output) => _output = output;
+
+        [Fact]
+        public async Task Version_information_can_be_requested_by_providing_the_version_option_without_other_arguments()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(DefaultCommand))
+                .AddCommand(typeof(NamedCommand))
+                .AddCommand(typeof(NamedSubCommand))
+                .UseVersionText("v6.9")
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(new[] {"--version"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().Be(0);
+            stdOutData.Should().Be("v6.9");
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Help_text_can_be_requested_by_providing_the_help_option()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(DefaultCommand))
+                .AddCommand(typeof(NamedCommand))
+                .AddCommand(typeof(NamedSubCommand))
+                .UseTitle("AppTitle")
+                .UseVersionText("AppVer")
+                .UseDescription("AppDesc")
+                .UseExecutableName("AppExe")
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"--help"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "AppTitle", "AppVer",
+                "AppDesc",
+                "Usage",
+                "AppExe", "[command]", "[options]",
+                "Options",
+                "-a|--option-a", "OptionA description.",
+                "-b|--option-b", "OptionB description.",
+                "-h|--help", "Shows help text.",
+                "--version", "Shows version information.",
+                "Commands",
+                "cmd", "NamedCommand description.",
+                "You can run", "to show help on a specific command."
+            );
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Help_text_can_be_requested_on_a_specific_named_command()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(DefaultCommand))
+                .AddCommand(typeof(NamedCommand))
+                .AddCommand(typeof(NamedSubCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"cmd", "--help"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "Description",
+                "NamedCommand description.",
+                "Usage",
+                "cmd", "[command]", "<param-a>", "[options]",
+                "Parameters",
+                "* param-a", "ParameterA description.",
+                "Options",
+                "-c|--option-c", "OptionC description.",
+                "-d|--option-d", "OptionD description.",
+                "-h|--help", "Shows help text.",
+                "Commands",
+                "sub", "SubCommand description.",
+                "You can run", "to show help on a specific command."
+            );
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Help_text_can_be_requested_on_a_specific_named_sub_command()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(DefaultCommand))
+                .AddCommand(typeof(NamedCommand))
+                .AddCommand(typeof(NamedSubCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"cmd", "sub", "--help"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "Description",
+                "SubCommand description.",
+                "Usage",
+                "cmd sub", "<param-b>", "<param-c>", "[options]",
+                "Parameters",
+                "* param-b", "ParameterB description.",
+                "* param-c", "ParameterC description.",
+                "Options",
+                "-e|--option-e", "OptionE description.",
+                "-h|--help", "Shows help text."
+            );
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Help_text_can_be_requested_without_specifying_command_even_if_default_command_is_not_defined()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(NamedCommand))
+                .AddCommand(typeof(NamedSubCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"--help"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "[command]",
+                "Options",
+                "-h|--help", "Shows help text.",
+                "--version", "Shows version information.",
+                "Commands",
+                "cmd", "NamedCommand description.",
+                "You can run", "to show help on a specific command."
+            );
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Help_text_shows_usage_format_which_lists_all_parameters()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(ParametersCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"cmd-with-params", "--help"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "cmd-with-params", "<first>", "<parameterb>", "<third list...>", "[options]"
+            );
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Help_text_shows_usage_format_which_lists_all_required_options()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(RequiredOptionsCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"cmd-with-req-opts", "--help"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "cmd-with-req-opts", "--option-f <value>", "--option-g <values...>", "[options]",
+                "Options",
+                "* -f|--option-f",
+                "* -g|--option-g",
+                "-h|--option-h"
+            );
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Help_text_shows_usage_format_which_lists_all_valid_values_for_enum_arguments()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(EnumArgumentsCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] { "cmd-with-enum-args", "--help" });
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "cmd-with-enum-args", "[options]",
+                "Parameters",
+                "value", "Valid values: Value1, Value2, Value3.",
+                "Options",
+                "* --value", "Enum option.", "Valid values: Value1, Value2, Value3.",
+                "--nullable-value", "Nullable enum option.", "Valid values: Value1, Value2, Value3."
+            );
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Help_text_lists_environment_variable_names_for_options_that_have_them_defined()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(EnvironmentVariableCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"cmd-with-env-vars", "--help"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "Options",
+                "* -a|--option-a", "Environment variable:", "ENV_OPT_A",
+                "-b|--option-b", "Environment variable:", "ENV_OPT_B"
+            );
+
+            _output.WriteLine(stdOutData);
+        }
+    }
+}

CliFx.Tests/RoutingSpecs.Commands.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+
+namespace CliFx.Tests
+{
+    public partial class RoutingSpecs
+    {
+        [Command]
+        private class DefaultCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console)
+            {
+                console.Output.WriteLine("Hello world!");
+                return default;
+            }
+        }
+
+        [Command("concat", Description = "Concatenate strings.")]
+        private class ConcatCommand : ICommand
+        {
+            [CommandOption('i', IsRequired = true, Description = "Input strings.")]
+            public IReadOnlyList<string> Inputs { get; set; } = Array.Empty<string>();
+
+            [CommandOption('s', Description = "String separator.")]
+            public string Separator { get; set; } = "";
+
+            public ValueTask ExecuteAsync(IConsole console)
+            {
+                console.Output.WriteLine(string.Join(Separator, Inputs));
+                return default;
+            }
+        }
+
+        [Command("div", Description = "Divide one number by another.")]
+        private class DivideCommand : ICommand
+        {
+            [CommandOption("dividend", 'D', IsRequired = true, Description = "The number to divide.")]
+            public double Dividend { get; set; } = 0;
+
+            [CommandOption("divisor", 'd', IsRequired = true, Description = "The number to divide by.")]
+            public double Divisor { get; set; } = 0;
+
+            public ValueTask ExecuteAsync(IConsole console)
+            {
+                console.Output.WriteLine(Dividend / Divisor);
+                return default;
+            }
+        }
+    }
+}

CliFx.Tests/RoutingSpecs.cs

@@ -0,0 +1,90 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Threading.Tasks;
+using FluentAssertions;
+using Xunit;
+
+namespace CliFx.Tests
+{
+    public partial class RoutingSpecs
+    {
+        [Fact]
+        public async Task Default_command_is_executed_if_provided_arguments_do_not_match_any_named_command()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(DefaultCommand))
+                .AddCommand(typeof(ConcatCommand))
+                .AddCommand(typeof(DivideCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                Array.Empty<string>(),
+                new Dictionary<string, string>());
+
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().Be(0);
+            stdOutData.Should().Be("Hello world!");
+        }
+
+        [Fact]
+        public async Task Help_text_is_printed_if_no_arguments_were_provided_and_default_command_is_not_defined()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(ConcatCommand))
+                .AddCommand(typeof(DivideCommand))
+                .UseConsole(console)
+                .UseDescription("This will be visible in help")
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                Array.Empty<string>(),
+                new Dictionary<string, string>());
+
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().Be(0);
+            stdOutData.Should().Contain("This will be visible in help");
+        }
+
+        [Fact]
+        public async Task Specific_named_command_is_executed_if_provided_arguments_match_its_name()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(DefaultCommand))
+                .AddCommand(typeof(ConcatCommand))
+                .AddCommand(typeof(DivideCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                new[] {"concat", "-i", "foo", "bar", "-s", ", "},
+                new Dictionary<string, string>());
+
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().Be(0);
+            stdOutData.Should().Be("foo, bar");
+        }
+    }
+}

CliFx.Tests/UtilitiesSpecs.cs

@@ -0,0 +1,63 @@
+using System.IO;
+using System.Linq;
+using CliFx.Utilities;
+using FluentAssertions;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace CliFx.Tests
+{
+    public class UtilitiesSpecs
+    {
+        private readonly ITestOutputHelper _output;
+
+        public UtilitiesSpecs(ITestOutputHelper output) => _output = output;
+
+        [Fact]
+        public void Progress_ticker_can_be_used_to_report_progress_to_console()
+        {
+            // Arrange
+            using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut, isOutputRedirected: false);
+
+            var ticker = console.CreateProgressTicker();
+
+            var progressValues = Enumerable.Range(0, 100).Select(p => p / 100.0).ToArray();
+            var progressStringValues = progressValues.Select(p => p.ToString("P2")).ToArray();
+
+            // Act
+            foreach (var progress in progressValues)
+                ticker.Report(progress);
+
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray());
+
+            // Assert
+            stdOutData.Should().ContainAll(progressStringValues);
+
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public void Progress_ticker_does_not_write_to_console_if_output_is_redirected()
+        {
+            // Arrange
+            using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var ticker = console.CreateProgressTicker();
+
+            var progressValues = Enumerable.Range(0, 100).Select(p => p / 100.0).ToArray();
+
+            // Act
+            foreach (var progress in progressValues)
+                ticker.Report(progress);
+
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray());
+
+            // Assert
+            stdOutData.Should().BeEmpty();
+
+            _output.WriteLine(stdOutData);
+        }
+    }
+}

CliFx.Tests/xunit.runner.json

@@ -0,0 +1,5 @@
+{
+  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+  "methodDisplayOptions": "all",
+  "methodDisplay": "method"
+}

CliFx.props

@@ -0,0 +1,11 @@
+<Project>
+
+  <PropertyGroup>
+    <Version>1.2</Version>
+    <Company>Tyrrrz</Company>
+    <Copyright>Copyright (C) Alexey Golub</Copyright>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

CliFx.sln

@@ -7,18 +7,23 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "CliFx", "CliFx\CliFx.csproj
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "CliFx.Tests", "CliFx.Tests\CliFx.Tests.csproj", "{268CF863-65A5-49BB-93CF-08972B7756DC}"
 EndProject
-Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "CliFx.Tests.Dummy", "CliFx.Tests.Dummy\CliFx.Tests.Dummy.csproj", "{4904B3EB-3286-4F1B-8B74-6FF051C8E787}"
-EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{3AAE8166-BB8E-49DA-844C-3A0EE6BD40A0}"
 	ProjectSection(SolutionItems) = preProject
 		Changelog.md = Changelog.md
+		CliFx.props = CliFx.props
 		License.txt = License.txt
 		Readme.md = Readme.md
 	EndProjectSection
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "CliFx.Benchmarks", "CliFx.Benchmarks\CliFx.Benchmarks.csproj", "{8ACD6DC2-D768-4850-9223-5B7C83A78513}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CliFx.Demo", "CliFx.Demo\CliFx.Demo.csproj", "{AAB6844C-BF71-448F-A11B-89AEE459AB15}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "CliFx.Demo", "CliFx.Demo\CliFx.Demo.csproj", "{AAB6844C-BF71-448F-A11B-89AEE459AB15}"
+EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "CliFx.Tests.Dummy", "CliFx.Tests.Dummy\CliFx.Tests.Dummy.csproj", "{F717347D-8656-44DA-A4A2-BE515E8C4655}"
+EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "CliFx.Analyzers", "CliFx.Analyzers\CliFx.Analyzers.csproj", "{F8460D69-F8CF-405C-A6ED-BED02A21DB42}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CliFx.Analyzers.Tests", "CliFx.Analyzers.Tests\CliFx.Analyzers.Tests.csproj", "{49878E75-2097-4C79-9151-B98A28FBB973}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -54,18 +59,6 @@ Global
 		{268CF863-65A5-49BB-93CF-08972B7756DC}.Release|x64.Build.0 = Release|Any CPU
 		{268CF863-65A5-49BB-93CF-08972B7756DC}.Release|x86.ActiveCfg = Release|Any CPU
 		{268CF863-65A5-49BB-93CF-08972B7756DC}.Release|x86.Build.0 = Release|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Debug|x64.ActiveCfg = Debug|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Debug|x64.Build.0 = Debug|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Debug|x86.ActiveCfg = Debug|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Debug|x86.Build.0 = Debug|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Release|Any CPU.Build.0 = Release|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Release|x64.ActiveCfg = Release|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Release|x64.Build.0 = Release|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Release|x86.ActiveCfg = Release|Any CPU
-		{4904B3EB-3286-4F1B-8B74-6FF051C8E787}.Release|x86.Build.0 = Release|Any CPU
 		{8ACD6DC2-D768-4850-9223-5B7C83A78513}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
 		{8ACD6DC2-D768-4850-9223-5B7C83A78513}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{8ACD6DC2-D768-4850-9223-5B7C83A78513}.Debug|x64.ActiveCfg = Debug|Any CPU
@@ -90,6 +83,42 @@ Global
 		{AAB6844C-BF71-448F-A11B-89AEE459AB15}.Release|x64.Build.0 = Release|Any CPU
 		{AAB6844C-BF71-448F-A11B-89AEE459AB15}.Release|x86.ActiveCfg = Release|Any CPU
 		{AAB6844C-BF71-448F-A11B-89AEE459AB15}.Release|x86.Build.0 = Release|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Debug|x64.Build.0 = Debug|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Debug|x86.Build.0 = Debug|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Release|Any CPU.Build.0 = Release|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Release|x64.ActiveCfg = Release|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Release|x64.Build.0 = Release|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Release|x86.ActiveCfg = Release|Any CPU
+		{F717347D-8656-44DA-A4A2-BE515E8C4655}.Release|x86.Build.0 = Release|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Debug|x64.Build.0 = Debug|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Debug|x86.Build.0 = Debug|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Release|Any CPU.Build.0 = Release|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Release|x64.ActiveCfg = Release|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Release|x64.Build.0 = Release|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Release|x86.ActiveCfg = Release|Any CPU
+		{F8460D69-F8CF-405C-A6ED-BED02A21DB42}.Release|x86.Build.0 = Release|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Debug|x64.Build.0 = Debug|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Debug|x86.Build.0 = Debug|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Release|Any CPU.Build.0 = Release|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Release|x64.ActiveCfg = Release|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Release|x64.Build.0 = Release|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Release|x86.ActiveCfg = Release|Any CPU
+		{49878E75-2097-4C79-9151-B98A28FBB973}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

CliFx/ApplicationConfiguration.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Collections.Generic;
+
+namespace CliFx
+{
+    /// <summary>
+    /// Configuration of an application.
+    /// </summary>
+    public class ApplicationConfiguration
+    {
+        /// <summary>
+        /// Command types defined in this application.
+        /// </summary>
+        public IReadOnlyList<Type> CommandTypes { get; }
+
+        /// <summary>
+        /// Whether debug mode is allowed in this application.
+        /// </summary>
+        public bool IsDebugModeAllowed { get; }
+
+        /// <summary>
+        /// Whether preview mode is allowed in this application.
+        /// </summary>
+        public bool IsPreviewModeAllowed { get; }
+
+        /// <summary>
+        /// Initializes an instance of <see cref="ApplicationConfiguration"/>.
+        /// </summary>
+        public ApplicationConfiguration(
+            IReadOnlyList<Type> commandTypes,
+            bool isDebugModeAllowed, bool isPreviewModeAllowed)
+        {
+            CommandTypes = commandTypes;
+            IsDebugModeAllowed = isDebugModeAllowed;
+            IsPreviewModeAllowed = isPreviewModeAllowed;
+        }
+    }
+}

CliFx/ApplicationMetadata.cs

@@ -1,6 +1,4 @@
-using CliFx.Internal;
+namespace CliFx
-
-namespace CliFx.Models
 {
     /// <summary>
     /// Metadata associated with an application.
@@ -25,17 +23,17 @@ namespace CliFx.Models
         /// <summary>
         /// Application description.
         /// </summary>
-        public string Description { get; }
+        public string? Description { get; }
 
         /// <summary>
         /// Initializes an instance of <see cref="ApplicationMetadata"/>.
         /// </summary>
-        public ApplicationMetadata(string title, string executableName, string versionText, string description)
+        public ApplicationMetadata(string title, string executableName, string versionText, string? description)
         {
-            Title = title.GuardNotNull(nameof(title));
+            Title = title;
-            ExecutableName = executableName.GuardNotNull(nameof(executableName));
+            ExecutableName = executableName;
-            VersionText = versionText.GuardNotNull(nameof(versionText));
+            VersionText = versionText;
-            Description = description; // can be null
+            Description = description;
         }
     }
 }

CliFx/Attributes/CommandAttribute.cs

@@ -10,27 +10,29 @@ namespace CliFx.Attributes
     {
         /// <summary>
         /// Command name.
+        /// If the name is not set, the command is treated as a default command, i.e. the one that gets executed when the user
+        /// does not specify a command name in the arguments.
+        /// All commands in an application must have different names. Likewise, only one command without a name is allowed.
         /// </summary>
-        public string Name { get; }
+        public string? Name { get; }
 
         /// <summary>
         /// Command description, which is used in help text.
         /// </summary>
-        public string Description { get; set; }
+        public string? Description { get; set; }
 
         /// <summary>
         /// Initializes an instance of <see cref="CommandAttribute"/>.
         /// </summary>
         public CommandAttribute(string name)
         {
-            Name = name; // can be null
+            Name = name;
         }
 
         /// <summary>
         /// Initializes an instance of <see cref="CommandAttribute"/>.
         /// </summary>
         public CommandAttribute()
-            : this(null)
         {
         }
     }

CliFx/Attributes/CommandOptionAttribute.cs

@@ -9,12 +9,16 @@ namespace CliFx.Attributes
     public class CommandOptionAttribute : Attribute
     {
         /// <summary>
-        /// Option name.
+        /// Option name (must be longer than a single character).
+        /// Either <see cref="Name"/> or <see cref="ShortName"/> must be set.
+        /// All options in a command must have different names (comparison is not case-sensitive).
         /// </summary>
-        public string Name { get; }
+        public string? Name { get; }
 
         /// <summary>
-        /// Option short name.
+        /// Option short name (single character).
+        /// Either <see cref="Name"/> or <see cref="ShortName"/> must be set.
+        /// All options in a command must have different short names (comparison is case-sensitive).
         /// </summary>
         public char? ShortName { get; }
 
@@ -26,15 +30,20 @@ namespace CliFx.Attributes
         /// <summary>
         /// Option description, which is used in help text.
         /// </summary>
-        public string Description { get; set; }
+        public string? Description { get; set; }
+
+        /// <summary>
+        /// Environment variable that will be used as fallback if no option value is specified.
+        /// </summary>
+        public string? EnvironmentVariableName { get; set; }
 
         /// <summary>
         /// Initializes an instance of <see cref="CommandOptionAttribute"/>.
         /// </summary>
-        public CommandOptionAttribute(string name, char? shortName)
+        private CommandOptionAttribute(string? name, char? shortName)
         {
-            Name = name; // can be null
+            Name = name;
-            ShortName = shortName; // can be null
+            ShortName = shortName;
         }
 
         /// <summary>
@@ -57,7 +66,7 @@ namespace CliFx.Attributes
         /// Initializes an instance of <see cref="CommandOptionAttribute"/>.
         /// </summary>
         public CommandOptionAttribute(char shortName)
-            : this(null, shortName)
+            : this(null, (char?) shortName)
         {
         }
     }

CliFx/Attributes/CommandParameterAttribute.cs

@@ -0,0 +1,37 @@
+using System;
+
+namespace CliFx.Attributes
+{
+    /// <summary>
+    /// Annotates a property that defines a command parameter.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Property)]
+    public class CommandParameterAttribute : Attribute
+    {
+        /// <summary>
+        /// Order of this parameter compared to other parameters.
+        /// All parameters in a command must have different order.
+        /// Parameter whose type is a non-scalar (e.g. array), must be the last in order and only one such parameter is allowed.
+        /// </summary>
+        public int Order { get; }
+
+        /// <summary>
+        /// Parameter name, which is only used in help text.
+        /// If this isn't specified, property name is used instead.
+        /// </summary>
+        public string? Name { get; set; }
+
+        /// <summary>
+        /// Parameter description, which is used in help text.
+        /// </summary>
+        public string? Description { get; set; }
+
+        /// <summary>
+        /// Initializes an instance of <see cref="CommandParameterAttribute"/>.
+        /// </summary>
+        public CommandParameterAttribute(int order)
+        {
+            Order = order;
+        }
+    }
+}

CliFx/CliApplication.cs

@@ -1,144 +1,232 @@
 using System;
+using System.Collections;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Threading.Tasks;
+using CliFx.Domain;
 using CliFx.Exceptions;
-using CliFx.Internal;
-using CliFx.Models;
-using CliFx.Services;
 
 namespace CliFx
 {
     /// <summary>
-    /// Default implementation of <see cref="ICliApplication"/>.
+    /// Command line application facade.
     /// </summary>
-    public class CliApplication : ICliApplication
+    public class CliApplication
     {
         private readonly ApplicationMetadata _metadata;
         private readonly ApplicationConfiguration _configuration;
-
         private readonly IConsole _console;
-        private readonly ICommandInputParser _commandInputParser;
+        private readonly ITypeActivator _typeActivator;
-        private readonly ICommandSchemaResolver _commandSchemaResolver;
+
-        private readonly ICommandFactory _commandFactory;
+        private readonly HelpTextWriter _helpTextWriter;
-        private readonly ICommandInitializer _commandInitializer;
-        private readonly IHelpTextRenderer _helpTextRenderer;
 
         /// <summary>
         /// Initializes an instance of <see cref="CliApplication"/>.
         /// </summary>
-        public CliApplication(ApplicationMetadata metadata, ApplicationConfiguration configuration,
+        public CliApplication(
-            IConsole console, ICommandInputParser commandInputParser, ICommandSchemaResolver commandSchemaResolver,
+            ApplicationMetadata metadata, ApplicationConfiguration configuration,
-            ICommandFactory commandFactory, ICommandInitializer commandInitializer, IHelpTextRenderer helpTextRenderer)
+            IConsole console, ITypeActivator typeActivator)
         {
-            _metadata = metadata.GuardNotNull(nameof(metadata));
+            _metadata = metadata;
-            _configuration = configuration.GuardNotNull(nameof(configuration));
+            _configuration = configuration;
+            _console = console;
+            _typeActivator = typeActivator;
 
-            _console = console.GuardNotNull(nameof(console));
+            _helpTextWriter = new HelpTextWriter(metadata, console);
-            _commandInputParser = commandInputParser.GuardNotNull(nameof(commandInputParser));
-            _commandSchemaResolver = commandSchemaResolver.GuardNotNull(nameof(commandSchemaResolver));
-            _commandFactory = commandFactory.GuardNotNull(nameof(commandFactory));
-            _commandInitializer = commandInitializer.GuardNotNull(nameof(commandInitializer));
-            _helpTextRenderer = helpTextRenderer.GuardNotNull(nameof(helpTextRenderer));
         }
 
-        /// <inheritdoc />
+        private async ValueTask<int?> HandleDebugDirectiveAsync(CommandLineInput commandLineInput)
-        public async Task<int> RunAsync(IReadOnlyList<string> commandLineArguments)
         {
-            commandLineArguments.GuardNotNull(nameof(commandLineArguments));
+            var isDebugMode = _configuration.IsDebugModeAllowed && commandLineInput.IsDebugDirectiveSpecified;
+            if (!isDebugMode)
+                return null;
 
+            _console.WithForegroundColor(ConsoleColor.Green, () =>
+                _console.Output.WriteLine($"Attach debugger to PID {Process.GetCurrentProcess().Id} to continue."));
+
+            while (!Debugger.IsAttached)
+                await Task.Delay(100);
+
+            return null;
+        }
+
+        private int? HandlePreviewDirective(ApplicationSchema applicationSchema, CommandLineInput commandLineInput)
+        {
+            var isPreviewMode = _configuration.IsPreviewModeAllowed && commandLineInput.IsPreviewDirectiveSpecified;
+            if (!isPreviewMode)
+                return null;
+
+            var commandSchema = applicationSchema.TryFindCommand(commandLineInput, out var argumentOffset);
+
+            _console.Output.WriteLine("Parser preview:");
+
+            // Command name
+            if (commandSchema != null && argumentOffset > 0)
+            {
+                _console.WithForegroundColor(ConsoleColor.Cyan, () =>
+                    _console.Output.Write(commandSchema.Name));
+
+                _console.Output.Write(' ');
+            }
+
+            // Parameters
+            foreach (var parameter in commandLineInput.UnboundArguments.Skip(argumentOffset))
+            {
+                _console.Output.Write('<');
+
+                _console.WithForegroundColor(ConsoleColor.White, () =>
+                    _console.Output.Write(parameter));
+
+                _console.Output.Write('>');
+                _console.Output.Write(' ');
+            }
+
+            // Options
+            foreach (var option in commandLineInput.Options)
+            {
+                _console.Output.Write('[');
+
+                _console.WithForegroundColor(ConsoleColor.White, () =>
+                    _console.Output.Write(option));
+
+                _console.Output.Write(']');
+                _console.Output.Write(' ');
+            }
+
+            _console.Output.WriteLine();
+
+            return 0;
+        }
+
+        private int? HandleVersionOption(CommandLineInput commandLineInput)
+        {
+            // Version option is available only on the default command (i.e. when arguments are not specified)
+            var shouldRenderVersion = !commandLineInput.UnboundArguments.Any() && commandLineInput.IsVersionOptionSpecified;
+            if (!shouldRenderVersion)
+                return null;
+
+            _console.Output.WriteLine(_metadata.VersionText);
+
+            return 0;
+        }
+
+        private int? HandleHelpOption(ApplicationSchema applicationSchema, CommandLineInput commandLineInput)
+        {
+            // Help is rendered either when it's requested or when the user provides no arguments and there is no default command
+            var shouldRenderHelp =
+                commandLineInput.IsHelpOptionSpecified ||
+                !applicationSchema.Commands.Any(c => c.IsDefault) && !commandLineInput.UnboundArguments.Any() && !commandLineInput.Options.Any();
+
+            if (!shouldRenderHelp)
+                return null;
+
+            // Get the command schema that matches the input or use a dummy default command as a fallback
+            var commandSchema =
+                applicationSchema.TryFindCommand(commandLineInput) ??
+                CommandSchema.StubDefaultCommand;
+
+            _helpTextWriter.Write(applicationSchema, commandSchema);
+
+            return 0;
+        }
+
+        private async ValueTask<int> HandleCommandExecutionAsync(
+            ApplicationSchema applicationSchema,
+            CommandLineInput commandLineInput,
+            IReadOnlyDictionary<string, string> environmentVariables)
+        {
+            await applicationSchema
+                .InitializeEntryPoint(commandLineInput, environmentVariables, _typeActivator)
+                .ExecuteAsync(_console);
+
+            return 0;
+        }
+
+        /// <summary>
+        /// Handle <see cref="CommandException"/>s differently from the rest because we want to
+        /// display it different based on whether we are showing the help text or not.
+        /// </summary>
+        private int HandleCliFxException(IReadOnlyList<string> commandLineArguments, CliFxException cfe)
+        {
+            var showHelp = cfe.ShowHelp;
+
+            var errorMessage = cfe.HasMessage
+                ? cfe.Message
+                : cfe.ToString();
+               
+            _console.WithForegroundColor(ConsoleColor.Red, () => _console.Error.WriteLine(errorMessage));            
+
+            if (showHelp)
+            {
+                var applicationSchema = ApplicationSchema.Resolve(_configuration.CommandTypes);
+                var commandLineInput = CommandLineInput.Parse(commandLineArguments);
+                var commandSchema = applicationSchema.TryFindCommand(commandLineInput) ??
+                    CommandSchema.StubDefaultCommand;
+                _helpTextWriter.Write(applicationSchema, commandSchema);
+            }
+
+            return cfe.ExitCode;
+        }
+
+        /// <summary>
+        /// Runs the application with specified command line arguments and environment variables, and returns the exit code.
+        /// </summary>
+        public async ValueTask<int> RunAsync(
+            IReadOnlyList<string> commandLineArguments,
+            IReadOnlyDictionary<string, string> environmentVariables)
+        {
             try
             {
-                // Get schemas for all available command types
+                var applicationSchema = ApplicationSchema.Resolve(_configuration.CommandTypes);
-                var availableCommandSchemas = _commandSchemaResolver.GetCommandSchemas(_configuration.CommandTypes);
+                var commandLineInput = CommandLineInput.Parse(commandLineArguments);
 
-                // Parse command input from arguments
+                return
-                var commandInput = _commandInputParser.ParseCommandInput(commandLineArguments);
+                    await HandleDebugDirectiveAsync(commandLineInput) ??
-
+                    HandlePreviewDirective(applicationSchema, commandLineInput) ??
-                // Find command schema matching the name specified in the input
+                    HandleVersionOption(commandLineInput) ??
-                var targetCommandSchema = availableCommandSchemas.FindByName(commandInput.CommandName);
+                    HandleHelpOption(applicationSchema, commandLineInput) ??
-
+                    await HandleCommandExecutionAsync(applicationSchema, commandLineInput, environmentVariables);
-                // Handle cases where requested command is not defined
+            }
-                if (targetCommandSchema == null)
+            catch (CliFxException cfe)
-                {
+            {
-                    var isError = false;
+                // We want to catch exceptions in order to print errors and return correct exit codes.
-
+                // Doing this also gets rid of the annoying Windows troubleshooting dialog that shows up on unhandled exceptions.
-                    // If specified a command - show error
+                var exitCode = HandleCliFxException(commandLineArguments, cfe);
-                    if (commandInput.IsCommandSpecified())
+                return exitCode;
-                    {
-                        isError = true;
-
-                        _console.WithForegroundColor(ConsoleColor.Red,
-                            () => _console.Error.WriteLine($"Specified command [{commandInput.CommandName}] is not defined."));
-                    }
-
-                    // Get parent command schema
-                    var parentCommandSchema = availableCommandSchemas.FindParent(commandInput.CommandName);
-
-                    // Show help for parent command if it's defined
-                    if (parentCommandSchema != null)
-                    {
-                        var helpTextSource = new HelpTextSource(_metadata, availableCommandSchemas, parentCommandSchema);
-                        _helpTextRenderer.RenderHelpText(_console, helpTextSource);
-                    }
-                    // Otherwise show help for a stub default command
-                    else
-                    {
-                        var helpTextSource = new HelpTextSource(_metadata,
-                            availableCommandSchemas.Concat(CommandSchema.StubDefaultCommand).ToArray(),
-                            CommandSchema.StubDefaultCommand);
-
-                        _helpTextRenderer.RenderHelpText(_console, helpTextSource);
-                    }
-
-                    return isError ? -1 : 0;
-                }
-
-                // Show version if it was requested without specifying a command
-                if (commandInput.IsVersionRequested() && !commandInput.IsCommandSpecified())
-                {
-                    _console.Output.WriteLine(_metadata.VersionText);
-
-                    return 0;
-                }
-
-                // Show help if it was requested
-                if (commandInput.IsHelpRequested())
-                {
-                    var helpTextSource = new HelpTextSource(_metadata, availableCommandSchemas, targetCommandSchema);
-                    _helpTextRenderer.RenderHelpText(_console, helpTextSource);
-
-                    return 0;
-                }
-
-                // Create an instance of the command
-                var command = _commandFactory.CreateCommand(targetCommandSchema);
-
-                // Populate command with options according to its schema
-                _commandInitializer.InitializeCommand(command, targetCommandSchema, commandInput);
-
-                // Execute command
-                await command.ExecuteAsync(_console);
-
-                return 0;
             }
             catch (Exception ex)
             {
-                // We want to catch exceptions in order to print errors and return correct exit codes.
+                // For all other errors, we just write the entire thing to stderr.
-                // Also, by doing this we get rid of the annoying Windows troubleshooting dialog that shows up on unhandled exceptions.
+                _console.WithForegroundColor(ConsoleColor.Red, () => _console.Error.WriteLine(ex.ToString()));
-
+                return ex.HResult;
-                // In case we catch a CliFx-specific exception, we want to just show the error message, not the stack trace.
-                // Stack trace isn't very useful to the user if the exception is not really coming from their code.
-
-                // CommandException is the same, but it also lets users specify exit code so we want to return that instead of default.
-
-                var message = ex is CliFxException && !ex.Message.IsNullOrWhiteSpace() ? ex.Message : ex.ToString();
-                var exitCode = ex is CommandException commandEx ? commandEx.ExitCode : ex.HResult;
-
-                _console.WithForegroundColor(ConsoleColor.Red, () => _console.Error.WriteLine(message));
-
-                return exitCode;
             }
         }
+
+        /// <summary>
+        /// Runs the application with specified command line arguments and returns the exit code.
+        /// Environment variables are retrieved automatically.
+        /// </summary>
+        public async ValueTask<int> RunAsync(IReadOnlyList<string> commandLineArguments)
+        {
+            var environmentVariables = Environment.GetEnvironmentVariables()
+                .Cast<DictionaryEntry>()
+                .ToDictionary(e => (string) e.Key, e => (string) e.Value, StringComparer.OrdinalIgnoreCase);
+
+            return await RunAsync(commandLineArguments, environmentVariables);
+        }
+
+        /// <summary>
+        /// Runs the application and returns the exit code.
+        /// Command line arguments and environment variables are retrieved automatically.
+        /// </summary>
+        public async ValueTask<int> RunAsync()
+        {
+            var commandLineArguments = Environment.GetCommandLineArgs()
+                .Skip(1)
+                .ToArray();
+
+            return await RunAsync(commandLineArguments);
+        }
     }
 }

CliFx/CliApplicationBuilder.cs

@@ -3,162 +3,198 @@ using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Reflection;
-using CliFx.Internal;
+using CliFx.Domain;
-using CliFx.Models;
-using CliFx.Services;
 
 namespace CliFx
 {
     /// <summary>
-    /// Default implementation of <see cref="ICliApplicationBuilder"/>.
+    /// Builds an instance of <see cref="CliApplication"/>.
     /// </summary>
-    public partial class CliApplicationBuilder : ICliApplicationBuilder
+    public partial class CliApplicationBuilder
     {
         private readonly HashSet<Type> _commandTypes = new HashSet<Type>();
 
-        private string _title;
+        private bool _isDebugModeAllowed = true;
-        private string _executableName;
+        private bool _isPreviewModeAllowed = true;
-        private string _versionText;
+        private string? _title;
-        private string _description;
+        private string? _executableName;
-        private IConsole _console;
+        private string? _versionText;
-        private ICommandFactory _commandFactory;
+        private string? _description;
+        private IConsole? _console;
+        private ITypeActivator? _typeActivator;
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplicationBuilder AddCommand(Type commandType)
+        /// Adds a command of specified type to the application.
+        /// </summary>
+        public CliApplicationBuilder AddCommand(Type commandType)
         {
-            commandType.GuardNotNull(nameof(commandType));
-
             _commandTypes.Add(commandType);
 
             return this;
         }
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplicationBuilder AddCommandsFrom(Assembly commandAssembly)
+        /// Adds multiple commands to the application.
+        /// </summary>
+        public CliApplicationBuilder AddCommands(IEnumerable<Type> commandTypes)
         {
-            commandAssembly.GuardNotNull(nameof(commandAssembly));
-
-            var commandTypes = commandAssembly.ExportedTypes.Where(t => t.Implements(typeof(ICommand)));
-
             foreach (var commandType in commandTypes)
                 AddCommand(commandType);
 
             return this;
         }
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplicationBuilder UseTitle(string title)
+        /// Adds commands from the specified assembly to the application.
+        /// Only adds public valid command types.
+        /// </summary>
+        public CliApplicationBuilder AddCommandsFrom(Assembly commandAssembly)
         {
-            _title = title.GuardNotNull(nameof(title));
+            foreach (var commandType in commandAssembly.ExportedTypes.Where(CommandSchema.IsCommandType))
+                AddCommand(commandType);
+
             return this;
         }
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplicationBuilder UseExecutableName(string executableName)
+        /// Adds commands from the specified assemblies to the application.
+        /// Only adds public valid command types.
+        /// </summary>
+        public CliApplicationBuilder AddCommandsFrom(IEnumerable<Assembly> commandAssemblies)
         {
-            _executableName = executableName.GuardNotNull(nameof(executableName));
+            foreach (var commandAssembly in commandAssemblies)
+                AddCommandsFrom(commandAssembly);
+
             return this;
         }
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplicationBuilder UseVersionText(string versionText)
+        /// Adds commands from the calling assembly to the application.
+        /// Only adds public valid command types.
+        /// </summary>
+        public CliApplicationBuilder AddCommandsFromThisAssembly() => AddCommandsFrom(Assembly.GetCallingAssembly());
+
+        /// <summary>
+        /// Specifies whether debug mode (enabled with [debug] directive) is allowed in the application.
+        /// </summary>
+        public CliApplicationBuilder AllowDebugMode(bool isAllowed = true)
         {
-            _versionText = versionText.GuardNotNull(nameof(versionText));
+            _isDebugModeAllowed = isAllowed;
             return this;
         }
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplicationBuilder UseDescription(string description)
+        /// Specifies whether preview mode (enabled with [preview] directive) is allowed in the application.
+        /// </summary>
+        public CliApplicationBuilder AllowPreviewMode(bool isAllowed = true)
         {
-            _description = description; // can be null
+            _isPreviewModeAllowed = isAllowed;
             return this;
         }
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplicationBuilder UseConsole(IConsole console)
+        /// Sets application title, which appears in the help text.
+        /// </summary>
+        public CliApplicationBuilder UseTitle(string title)
         {
-            _console = console.GuardNotNull(nameof(console));
+            _title = title;
             return this;
         }
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplicationBuilder UseCommandFactory(ICommandFactory factory)
+        /// Sets application executable name, which appears in the help text.
+        /// </summary>
+        public CliApplicationBuilder UseExecutableName(string executableName)
         {
-            _commandFactory = factory.GuardNotNull(nameof(factory));
+            _executableName = executableName;
             return this;
         }
 
-        private void SetFallbackValues()
+        /// <summary>
+        /// Sets application version text, which appears in the help text and when the user requests version information.
+        /// </summary>
+        public CliApplicationBuilder UseVersionText(string versionText)
         {
-            if (_title.IsNullOrWhiteSpace())
+            _versionText = versionText;
-            {
+            return this;
-                // Entry assembly is null in tests
-                UseTitle(EntryAssembly?.GetName().Name ?? "App");
-            }
-
-            if (_executableName.IsNullOrWhiteSpace())
-            {
-                // Entry assembly is null in tests
-                var entryAssemblyLocation = EntryAssembly?.Location;
-
-                // Set different executable name depending on location
-                if (!entryAssemblyLocation.IsNullOrWhiteSpace())
-                {
-                    // Prepend 'dotnet' to assembly file name if the entry assembly is a dll file (extension needs to be kept)
-                    if (string.Equals(Path.GetExtension(entryAssemblyLocation), ".dll", StringComparison.OrdinalIgnoreCase))
-                    {
-                        UseExecutableName("dotnet " + Path.GetFileName(entryAssemblyLocation));
-                    }
-                    // Otherwise just use assembly file name without extension
-                    else
-                    {
-                        UseExecutableName(Path.GetFileNameWithoutExtension(entryAssemblyLocation));
-                    }
-                }
-                // If location is null then just use a stub
-                else
-                {
-                    UseExecutableName("app");
-                }
-            }
-
-            if (_versionText.IsNullOrWhiteSpace())
-            {
-                // Entry assembly is null in tests
-                UseVersionText(EntryAssembly?.GetName().Version.ToString() ?? "1.0");
-            }
-
-            if (_console == null)
-            {
-                UseConsole(new SystemConsole());
-            }
-
-            if (_commandFactory == null)
-            {
-                UseCommandFactory(new CommandFactory());
-            }
         }
 
-        /// <inheritdoc />
+        /// <summary>
-        public ICliApplication Build()
+        /// Sets application description, which appears in the help text.
+        /// </summary>
+        public CliApplicationBuilder UseDescription(string? description)
         {
-            // Use defaults for required parameters that were not configured
+            _description = description;
-            SetFallbackValues();
+            return this;
+        }
+
+        /// <summary>
+        /// Configures the application to use the specified implementation of <see cref="IConsole"/>.
+        /// </summary>
+        public CliApplicationBuilder UseConsole(IConsole console)
+        {
+            _console = console;
+            return this;
+        }
+
+        /// <summary>
+        /// Configures the application to use the specified implementation of <see cref="ITypeActivator"/>.
+        /// </summary>
+        public CliApplicationBuilder UseTypeActivator(ITypeActivator typeActivator)
+        {
+            _typeActivator = typeActivator;
+            return this;
+        }
+
+        /// <summary>
+        /// Configures the application to use the specified function for activating types.
+        /// </summary>
+        public CliApplicationBuilder UseTypeActivator(Func<Type, object> typeActivator) =>
+            UseTypeActivator(new DelegateTypeActivator(typeActivator));
+
+        /// <summary>
+        /// Creates an instance of <see cref="CliApplication"/> using configured parameters.
+        /// Default values are used in place of parameters that were not specified.
+        /// </summary>
+        public CliApplication Build()
+        {
+            _title ??= GetDefaultTitle() ?? "App";
+            _executableName ??= GetDefaultExecutableName() ?? "app";
+            _versionText ??= GetDefaultVersionText() ?? "v1.0";
+            _console ??= new SystemConsole();
+            _typeActivator ??= new DefaultTypeActivator();
 
-            // Project parameters to expected types
             var metadata = new ApplicationMetadata(_title, _executableName, _versionText, _description);
-            var configuration = new ApplicationConfiguration(_commandTypes.ToArray());
+            var configuration = new ApplicationConfiguration(_commandTypes.ToArray(), _isDebugModeAllowed, _isPreviewModeAllowed);
 
-            return new CliApplication(metadata, configuration,
+            return new CliApplication(metadata, configuration, _console, _typeActivator);
-                _console, new CommandInputParser(), new CommandSchemaResolver(),
-                _commandFactory, new CommandInitializer(), new HelpTextRenderer());
         }
     }
 
     public partial class CliApplicationBuilder
     {
-        private static readonly Lazy<Assembly> LazyEntryAssembly = new Lazy<Assembly>(Assembly.GetEntryAssembly);
+        private static readonly Lazy<Assembly?> LazyEntryAssembly = new Lazy<Assembly?>(Assembly.GetEntryAssembly);
 
-        private static Assembly EntryAssembly => LazyEntryAssembly.Value;
+        // Entry assembly is null in tests
+        private static Assembly? EntryAssembly => LazyEntryAssembly.Value;
+
+        private static string? GetDefaultTitle() => EntryAssembly?.GetName().Name;
+
+        private static string? GetDefaultExecutableName()
+        {
+            var entryAssemblyLocation = EntryAssembly?.Location;
+
+            // The assembly can be an executable or a dll, depending on how it was packaged
+            var isDll = string.Equals(Path.GetExtension(entryAssemblyLocation), ".dll", StringComparison.OrdinalIgnoreCase);
+
+            return isDll
+                ? "dotnet " + Path.GetFileName(entryAssemblyLocation)
+                : Path.GetFileNameWithoutExtension(entryAssemblyLocation);
+        }
+
+        private static string? GetDefaultVersionText() =>
+            EntryAssembly != null
+                ? $"v{EntryAssembly.GetName().Version}"
+                : null;
     }
 }

CliFx/CliFx.csproj

@@ -1,23 +1,60 @@
 <Project Sdk="Microsoft.NET.Sdk">
+  <Import Project="../CliFx.props" />
 
   <PropertyGroup>
-    <TargetFrameworks>net45;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>netstandard2.1;netstandard2.0</TargetFrameworks>
-    <LangVersion>latest</LangVersion>
-    <Version>0.0.2</Version>
-    <Company>Tyrrrz</Company>
     <Authors>$(Company)</Authors>
-    <Copyright>Copyright (C) Alexey Golub</Copyright>
     <Description>Declarative framework for CLI applications</Description>
     <PackageTags>command line executable interface framework parser arguments net core</PackageTags>
     <PackageProjectUrl>https://github.com/Tyrrrz/CliFx</PackageProjectUrl>
     <PackageReleaseNotes>https://github.com/Tyrrrz/CliFx/blob/master/Changelog.md</PackageReleaseNotes>
-    <PackageIconUrl>https://raw.githubusercontent.com/Tyrrrz/CliFx/master/favicon.png</PackageIconUrl>
+    <PackageIcon>favicon.png</PackageIcon>
-    <PackageLicenseExpression>LGPL-3.0-only</PackageLicenseExpression>
+    <PackageLicenseExpression>MIT</PackageLicenseExpression>
-    <RepositoryUrl>https://github.com/Tyrrrz/CliFx</RepositoryUrl>
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
-    <RepositoryType>git</RepositoryType>
+    <PublishRepositoryUrl>true</PublishRepositoryUrl>
-    <PackageRequireLicenseAcceptance>True</PackageRequireLicenseAcceptance>
+    <EmbedUntrackedSources>true</EmbedUntrackedSources>
-    <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
+    <IncludeSymbols>true</IncludeSymbols>
-    <DocumentationFile>bin/$(Configuration)/$(TargetFramework)/$(AssemblyName).xml</DocumentationFile>
+    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+    <TargetsForTfmSpecificContentInPackage>$(TargetsForTfmSpecificContentInPackage);CopyAnalyzerToPackage</TargetsForTfmSpecificContentInPackage>
   </PropertyGroup>
 
+  <!-- Disable nullability warnings on older frameworks because there is no nullability info for BCL -->
+  <PropertyGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <Nullable>annotations</Nullable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleToAttribute">
+      <_Parameter1>$(AssemblyName).Tests</_Parameter1>
+    </AssemblyAttribute>
+    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleToAttribute">
+      <_Parameter1>$(AssemblyName).Analyzers</_Parameter1>
+    </AssemblyAttribute>
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="all" />
+    <PackageReference Include="Nullable" Version="1.2.1" PrivateAssets="all" />
+  </ItemGroup>
+
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.3" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Include="../favicon.png" Pack="true" PackagePath="" />
+  </ItemGroup>
+
+  <!-- The following item group and target ensure that the analyzer project is copied into the output NuGet package -->
+  <ItemGroup>
+    <ProjectReference Include="..\CliFx.Analyzers\CliFx.Analyzers.csproj" ReferenceOutputAssembly="true" IncludeAssets="CliFx.Analyzers.dll" />
+  </ItemGroup>
+
+  <Target Name="CopyAnalyzerToPackage">
+    <ItemGroup>
+      <TfmSpecificPackageFile Include="$(OutDir)/CliFx.Analyzers.dll" PackagePath="analyzers/dotnet/cs" BuildAction="none" />
+    </ItemGroup>
+  </Target>
+
 </Project>

CliFx/DefaultTypeActivator.cs

@@ -0,0 +1,25 @@
+using System;
+using System.Text;
+using CliFx.Exceptions;
+
+namespace CliFx
+{
+    /// <summary>
+    /// Type activator that uses the <see cref="Activator"/> class to instantiate objects.
+    /// </summary>
+    public class DefaultTypeActivator : ITypeActivator
+    {
+        /// <inheritdoc />
+        public object CreateInstance(Type type)
+        {
+            try
+            {
+                return Activator.CreateInstance(type);
+            }
+            catch (Exception ex)
+            {
+                throw CliFxException.DefaultActivatorFailed(type, ex);
+            }
+        }
+    }
+}

CliFx/DelegateTypeActivator.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Text;
+using CliFx.Exceptions;
+
+namespace CliFx
+{
+    /// <summary>
+    /// Type activator that uses the specified delegate to instantiate objects.
+    /// </summary>
+    public class DelegateTypeActivator : ITypeActivator
+    {
+        private readonly Func<Type, object> _func;
+
+        /// <summary>
+        /// Initializes an instance of <see cref="DelegateTypeActivator"/>.
+        /// </summary>
+        public DelegateTypeActivator(Func<Type, object> func) => _func = func;
+
+        /// <inheritdoc />
+        public object CreateInstance(Type type) =>
+            _func(type) ?? throw CliFxException.DelegateActivatorReceivedNull(type);
+    }
+}

CliFx/Domain/ApplicationSchema.cs

@@ -0,0 +1,252 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using CliFx.Exceptions;
+
+namespace CliFx.Domain
+{
+    internal partial class ApplicationSchema
+    {
+        public IReadOnlyList<CommandSchema> Commands { get; }
+
+        public ApplicationSchema(IReadOnlyList<CommandSchema> commands)
+        {
+            Commands = commands;
+        }
+
+        public CommandSchema? TryFindParentCommand(string? childCommandName)
+        {
+            // Default command has no parent
+            if (string.IsNullOrWhiteSpace(childCommandName))
+                return null;
+
+            // Try to find the parent command by repeatedly biting off chunks of its name
+            var route = childCommandName.Split(' ');
+            for (var i = route.Length - 1; i >= 1; i--)
+            {
+                var potentialParentCommandName = string.Join(" ", route.Take(i));
+                var matchingParentCommand = Commands.FirstOrDefault(c => c.MatchesName(potentialParentCommandName));
+
+                if (matchingParentCommand != null)
+                    return matchingParentCommand;
+            }
+
+            // If there's no parent - fall back to default command
+            return Commands.FirstOrDefault(c => c.IsDefault);
+        }
+
+        public IReadOnlyList<CommandSchema> GetChildCommands(string? parentCommandName) =>
+            !string.IsNullOrWhiteSpace(parentCommandName) || Commands.Any(c => c.IsDefault)
+                ? Commands.Where(c => TryFindParentCommand(c.Name)?.MatchesName(parentCommandName) == true).ToArray()
+                : Commands.Where(c => !string.IsNullOrWhiteSpace(c.Name) && TryFindParentCommand(c.Name) == null).ToArray();
+
+        // TODO: this out parameter is not a really nice design
+        public CommandSchema? TryFindCommand(CommandLineInput commandLineInput, out int argumentOffset)
+        {
+            // Try to find the command that contains the most of the input arguments in its name
+            for (var i = commandLineInput.UnboundArguments.Count; i >= 0; i--)
+            {
+                var potentialCommandName = string.Join(" ", commandLineInput.UnboundArguments.Take(i));
+                var matchingCommand = Commands.FirstOrDefault(c => c.MatchesName(potentialCommandName));
+
+                if (matchingCommand != null)
+                {
+                    argumentOffset = i;
+                    return matchingCommand;
+                }
+            }
+
+            argumentOffset = 0;
+            return Commands.FirstOrDefault(c => c.IsDefault);
+        }
+
+        public CommandSchema? TryFindCommand(CommandLineInput commandLineInput) =>
+            TryFindCommand(commandLineInput, out _);
+
+        public ICommand InitializeEntryPoint(
+            CommandLineInput commandLineInput,
+            IReadOnlyDictionary<string, string> environmentVariables,
+            ITypeActivator activator)
+        {
+            var command = TryFindCommand(commandLineInput, out var argumentOffset) ??
+                          throw CliFxException.CannotFindMatchingCommand(commandLineInput);
+
+            var parameterInputs = argumentOffset == 0
+                ? commandLineInput.UnboundArguments.ToArray()
+                : commandLineInput.UnboundArguments.Skip(argumentOffset).ToArray();
+
+            return command.CreateInstance(parameterInputs, commandLineInput.Options, environmentVariables, activator);
+        }
+
+        public ICommand InitializeEntryPoint(
+            CommandLineInput commandLineInput,
+            IReadOnlyDictionary<string, string> environmentVariables) =>
+            InitializeEntryPoint(commandLineInput, environmentVariables, new DefaultTypeActivator());
+
+        public ICommand InitializeEntryPoint(CommandLineInput commandLineInput) =>
+            InitializeEntryPoint(commandLineInput, new Dictionary<string, string>());
+
+        public override string ToString() => string.Join(Environment.NewLine, Commands);
+    }
+
+    internal partial class ApplicationSchema
+    {
+        private static void ValidateParameters(CommandSchema command)
+        {
+            var duplicateOrderGroup = command.Parameters
+                .GroupBy(a => a.Order)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateOrderGroup != null)
+            {
+                throw CliFxException.CommandParametersDuplicateOrder(
+                    command,
+                    duplicateOrderGroup.Key,
+                    duplicateOrderGroup.ToArray());
+            }
+
+            var duplicateNameGroup = command.Parameters
+                .Where(a => !string.IsNullOrWhiteSpace(a.Name))
+                .GroupBy(a => a.Name!, StringComparer.OrdinalIgnoreCase)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateNameGroup != null)
+            {
+                throw CliFxException.CommandParametersDuplicateName(
+                    command,
+                    duplicateNameGroup.Key,
+                    duplicateNameGroup.ToArray());
+            }
+
+            var nonScalarParameters = command.Parameters
+                .Where(p => !p.IsScalar)
+                .ToArray();
+
+            if (nonScalarParameters.Length > 1)
+            {
+                throw CliFxException.CommandParametersTooManyNonScalar(
+                    command,
+                    nonScalarParameters);
+            }
+
+            var nonLastNonScalarParameter = command.Parameters
+                .OrderByDescending(a => a.Order)
+                .Skip(1)
+                .LastOrDefault(p => !p.IsScalar);
+
+            if (nonLastNonScalarParameter != null)
+            {
+                throw CliFxException.CommandParametersNonLastNonScalar(
+                    command,
+                    nonLastNonScalarParameter);
+            }
+        }
+
+        private static void ValidateOptions(CommandSchema command)
+        {
+            var noNameGroup = command.Options
+                .Where(o => o.ShortName == null && string.IsNullOrWhiteSpace(o.Name))
+                .ToArray();
+
+            if (noNameGroup.Any())
+            {
+                throw CliFxException.CommandOptionsNoName(
+                    command,
+                    noNameGroup.ToArray());
+            }
+
+            var invalidLengthNameGroup = command.Options
+                .Where(o => !string.IsNullOrWhiteSpace(o.Name))
+                .Where(o => o.Name!.Length <= 1)
+                .ToArray();
+
+            if (invalidLengthNameGroup.Any())
+            {
+                throw CliFxException.CommandOptionsInvalidLengthName(
+                    command,
+                    invalidLengthNameGroup);
+            }
+
+            var duplicateNameGroup = command.Options
+                .Where(o => !string.IsNullOrWhiteSpace(o.Name))
+                .GroupBy(o => o.Name!, StringComparer.OrdinalIgnoreCase)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateNameGroup != null)
+            {
+                throw CliFxException.CommandOptionsDuplicateName(
+                    command,
+                    duplicateNameGroup.Key,
+                    duplicateNameGroup.ToArray());
+            }
+
+            var duplicateShortNameGroup = command.Options
+                .Where(o => o.ShortName != null)
+                .GroupBy(o => o.ShortName!.Value)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateShortNameGroup != null)
+            {
+                throw CliFxException.CommandOptionsDuplicateShortName(
+                    command,
+                    duplicateShortNameGroup.Key,
+                    duplicateShortNameGroup.ToArray());
+            }
+
+            var duplicateEnvironmentVariableNameGroup = command.Options
+                .Where(o => !string.IsNullOrWhiteSpace(o.EnvironmentVariableName))
+                .GroupBy(o => o.EnvironmentVariableName!, StringComparer.OrdinalIgnoreCase)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateEnvironmentVariableNameGroup != null)
+            {
+                throw CliFxException.CommandOptionsDuplicateEnvironmentVariableName(
+                    command,
+                    duplicateEnvironmentVariableNameGroup.Key,
+                    duplicateEnvironmentVariableNameGroup.ToArray());
+            }
+        }
+
+        private static void ValidateCommands(IReadOnlyList<CommandSchema> commands)
+        {
+            if (!commands.Any())
+            {
+                throw CliFxException.CommandsNotRegistered();
+            }
+
+            var duplicateNameGroup = commands
+                .GroupBy(c => c.Name, StringComparer.OrdinalIgnoreCase)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateNameGroup != null)
+            {
+                if (!string.IsNullOrWhiteSpace(duplicateNameGroup.Key))
+                    throw CliFxException.CommandsDuplicateName(
+                        duplicateNameGroup.Key,
+                        duplicateNameGroup.ToArray());
+
+                throw CliFxException.CommandsTooManyDefaults(duplicateNameGroup.ToArray());
+            }
+        }
+
+        public static ApplicationSchema Resolve(IReadOnlyList<Type> commandTypes)
+        {
+            var commands = new List<CommandSchema>();
+
+            foreach (var commandType in commandTypes)
+            {
+                var command = CommandSchema.TryResolve(commandType) ??
+                              throw CliFxException.InvalidCommandType(commandType);
+
+                ValidateParameters(command);
+                ValidateOptions(command);
+
+                commands.Add(command);
+            }
+
+            ValidateCommands(commands);
+
+            return new ApplicationSchema(commands);
+        }
+    }
+}