Update version

Closes #19

.github/workflows/CD.yml

@@ -0,0 +1,27 @@
+name: CD
+
+on:
+  push:
+    tags:
+    - '*'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v1
+
+    - name: Install .NET Core
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.0.100
+
+    - 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}}
+        dotnet nuget push CliFx/bin/Release/*.snupkg -s nuget.org -k ${{secrets.NUGET_TOKEN}}

.github/workflows/CI.yml

@@ -0,0 +1,22 @@
+name: CI
+
+on: [push, pull_request]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v1
+
+    - name: Install .NET Core
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: 3.0.100
+
+    - name: Build & test
+      run: dotnet test --configuration Release
+
+    - name: Coverage
+      run: curl -s https://codecov.io/bash | bash -s -- -f CliFx.Tests/bin/Release/Coverage.xml -t ${{secrets.CODECOV_TOKEN}} -Z

.gitignore

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

CliFx.Benchmarks/Benchmark.cs

@@ -8,7 +8,7 @@ namespace CliFx.Benchmarks
     [RankColumn]
     public class Benchmark
     {
-        private static readonly string[] Arguments = { "--str", "hello world", "-i", "13", "-b" };
+        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);
@@ -19,16 +19,17 @@ namespace CliFx.Benchmarks
         [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")]
+        [Benchmark(Description = "CommandLineParser")]
         public void ExecuteWithCommandLineParser()
         {
-            var parsed = CommandLine.Parser.Default.ParseArguments(Arguments, typeof(CommandLineParserCommand));
+            var parsed = new CommandLine.Parser().ParseArguments(Arguments, typeof(CommandLineParserCommand));
             CommandLine.ParserResultExtensions.WithParsed<CommandLineParserCommand>(parsed, 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();
     }
 }

CliFx.Benchmarks/CliFx.Benchmarks.csproj

@@ -2,12 +2,13 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
-    <LangVersion>latest</LangVersion>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
     <PackageReference Include="BenchmarkDotNet" Version="0.11.5" />
+    <PackageReference Include="clipr" Version="1.6.1" />
     <PackageReference Include="CommandLineParser" Version="2.6.0" />
     <PackageReference Include="McMaster.Extensions.CommandLineUtils" Version="2.3.4" />
     <PackageReference Include="PowerArgs" Version="3.6.0" />

CliFx.Benchmarks/Commands/CliFxCommand.cs

@@ -8,7 +8,7 @@ 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; }

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/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.Demo/CliFx.Demo.csproj

@@ -2,8 +2,8 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>netcoreapp2.2</TargetFramework>
-    <LangVersion>latest</LangVersion>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>

CliFx.Demo/Commands/BookAddCommand.cs

@@ -24,7 +24,7 @@ namespace CliFx.Demo.Commands
         public DateTimeOffset Published { get; set; }
 
         [CommandOption("isbn", 'n', Description = "Book ISBN.")]
-        public Isbn Isbn { get; set; }
+        public Isbn? Isbn { get; set; }
 
         public BookAddCommand(LibraryService libraryService)
         {

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 ConfigureServices()
         {
             // We use Microsoft.Extensions.DependencyInjection for injecting dependencies in commands
             var services = new ServiceCollection();
@@ -21,7 +22,12 @@ namespace CliFx.Demo
             services.AddTransient<BookRemoveCommand>();
             services.AddTransient<BookListCommand>();
 
-            var serviceProvider = services.BuildServiceProvider();
+            return services.BuildServiceProvider();
+        }
+
+        public static Task<int> Main(string[] args)
+        {
+            var serviceProvider = ConfigureServices();
 
             return new CliApplicationBuilder()
                 .AddCommandsFromThisAssembly()

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

@@ -1,14 +0,0 @@
-<Project Sdk="Microsoft.NET.Sdk">
-
-  <PropertyGroup>
-    <OutputType>Exe</OutputType>
-    <TargetFramework>net46</TargetFramework>
-    <Version>1.2.3.4</Version>
-    <LangVersion>latest</LangVersion>
-  </PropertyGroup>
-
-  <ItemGroup>
-    <ProjectReference Include="..\CliFx\CliFx.csproj" />
-  </ItemGroup>
-
-</Project>

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/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/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 +0,0 @@
-using System.Globalization;
-using System.Threading.Tasks;
-
-namespace CliFx.Tests.Dummy
-{
-    public static class Program
-    {
-        public static Task<int> Main(string[] args)
-        {
-            // 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()
-                .AddCommandsFromThisAssembly()
-                .UseDescription("Dummy program used for E2E tests.")
-                .Build()
-                .RunAsync(args);
-        }
-    }
-}

CliFx.Tests/CliApplicationBuilderTests.cs

@@ -0,0 +1,51 @@
+using NUnit.Framework;
+using System;
+using System.IO;
+using CliFx.Services;
+using CliFx.Tests.Stubs;
+using CliFx.Tests.TestCommands;
+
+namespace CliFx.Tests
+{
+    [TestFixture]
+    public class CliApplicationBuilderTests
+    {
+        // Make sure all builder methods work
+        [Test]
+        public void All_Smoke_Test()
+        {
+            // Arrange
+            var builder = new CliApplicationBuilder();
+
+            // Act
+            builder
+                .AddCommand(typeof(HelloWorldDefaultCommand))
+                .AddCommandsFrom(typeof(HelloWorldDefaultCommand).Assembly)
+                .AddCommands(new[] {typeof(HelloWorldDefaultCommand)})
+                .AddCommandsFrom(new[] {typeof(HelloWorldDefaultCommand).Assembly})
+                .AddCommandsFromThisAssembly()
+                .AllowDebugMode()
+                .AllowPreviewMode()
+                .UseTitle("test")
+                .UseExecutableName("test")
+                .UseVersionText("test")
+                .UseDescription("test")
+                .UseConsole(new VirtualConsole(TextWriter.Null))
+                .UseCommandFactory(schema => (ICommand) Activator.CreateInstance(schema.Type!)!)
+                .UseCommandOptionInputConverter(new CommandOptionInputConverter())
+                .UseEnvironmentVariablesProvider(new EnvironmentVariablesProviderStub())
+                .Build();
+        }
+
+        // Make sure builder can produce an application with no parameters specified
+        [Test]
+        public void Build_Test()
+        {
+            // Arrange
+            var builder = new CliApplicationBuilder();
+
+            // Act
+            builder.Build();
+        }
+    }
+}

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

@@ -1,76 +1,123 @@
-using System;
+using FluentAssertions;
+using NUnit.Framework;
+using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Threading;
 using System.Threading.Tasks;
 using CliFx.Services;
-using FluentAssertions;
-using NUnit.Framework;
+using CliFx.Tests.Stubs;
+using CliFx.Tests.TestCommands;
 
 namespace CliFx.Tests
 {
     [TestFixture]
-    public partial class CliApplicationTests
+    public class CliApplicationTests
     {
+        private const string TestVersionText = "v1.0";
+
         private static IEnumerable<TestCaseData> GetTestCases_RunAsync()
         {
             yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
+                new[] {typeof(HelloWorldDefaultCommand)},
                 new string[0],
-                "DefaultCommand executed."
+                "Hello world."
             );
 
             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]
+                new[] {typeof(ConcatCommand)},
+                new[] {"concat", "-i", "foo", "-i", "bar", "-s", " "},
+                "foo bar"
             );
 
             yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new[] {"-h"}
+                new[] {typeof(ConcatCommand)},
+                new[] {"concat", "-i", "one", "two", "three", "-s", ", "},
+                "one, two, three"
             );
 
             yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new[] {"--help"}
+                new[] {typeof(DivideCommand)},
+                new[] {"div", "-D", "24", "-d", "8"},
+                "3"
             );
 
             yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new[] {"--version"}
+                new[] {typeof(HelloWorldDefaultCommand)},
+                new[] {"--version"},
+                TestVersionText
             );
 
             yield return new TestCaseData(
-                new[] {typeof(NamedCommand)},
-                new string[0]
+                new[] {typeof(ConcatCommand)},
+                new[] {"--version"},
+                TestVersionText
             );
 
             yield return new TestCaseData(
-                new[] {typeof(NamedCommand)},
-                new[] {"cmd", "-h"}
+                new[] {typeof(HelloWorldDefaultCommand)},
+                new[] {"-h"},
+                null
             );
 
             yield return new TestCaseData(
-                new[] {typeof(FaultyCommand1)},
-                new[] {"faulty1", "-h"}
+                new[] {typeof(HelloWorldDefaultCommand)},
+                new[] {"--help"},
+                null
             );
 
             yield return new TestCaseData(
-                new[] {typeof(FaultyCommand2)},
-                new[] {"faulty2", "-h"}
+                new[] {typeof(ConcatCommand)},
+                new string[0],
+                null
             );
 
             yield return new TestCaseData(
-                new[] {typeof(FaultyCommand3)},
-                new[] {"faulty3", "-h"}
+                new[] {typeof(ConcatCommand)},
+                new[] {"-h"},
+                null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(ConcatCommand)},
+                new[] {"--help"},
+                null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(ConcatCommand)},
+                new[] {"concat", "-h"},
+                null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(ExceptionCommand)},
+                new[] {"exc", "-h"},
+                null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(CommandExceptionCommand)},
+                new[] {"exc", "-h"},
+                null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(ConcatCommand)},
+                new[] {"[preview]"},
+                null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(ExceptionCommand)},
+                new[] {"exc", "[preview]"},
+                null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(ConcatCommand)},
+                new[] {"concat", "[preview]", "-o", "value"},
+                null
             );
         }
 
@@ -78,97 +125,136 @@ namespace CliFx.Tests
         {
             yield return new TestCaseData(
                 new Type[0],
-                new string[0]
+                new string[0],
+                null, null
             );
 
             yield return new TestCaseData(
-                new[] {typeof(DefaultCommand)},
-                new[] {"non-existing"}
+                new[] {typeof(ConcatCommand)},
+                new[] {"non-existing"},
+                null, null
             );
 
             yield return new TestCaseData(
-                new[] {typeof(FaultyCommand1)},
-                new[] {"faulty1"}
+                new[] {typeof(ExceptionCommand)},
+                new[] {"exc"},
+                null, null
             );
 
             yield return new TestCaseData(
-                new[] {typeof(FaultyCommand2)},
-                new[] {"faulty2"}
+                new[] {typeof(CommandExceptionCommand)},
+                new[] {"exc"},
+                null, null
             );
 
             yield return new TestCaseData(
-                new[] {typeof(FaultyCommand3)},
-                new[] {"faulty3"}
+                new[] {typeof(CommandExceptionCommand)},
+                new[] {"exc"},
+                null, null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(CommandExceptionCommand)},
+                new[] {"exc", "-m", "foo bar"},
+                "foo bar", null
+            );
+
+            yield return new TestCaseData(
+                new[] {typeof(CommandExceptionCommand)},
+                new[] {"exc", "-m", "foo bar", "-c", "666"},
+                "foo bar", 666
             );
         }
 
         [Test]
         [TestCaseSource(nameof(GetTestCases_RunAsync))]
-        public async Task RunAsync_Test(IReadOnlyList<Type> commandTypes, IReadOnlyList<string> commandLineArguments, string expectedStdOut)
+        public async Task RunAsync_Test(IReadOnlyList<Type> commandTypes, IReadOnlyList<string> commandLineArguments,
+            string? expectedStdOut = null)
         {
             // Arrange
-            using (var stdout = new StringWriter())
-            {
-                var console = new VirtualConsole(stdout);
+            await using var stdoutStream = new StringWriter();
 
-                var application = new CliApplicationBuilder()
-                    .AddCommands(commandTypes)
-                    .UseConsole(console)
-                    .Build();
+            var console = new VirtualConsole(stdoutStream);
+            var environmentVariablesProvider = new EnvironmentVariablesProviderStub();
 
-                // Act
-                var exitCode = await application.RunAsync(commandLineArguments);
+            var application = new CliApplicationBuilder()
+                .AddCommands(commandTypes)
+                .UseVersionText(TestVersionText)
+                .UseConsole(console)
+                .UseEnvironmentVariablesProvider(environmentVariablesProvider)
+                .Build();
 
-                // Assert
-                exitCode.Should().Be(0);
-                stdout.ToString().Trim().Should().Be(expectedStdOut);
-            }
-        }
+            // Act
+            var exitCode = await application.RunAsync(commandLineArguments);
+            var stdOut = stdoutStream.ToString().Trim();
 
-        [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);
+            // Assert
+            exitCode.Should().Be(0);
 
-                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();
-            }
+            if (expectedStdOut != null)
+                stdOut.Should().Be(expectedStdOut);
+            else
+                stdOut.Should().NotBeNullOrWhiteSpace();
         }
 
         [Test]
         [TestCaseSource(nameof(GetTestCases_RunAsync_Negative))]
-        public async Task RunAsync_Negative_Test(IReadOnlyList<Type> commandTypes, IReadOnlyList<string> commandLineArguments)
+        public async Task RunAsync_Negative_Test(IReadOnlyList<Type> commandTypes, IReadOnlyList<string> commandLineArguments,
+            string? expectedStdErr = null, int? expectedExitCode = null)
         {
             // Arrange
-            using (var stderr = new StringWriter())
-            {
-                var console = new VirtualConsole(TextWriter.Null, stderr);
+            await using var stderrStream = new StringWriter();
 
-                var application = new CliApplicationBuilder()
-                    .AddCommands(commandTypes)
-                    .UseConsole(console)
-                    .Build();
+            var console = new VirtualConsole(TextWriter.Null, stderrStream);
+            var environmentVariablesProvider = new EnvironmentVariablesProviderStub();
 
-                // Act
-                var exitCode = await application.RunAsync(commandLineArguments);
+            var application = new CliApplicationBuilder()
+                .AddCommands(commandTypes)
+                .UseVersionText(TestVersionText)
+                .UseEnvironmentVariablesProvider(environmentVariablesProvider)
+                .UseConsole(console)
+                .Build();
 
-                // Assert
+            // Act
+            var exitCode = await application.RunAsync(commandLineArguments);
+            var stderr = stderrStream.ToString().Trim();
+
+            // Assert
+            if (expectedExitCode != null)
+                exitCode.Should().Be(expectedExitCode);
+            else
                 exitCode.Should().NotBe(0);
-                stderr.ToString().Should().NotBeNullOrWhiteSpace();
-            }
+
+            if (expectedStdErr != null)
+                stderr.Should().Be(expectedStdErr);
+            else
+                stderr.Should().NotBeNullOrWhiteSpace();
+        }
+
+        [Test]
+        public async Task RunAsync_Cancellation_Test()
+        {
+            // Arrange
+            using var cancellationTokenSource = new CancellationTokenSource();
+            await using var stdoutStream = new StringWriter();
+
+            var console = new VirtualConsole(stdoutStream, cancellationTokenSource.Token);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(CancellableCommand))
+                .UseConsole(console)
+                .Build();
+            var args = new[] {"cancel"};
+
+            // Act
+            var runTask = application.RunAsync(args);
+            cancellationTokenSource.Cancel();
+            var exitCode = await runTask.ConfigureAwait(false);
+            var stdOut = stdoutStream.ToString().Trim();
+
+            // Assert
+            exitCode.Should().Be(-2146233029);
+            stdOut.Should().Be("Printed");
         }
     }
 }

CliFx.Tests/CliFx.Tests.csproj

@@ -1,29 +1,24 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>net46</TargetFramework>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
     <IsPackable>false</IsPackable>
     <IsTestProject>true</IsTestProject>
     <CollectCoverage>true</CollectCoverage>
     <CoverletOutputFormat>opencover</CoverletOutputFormat>
     <CoverletOutput>bin/$(Configuration)/Coverage.xml</CoverletOutput>
-    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="FluentAssertions" Version="5.8.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
+    <PackageReference Include="FluentAssertions" Version="5.9.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.4.0" />
     <PackageReference Include="NUnit" Version="3.12.0" />
-    <PackageReference Include="NUnit3TestAdapter" Version="3.14.0" />
-    <PackageReference Include="CliWrap" Version="2.3.1" />
-    <PackageReference Include="coverlet.msbuild" Version="2.6.3">
-      <PrivateAssets>all</PrivateAssets>
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
-    </PackageReference>
+    <PackageReference Include="NUnit3TestAdapter" Version="3.15.1" />
+    <PackageReference Include="coverlet.msbuild" Version="2.7.0" PrivateAssets="all" />
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\CliFx.Tests.Dummy\CliFx.Tests.Dummy.csproj" />
     <ProjectReference Include="..\CliFx\CliFx.csproj" />
   </ItemGroup>
 

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/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/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/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/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/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/Services/CommandFactoryTests.cs

@@ -3,20 +3,21 @@ using System.Collections.Generic;
 using System.Linq;
 using CliFx.Models;
 using CliFx.Services;
+using CliFx.Tests.TestCommands;
 using FluentAssertions;
 using NUnit.Framework;
 
-namespace CliFx.Tests
+namespace CliFx.Tests.Services
 {
     [TestFixture]
-    public partial class CommandFactoryTests
+    public 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)));
+            yield return new TestCaseData(GetCommandSchema(typeof(HelloWorldDefaultCommand)));
         }
 
         [Test]

CliFx.Tests/Services/CommandInitializerTests.cs

@@ -0,0 +1,174 @@
+using FluentAssertions;
+using NUnit.Framework;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using CliFx.Exceptions;
+using CliFx.Models;
+using CliFx.Services;
+using CliFx.Tests.TestCommands;
+using CliFx.Tests.Stubs;
+using System.IO;
+
+namespace CliFx.Tests.Services
+{
+    [TestFixture]
+    public 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 DivideCommand(),
+                GetCommandSchema(typeof(DivideCommand)),
+                new CommandInput("div", new[]
+                {
+                    new CommandOptionInput("dividend", "13"),
+                    new CommandOptionInput("divisor", "8")
+                }),
+                new DivideCommand { Dividend = 13, Divisor = 8 }
+            );
+
+            yield return new TestCaseData(
+                new DivideCommand(),
+                GetCommandSchema(typeof(DivideCommand)),
+                new CommandInput("div", new[]
+                {
+                    new CommandOptionInput("dividend", "13"),
+                    new CommandOptionInput("d", "8")
+                }),
+                new DivideCommand { Dividend = 13, Divisor = 8 }
+            );
+
+            yield return new TestCaseData(
+                new DivideCommand(),
+                GetCommandSchema(typeof(DivideCommand)),
+                new CommandInput("div", new[]
+                {
+                    new CommandOptionInput("D", "13"),
+                    new CommandOptionInput("d", "8")
+                }),
+                new DivideCommand { Dividend = 13, Divisor = 8 }
+            );
+
+            yield return new TestCaseData(
+                new ConcatCommand(),
+                GetCommandSchema(typeof(ConcatCommand)),
+                new CommandInput("concat", new[]
+                {
+                    new CommandOptionInput("i", new[] {"foo", " ", "bar"})
+                }),
+                new ConcatCommand { Inputs = new[] { "foo", " ", "bar" } }
+            );
+
+            yield return new TestCaseData(
+                new ConcatCommand(),
+                GetCommandSchema(typeof(ConcatCommand)),
+                new CommandInput("concat", new[]
+                {
+                    new CommandOptionInput("i", new[] {"foo", "bar"}),
+                    new CommandOptionInput("s", " ")
+                }),
+                new ConcatCommand { Inputs = new[] { "foo", "bar" }, Separator = " " }
+            );
+
+            //Will read a value from environment variables because none is supplied via CommandInput
+            yield return new TestCaseData(
+                new EnvironmentVariableCommand(),
+                GetCommandSchema(typeof(EnvironmentVariableCommand)),
+                new CommandInput(null, new CommandOptionInput[0], EnvironmentVariablesProviderStub.EnvironmentVariables),
+                new EnvironmentVariableCommand { Option = "A" }
+            );
+
+            //Will read multiple values from environment variables because none is supplied via CommandInput
+            yield return new TestCaseData(
+                new EnvironmentVariableWithMultipleValuesCommand(),
+                GetCommandSchema(typeof(EnvironmentVariableWithMultipleValuesCommand)),
+                new CommandInput(null, new CommandOptionInput[0], EnvironmentVariablesProviderStub.EnvironmentVariables),
+                new EnvironmentVariableWithMultipleValuesCommand { Option = new[] { "A", "B", "C" } }
+            );
+
+            //Will not read a value from environment variables because one is supplied via CommandInput
+            yield return new TestCaseData(
+                new EnvironmentVariableCommand(),
+                GetCommandSchema(typeof(EnvironmentVariableCommand)),
+                new CommandInput(null, new[]
+                {
+                    new CommandOptionInput("opt", new[] { "X" })
+                },
+                EnvironmentVariablesProviderStub.EnvironmentVariables),
+                new EnvironmentVariableCommand { Option = "X" }
+            );
+
+            //Will not split environment variable values because underlying property is not a collection
+            yield return new TestCaseData(
+                new EnvironmentVariableWithoutCollectionPropertyCommand(),
+                GetCommandSchema(typeof(EnvironmentVariableWithoutCollectionPropertyCommand)),
+                new CommandInput(null, new CommandOptionInput[0], EnvironmentVariablesProviderStub.EnvironmentVariables),
+                new EnvironmentVariableWithoutCollectionPropertyCommand { Option = $"A{Path.PathSeparator}B{Path.PathSeparator}C{Path.PathSeparator}" }
+            );
+        }
+
+        private static IEnumerable<TestCaseData> GetTestCases_InitializeCommand_Negative()
+        {
+            yield return new TestCaseData(
+                new DivideCommand(),
+                GetCommandSchema(typeof(DivideCommand)),
+                new CommandInput("div")
+            );
+
+            yield return new TestCaseData(
+                new DivideCommand(),
+                GetCommandSchema(typeof(DivideCommand)),
+                new CommandInput("div", new[]
+                {
+                    new CommandOptionInput("D", "13")
+                })
+            );
+
+            yield return new TestCaseData(
+                new ConcatCommand(),
+                GetCommandSchema(typeof(ConcatCommand)),
+                new CommandInput("concat")
+            );
+
+            yield return new TestCaseData(
+                new ConcatCommand(),
+                GetCommandSchema(typeof(ConcatCommand)),
+                new CommandInput("concat", new[]
+                {
+                    new CommandOptionInput("s", "_")
+                })
+            );
+        }
+
+        [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<CliFxException>();
+        }
+    }
+}

CliFx.Tests/Services/CommandInputParserTests.cs

@@ -0,0 +1,255 @@
+using FluentAssertions;
+using NUnit.Framework;
+using System.Collections.Generic;
+using CliFx.Models;
+using CliFx.Services;
+using CliFx.Tests.Stubs;
+
+namespace CliFx.Tests.Services
+{
+    [TestFixture]
+    public class CommandInputParserTests
+    {
+        private static IEnumerable<TestCaseData> GetTestCases_ParseCommandInput()
+        {
+            yield return new TestCaseData(new string[0], CommandInput.Empty, new EmptyEnvironmentVariablesProviderStub());
+
+            yield return new TestCaseData(
+                new[] { "--option", "value" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("option", "value")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "--option1", "value1", "--option2", "value2" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("option1", "value1"),
+                    new CommandOptionInput("option2", "value2")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "--option", "value1", "value2" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("option", new[] {"value1", "value2"})
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "--option", "value1", "--option", "value2" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("option", new[] {"value1", "value2"})
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "-a", "value" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("a", "value")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "-a", "value1", "-b", "value2" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("a", "value1"),
+                    new CommandOptionInput("b", "value2")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "-a", "value1", "value2" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("a", new[] {"value1", "value2"})
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "-a", "value1", "-a", "value2" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("a", new[] {"value1", "value2"})
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "--option1", "value1", "-b", "value2" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("option1", "value1"),
+                    new CommandOptionInput("b", "value2")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "--switch" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("switch")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "--switch1", "--switch2" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("switch1"),
+                    new CommandOptionInput("switch2")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "-s" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("s")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "-a", "-b" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("a"),
+                    new CommandOptionInput("b")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "-ab" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("a"),
+                    new CommandOptionInput("b")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "-ab", "value" },
+                new CommandInput(new[]
+                {
+                    new CommandOptionInput("a"),
+                    new CommandOptionInput("b", "value")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "command" },
+                new CommandInput("command"),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "command", "--option", "value" },
+                new CommandInput("command", new[]
+                {
+                    new CommandOptionInput("option", "value")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "long", "command", "name" },
+                new CommandInput("long command name"),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "long", "command", "name", "--option", "value" },
+                new CommandInput("long command name", new[]
+                {
+                    new CommandOptionInput("option", "value")
+                }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "[debug]" },
+                new CommandInput(null,
+                    new[] { "debug" },
+                    new CommandOptionInput[0]),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "[debug]", "[preview]" },
+                new CommandInput(null,
+                    new[] { "debug", "preview" },
+                    new CommandOptionInput[0]),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "[debug]", "[preview]", "-o", "value" },
+                new CommandInput(null,
+                    new[] { "debug", "preview" },
+                    new[]
+                    {
+                        new CommandOptionInput("o", "value")
+                    }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "command", "[debug]", "[preview]", "-o", "value" },
+                new CommandInput("command",
+                    new[] { "debug", "preview" },
+                    new[]
+                    {
+                        new CommandOptionInput("o", "value")
+                    }),
+                new EmptyEnvironmentVariablesProviderStub()
+            );
+
+            yield return new TestCaseData(
+                new[] { "command", "[debug]", "[preview]", "-o", "value" },
+                new CommandInput("command",
+                    new[] { "debug", "preview" },
+                    new[]
+                    {
+                        new CommandOptionInput("o", "value")
+                    },
+                    EnvironmentVariablesProviderStub.EnvironmentVariables),
+                new EnvironmentVariablesProviderStub()
+            );
+        }
+
+        [Test]
+        [TestCaseSource(nameof(GetTestCases_ParseCommandInput))]
+        public void ParseCommandInput_Test(IReadOnlyList<string> commandLineArguments,
+            CommandInput expectedCommandInput, IEnvironmentVariablesProvider environmentVariablesProvider)
+        {
+            // Arrange
+            var parser = new CommandInputParser(environmentVariablesProvider);
+
+            // Act
+            var commandInput = parser.ParseCommandInput(commandLineArguments);
+
+            // Assert
+            commandInput.Should().BeEquivalentTo(expectedCommandInput);
+        }
+    }
+}

CliFx.Tests/Services/CommandOptionInputConverterTests.cs

@@ -5,13 +5,14 @@ using System.Globalization;
 using CliFx.Exceptions;
 using CliFx.Models;
 using CliFx.Services;
+using CliFx.Tests.TestCustomTypes;
 using FluentAssertions;
 using NUnit.Framework;
 
-namespace CliFx.Tests
+namespace CliFx.Tests.Services
 {
     [TestFixture]
-    public partial class CommandOptionInputConverterTests
+    public class CommandOptionInputConverterTests
     {
         private static IEnumerable<TestCaseData> GetTestCases_ConvertOptionInput()
         {
@@ -213,6 +214,12 @@ namespace CliFx.Tests
                 new[] {47, 69}
             );
 
+            yield return new TestCaseData(
+                new CommandOptionInput("option", new[] {"47"}),
+                typeof(int[]),
+                new[] {47}
+            );
+
             yield return new TestCaseData(
                 new CommandOptionInput("option", new[] {"value1", "value3"}),
                 typeof(TestEnum[]),
@@ -269,15 +276,26 @@ namespace CliFx.Tests
                 typeof(int)
             );
 
+            yield return new TestCaseData(
+                new CommandOptionInput("option", new[] {"123", "456"}),
+                typeof(int)
+            );
+
+            yield return new TestCaseData(
+                new CommandOptionInput("option"),
+                typeof(int)
+            );
+
             yield return new TestCaseData(
                 new CommandOptionInput("option", "123"),
-                typeof(NonStringParseable)
+                typeof(TestNonStringParseable)
             );
         }
 
         [Test]
         [TestCaseSource(nameof(GetTestCases_ConvertOptionInput))]
-        public void ConvertOptionInput_Test(CommandOptionInput optionInput, Type targetType, object expectedConvertedValue)
+        public void ConvertOptionInput_Test(CommandOptionInput optionInput, Type targetType,
+            object expectedConvertedValue)
         {
             // Arrange
             var converter = new CommandOptionInputConverter();
@@ -299,7 +317,7 @@ namespace CliFx.Tests
 
             // Act & Assert
             converter.Invoking(c => c.ConvertOptionInput(optionInput, targetType))
-                .Should().ThrowExactly<InvalidCommandOptionInputException>();
+                .Should().ThrowExactly<CliFxException>();
         }
     }
 }

CliFx.Tests/Services/CommandSchemaResolverTests.cs

@@ -0,0 +1,116 @@
+using FluentAssertions;
+using NUnit.Framework;
+using System;
+using System.Collections.Generic;
+using CliFx.Exceptions;
+using CliFx.Models;
+using CliFx.Services;
+using CliFx.Tests.TestCommands;
+
+namespace CliFx.Tests.Services
+{
+    [TestFixture]
+    public class CommandSchemaResolverTests
+    {
+        private static IEnumerable<TestCaseData> GetTestCases_GetCommandSchemas()
+        {
+            yield return new TestCaseData(
+                new[] { typeof(DivideCommand), typeof(ConcatCommand), typeof(EnvironmentVariableCommand) },
+                new[]
+                {
+                    new CommandSchema(typeof(DivideCommand), "div", "Divide one number by another.",
+                        new[]
+                        {
+                            new CommandOptionSchema(typeof(DivideCommand).GetProperty(nameof(DivideCommand.Dividend)),
+                                "dividend", 'D', true, "The number to divide.", null),
+                            new CommandOptionSchema(typeof(DivideCommand).GetProperty(nameof(DivideCommand.Divisor)),
+                                "divisor", 'd', true, "The number to divide by.", null)
+                        }),
+                    new CommandSchema(typeof(ConcatCommand), "concat", "Concatenate strings.",
+                        new[]
+                        {
+                            new CommandOptionSchema(typeof(ConcatCommand).GetProperty(nameof(ConcatCommand.Inputs)),
+                                null, 'i', true, "Input strings.", null),
+                            new CommandOptionSchema(typeof(ConcatCommand).GetProperty(nameof(ConcatCommand.Separator)),
+                                null, 's', false, "String separator.", null)
+                        }),
+                    new CommandSchema(typeof(EnvironmentVariableCommand), null, "Reads option values from environment variables.",
+                        new[]
+                        {
+                            new CommandOptionSchema(typeof(EnvironmentVariableCommand).GetProperty(nameof(EnvironmentVariableCommand.Option)),
+                                "opt", null, false, null, "ENV_SINGLE_VALUE")
+                        }
+                    )
+                }
+            );
+
+            yield return new TestCaseData(
+                new[] { typeof(HelloWorldDefaultCommand) },
+                new[]
+                {
+                    new CommandSchema(typeof(HelloWorldDefaultCommand), null, null, new CommandOptionSchema[0])
+                }
+            );
+        }
+
+        private static IEnumerable<TestCaseData> GetTestCases_GetCommandSchemas_Negative()
+        {
+            yield return new TestCaseData(new object[]
+            {
+                new Type[0]
+            });
+
+            yield return new TestCaseData(new object[]
+            {
+                new[] {typeof(NonImplementedCommand)}
+            });
+
+            yield return new TestCaseData(new object[]
+            {
+                new[] {typeof(NonAnnotatedCommand)}
+            });
+
+            yield return new TestCaseData(new object[]
+            {
+                new[] {typeof(DuplicateOptionNamesCommand)}
+            });
+
+            yield return new TestCaseData(new object[]
+            {
+                new[] {typeof(DuplicateOptionShortNamesCommand)}
+            });
+
+            yield return new TestCaseData(new object[]
+            {
+                new[] {typeof(ExceptionCommand), typeof(CommandExceptionCommand)}
+            });
+        }
+
+        [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<CliFxException>();
+        }
+    }
+}

CliFx.Tests/Services/DelegateCommandFactoryTests.cs

@@ -3,13 +3,14 @@ using System.Collections.Generic;
 using System.Linq;
 using CliFx.Models;
 using CliFx.Services;
+using CliFx.Tests.TestCommands;
 using FluentAssertions;
 using NUnit.Framework;
 
-namespace CliFx.Tests
+namespace CliFx.Tests.Services
 {
     [TestFixture]
-    public partial class DelegateCommandFactoryTests
+    public class DelegateCommandFactoryTests
     {
         private static CommandSchema GetCommandSchema(Type commandType) =>
             new CommandSchemaResolver().GetCommandSchemas(new[] {commandType}).Single();
@@ -17,8 +18,8 @@ namespace CliFx.Tests
         private static IEnumerable<TestCaseData> GetTestCases_CreateCommand()
         {
             yield return new TestCaseData(
-                new Func<CommandSchema, ICommand>(schema => (ICommand) Activator.CreateInstance(schema.Type)),
-                GetCommandSchema(typeof(TestCommand))
+                new Func<CommandSchema, ICommand>(schema => (ICommand) Activator.CreateInstance(schema.Type!)!),
+                GetCommandSchema(typeof(HelloWorldDefaultCommand))
             );
         }
 

CliFx.Tests/Services/HelpTextRendererTests.cs

@@ -4,13 +4,14 @@ using System.IO;
 using System.Linq;
 using CliFx.Models;
 using CliFx.Services;
+using CliFx.Tests.TestCommands;
 using FluentAssertions;
 using NUnit.Framework;
 
-namespace CliFx.Tests
+namespace CliFx.Tests.Services
 {
     [TestFixture]
-    public partial class HelpTextRendererTests
+    public class HelpTextRendererTests
     {
         private static HelpTextSource CreateHelpTextSource(IReadOnlyList<Type> availableCommandTypes, Type targetCommandType)
         {
@@ -27,11 +28,13 @@ namespace CliFx.Tests
         {
             yield return new TestCaseData(
                 CreateHelpTextSource(
-                    new[] {typeof(DefaultCommand), typeof(NamedCommand), typeof(NamedSubCommand)},
-                    typeof(DefaultCommand)),
+                    new[] {typeof(HelpDefaultCommand), typeof(HelpNamedCommand), typeof(HelpSubCommand)},
+                    typeof(HelpDefaultCommand)),
 
                 new[]
                 {
+                    "Description",
+                    "HelpDefaultCommand description.",
                     "Usage",
                     "[command]", "[options]",
                     "Options",
@@ -40,20 +43,20 @@ namespace CliFx.Tests
                     "-h|--help", "Shows help text.",
                     "--version", "Shows version information.",
                     "Commands",
-                    "cmd", "NamedCommand description.",
+                    "cmd", "HelpNamedCommand 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[] {typeof(HelpDefaultCommand), typeof(HelpNamedCommand), typeof(HelpSubCommand)},
+                    typeof(HelpNamedCommand)),
 
                 new[]
                 {
                     "Description",
-                    "NamedCommand description.",
+                    "HelpNamedCommand description.",
                     "Usage",
                     "cmd", "[command]", "[options]",
                     "Options",
@@ -61,20 +64,20 @@ namespace CliFx.Tests
                     "-d|--option-d", "OptionD description.",
                     "-h|--help", "Shows help text.",
                     "Commands",
-                    "sub", "NamedSubCommand description.",
+                    "sub", "HelpSubCommand 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[] {typeof(HelpDefaultCommand), typeof(HelpNamedCommand), typeof(HelpSubCommand)},
+                    typeof(HelpSubCommand)),
 
                 new[]
                 {
                     "Description",
-                    "NamedSubCommand description.",
+                    "HelpSubCommand description.",
                     "Usage",
                     "cmd sub", "[options]",
                     "Options",
@@ -86,20 +89,20 @@ namespace CliFx.Tests
 
         [Test]
         [TestCaseSource(nameof(GetTestCases_RenderHelpText))]
-        public void RenderHelpText_Test(HelpTextSource source, IReadOnlyList<string> expectedSubstrings)
+        public void RenderHelpText_Test(HelpTextSource source,
+            IReadOnlyList<string> expectedSubstrings)
         {
             // Arrange
-            using (var stdout = new StringWriter())
-            {
-                var renderer = new HelpTextRenderer();
-                var console = new VirtualConsole(stdout);
+            using var stdout = new StringWriter();
 
-                // Act
-                renderer.RenderHelpText(console, source);
+            var console = new VirtualConsole(stdout);
+            var renderer = new HelpTextRenderer();
 
-                // Assert
-                stdout.ToString().Should().ContainAll(expectedSubstrings);
-            }
+            // Act
+            renderer.RenderHelpText(console, source);
+
+            // Assert
+            stdout.ToString().Should().ContainAll(expectedSubstrings);
         }
     }
 }

CliFx.Tests/Services/SystemConsoleTests.cs

@@ -0,0 +1,41 @@
+using System;
+using CliFx.Services;
+using FluentAssertions;
+using NUnit.Framework;
+
+namespace CliFx.Tests.Services
+{
+    [TestFixture]
+    public class SystemConsoleTests
+    {
+        [TearDown]
+        public void TearDown()
+        {
+            // Reset console color so it doesn't carry on into next tests
+            Console.ResetColor();
+        }
+        
+        // Make sure console correctly wraps around System.Console
+        [Test]
+        public void All_Smoke_Test()
+        {
+            // Arrange
+            var console = new SystemConsole();
+
+            // Act
+            console.ResetColor();
+            console.ForegroundColor = ConsoleColor.DarkMagenta;
+            console.BackgroundColor = ConsoleColor.DarkMagenta;
+
+            // Assert
+            console.Input.Should().BeSameAs(Console.In);
+            console.IsInputRedirected.Should().Be(Console.IsInputRedirected);
+            console.Output.Should().BeSameAs(Console.Out);
+            console.IsOutputRedirected.Should().Be(Console.IsOutputRedirected);
+            console.Error.Should().BeSameAs(Console.Error);
+            console.IsErrorRedirected.Should().Be(Console.IsErrorRedirected);
+            console.ForegroundColor.Should().Be(Console.ForegroundColor);
+            console.BackgroundColor.Should().Be(Console.BackgroundColor);
+        }
+    }
+}

CliFx.Tests/Services/VirtualConsoleTests.cs

@@ -0,0 +1,42 @@
+using System;
+using System.IO;
+using CliFx.Services;
+using FluentAssertions;
+using NUnit.Framework;
+
+namespace CliFx.Tests.Services
+{
+    [TestFixture]
+    public class VirtualConsoleTests
+    {
+        // Make sure console uses specified streams and doesn't leak to System.Console
+        [Test]
+        public void All_Smoke_Test()
+        {
+            // Arrange
+            using var stdin = new StringReader("hello world");
+            using var stdout = new StringWriter();
+            using var stderr = new StringWriter();
+
+            var console = new VirtualConsole(stdin, stdout, stderr);
+
+            // Act
+            console.ResetColor();
+            console.ForegroundColor = ConsoleColor.DarkMagenta;
+            console.BackgroundColor = ConsoleColor.DarkMagenta;
+
+            // Assert
+            console.Input.Should().BeSameAs(stdin);
+            console.Input.Should().NotBeSameAs(Console.In);
+            console.IsInputRedirected.Should().BeTrue();
+            console.Output.Should().BeSameAs(stdout);
+            console.Output.Should().NotBeSameAs(Console.Out);
+            console.IsOutputRedirected.Should().BeTrue();
+            console.Error.Should().BeSameAs(stderr);
+            console.Error.Should().NotBeSameAs(Console.Error);
+            console.IsErrorRedirected.Should().BeTrue();
+            console.ForegroundColor.Should().NotBe(Console.ForegroundColor);
+            console.BackgroundColor.Should().NotBe(Console.BackgroundColor);
+        }
+    }
+}

CliFx.Tests/Stubs/EmptyEnvironmentVariablesProviderStub.cs

@@ -0,0 +1,10 @@
+using System.Collections.Generic;
+using CliFx.Services;
+
+namespace CliFx.Tests.Stubs
+{
+    public class EmptyEnvironmentVariablesProviderStub : IEnvironmentVariablesProvider
+    {
+        public IReadOnlyDictionary<string, string> GetEnvironmentVariables() => new Dictionary<string, string>();
+    }
+}

CliFx.Tests/Stubs/EnvironmentVariablesProviderStub.cs

@@ -0,0 +1,18 @@
+using System.Collections.Generic;
+using System.IO;
+using CliFx.Services;
+
+namespace CliFx.Tests.Stubs
+{
+    public class EnvironmentVariablesProviderStub : IEnvironmentVariablesProvider
+    {
+        public static readonly Dictionary<string, string> EnvironmentVariables = new Dictionary<string, string>
+        {
+            ["ENV_SINGLE_VALUE"] = "A",
+            ["ENV_MULTIPLE_VALUES"] = $"A{Path.PathSeparator}B{Path.PathSeparator}C{Path.PathSeparator}",
+            ["ENV_ESCAPED_MULTIPLE_VALUES"] = $"\"A{Path.PathSeparator}B{Path.PathSeparator}C{Path.PathSeparator}\""
+        };
+
+        public IReadOnlyDictionary<string, string> GetEnvironmentVariables() => EnvironmentVariables;
+    }
+}

CliFx.Tests/TestCommands/CancellableCommand.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command("cancel")]
+    public class CancellableCommand : ICommand
+    {
+        public async Task ExecuteAsync(IConsole console)
+        {
+            await Task.Yield();
+
+            console.Output.WriteLine("Printed");
+
+            await Task.Delay(TimeSpan.FromSeconds(1), console.GetCancellationToken()).ConfigureAwait(false);
+
+            console.Output.WriteLine("Never printed");
+        }
+    }
+}

CliFx.Tests/TestCommands/CommandExceptionCommand.cs

@@ -0,0 +1,19 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Exceptions;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command("exc")]
+    public class CommandExceptionCommand : ICommand
+    {
+        [CommandOption("code", 'c')]
+        public int ExitCode { get; set; } = 1337;
+        
+        [CommandOption("msg", 'm')]
+        public string? Message { get; set; }
+        
+        public Task ExecuteAsync(IConsole console) => throw new CommandException(Message, ExitCode);
+    }
+}

CliFx.Tests/TestCommands/ConcatCommand.cs

@@ -0,0 +1,23 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command("concat", Description = "Concatenate strings.")]
+    public class ConcatCommand : ICommand
+    {
+        [CommandOption('i', IsRequired = true, Description = "Input strings.")]
+        public IReadOnlyList<string> Inputs { get; set; }
+
+        [CommandOption('s', Description = "String separator.")]
+        public string Separator { get; set; } = ""; 
+        
+        public Task ExecuteAsync(IConsole console)
+        {
+            console.Output.WriteLine(string.Join(Separator, Inputs));
+            return Task.CompletedTask;
+        }
+    }
+}

CliFx.Tests/TestCommands/DivideCommand.cs

@@ -0,0 +1,25 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command("div", Description = "Divide one number by another.")]
+    public class DivideCommand : ICommand
+    {
+        [CommandOption("dividend", 'D', IsRequired = true, Description = "The number to divide.")]
+        public double Dividend { get; set; }
+
+        [CommandOption("divisor", 'd', IsRequired = true, Description = "The number to divide by.")]
+        public double Divisor { get; set; }
+
+        // This property should be ignored by resolver
+        public bool NotAnOption { get; set; }
+        
+        public Task ExecuteAsync(IConsole console)
+        {
+            console.Output.WriteLine(Dividend / Divisor);
+            return Task.CompletedTask;
+        }
+    }
+}

CliFx.Tests/TestCommands/DuplicateOptionNamesCommand.cs

@@ -0,0 +1,18 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command]
+    public class DuplicateOptionNamesCommand : ICommand
+    {
+        [CommandOption("fruits")]
+        public string? Apples { get; set; }
+        
+        [CommandOption("fruits")]
+        public string? Oranges { get; set; }
+        
+        public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+    }
+}

CliFx.Tests/TestCommands/DuplicateOptionShortNamesCommand.cs

@@ -0,0 +1,18 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command]
+    public class DuplicateOptionShortNamesCommand : ICommand
+    {
+        [CommandOption('f')]
+        public string? Apples { get; set; }
+        
+        [CommandOption('f')]
+        public string? Oranges { get; set; }
+        
+        public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+    }
+}

CliFx.Tests/TestCommands/EnvironmentVariableCommand.cs

@@ -0,0 +1,15 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+	[Command(Description = "Reads option values from environment variables.")]
+	public class EnvironmentVariableCommand : ICommand
+	{
+		[CommandOption("opt", EnvironmentVariableName = "ENV_SINGLE_VALUE")]
+		public string? Option { get; set; }
+
+		public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+	}
+}

CliFx.Tests/TestCommands/EnvironmentVariableWithMultipleValuesCommand.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+	[Command(Description = "Reads multiple option values from environment variables.")]
+	public class EnvironmentVariableWithMultipleValuesCommand : ICommand
+	{
+		[CommandOption("opt", EnvironmentVariableName = "ENV_MULTIPLE_VALUES")]
+		public IEnumerable<string>? Option { get; set; }
+
+		public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+	}
+}

CliFx.Tests/TestCommands/EnvironmentVariableWithoutCollectionPropertyCommand.cs

@@ -0,0 +1,15 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command(Description = "Reads one option value from environment variables because target property is not a collection.")]
+    public class EnvironmentVariableWithoutCollectionPropertyCommand : ICommand
+    {
+        [CommandOption("opt", EnvironmentVariableName = "ENV_MULTIPLE_VALUES")]
+        public string? Option { get; set; }
+
+        public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+    }
+}

CliFx.Tests/TestCommands/ExceptionCommand.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command("exc")]
+    public class ExceptionCommand : ICommand
+    {
+        [CommandOption("msg", 'm')]
+        public string? Message { get; set; }
+        
+        public Task ExecuteAsync(IConsole console) => throw new Exception(Message);
+    }
+}

CliFx.Tests/TestCommands/HelloWorldDefaultCommand.cs

@@ -0,0 +1,16 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command]
+    public class HelloWorldDefaultCommand : ICommand
+    {
+        public Task ExecuteAsync(IConsole console)
+        {
+            console.Output.WriteLine("Hello world.");
+            return Task.CompletedTask;
+        }
+    }
+}

CliFx.Tests/TestCommands/HelpDefaultCommand.cs

@@ -0,0 +1,18 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command(Description = "HelpDefaultCommand description.")]
+    public class HelpDefaultCommand : 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;
+    }
+}

CliFx.Tests/TestCommands/HelpNamedCommand.cs

@@ -0,0 +1,18 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command("cmd", Description = "HelpNamedCommand description.")]
+    public class HelpNamedCommand : 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;
+    }
+}

CliFx.Tests/TestCommands/HelpSubCommand.cs

@@ -0,0 +1,15 @@
+using System.Threading.Tasks;
+using CliFx.Attributes;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command("cmd sub", Description = "HelpSubCommand description.")]
+    public class HelpSubCommand : ICommand
+    {
+        [CommandOption("option-e", 'e', Description = "OptionE description.")]
+        public string? OptionE { get; set; }
+
+        public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+    }
+}

CliFx.Tests/TestCommands/NonAnnotatedCommand.cs

@@ -0,0 +1,10 @@
+using System.Threading.Tasks;
+using CliFx.Services;
+
+namespace CliFx.Tests.TestCommands
+{
+    public class NonAnnotatedCommand : ICommand
+    {
+        public Task ExecuteAsync(IConsole console) => Task.CompletedTask;
+    }
+}

CliFx.Tests/TestCommands/NonImplementedCommand.cs

@@ -0,0 +1,9 @@
+using CliFx.Attributes;
+
+namespace CliFx.Tests.TestCommands
+{
+    [Command]
+    public class NonImplementedCommand
+    {
+    }
+}

CliFx.Tests/TestCustomTypes/TestEnum.cs

@@ -0,0 +1,9 @@
+namespace CliFx.Tests.TestCustomTypes
+{
+    public enum TestEnum
+    {
+        Value1,
+        Value2,
+        Value3
+    }
+}

CliFx.Tests/TestCustomTypes/TestNonStringParseable.cs

@@ -0,0 +1,12 @@
+namespace CliFx.Tests.TestCustomTypes
+{
+    public class TestNonStringParseable
+    {
+        public int Value { get; }
+
+        public TestNonStringParseable(int value)
+        {
+            Value = value;
+        }
+    }
+}

CliFx.Tests/TestCustomTypes/TestStringConstructable.cs

@@ -0,0 +1,12 @@
+namespace CliFx.Tests.TestCustomTypes
+{
+    public class TestStringConstructable
+    {
+        public string Value { get; }
+
+        public TestStringConstructable(string value)
+        {
+            Value = value;
+        }
+    }
+}

CliFx.Tests/TestCustomTypes/TestStringParseable.cs

@@ -0,0 +1,14 @@
+namespace CliFx.Tests.TestCustomTypes
+{
+    public class TestStringParseable
+    {
+        public string Value { get; }
+
+        private TestStringParseable(string value)
+        {
+            Value = value;
+        }
+
+        public static TestStringParseable Parse(string value) => new TestStringParseable(value);
+    }
+}

CliFx.Tests/TestCustomTypes/TestStringParseableWithFormatProvider.cs

@@ -0,0 +1,17 @@
+using System;
+
+namespace CliFx.Tests.TestCustomTypes
+{
+    public class TestStringParseableWithFormatProvider
+    {
+        public string Value { get; }
+
+        private TestStringParseableWithFormatProvider(string value)
+        {
+            Value = value;
+        }
+
+        public static TestStringParseableWithFormatProvider Parse(string value, IFormatProvider formatProvider) =>
+            new TestStringParseableWithFormatProvider(value + " " + formatProvider);
+    }
+}

CliFx.Tests/Utilities/ProgressTickerTests.cs

@@ -0,0 +1,55 @@
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using CliFx.Services;
+using CliFx.Utilities;
+using FluentAssertions;
+using NUnit.Framework;
+
+namespace CliFx.Tests.Utilities
+{
+    [TestFixture]
+    public class ProgressTickerTests
+    {
+        [Test]
+        public void Report_Test()
+        {
+            // Arrange
+            var formatProvider = CultureInfo.InvariantCulture;
+
+            using var stdout = new StringWriter(formatProvider);
+
+            var console = new VirtualConsole(TextReader.Null, false, stdout, false, TextWriter.Null, 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", formatProvider)).ToArray();
+
+            // Act
+            foreach (var progress in progressValues)
+                ticker.Report(progress);
+
+            // Assert
+            stdout.ToString().Should().ContainAll(progressStringValues);
+        }
+
+        [Test]
+        public void Report_Redirected_Test()
+        {
+            // Arrange
+            using var stdout = new StringWriter();
+
+            var console = new VirtualConsole(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);
+
+            // Assert
+            stdout.ToString().Should().BeEmpty();
+        }
+    }
+}

CliFx.sln

@@ -7,8 +7,6 @@ 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
@@ -18,7 +16,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution
 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
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -54,18 +52,6 @@ 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

CliFx/Attributes/CommandAttribute.cs

@@ -10,27 +10,27 @@ namespace CliFx.Attributes
     {
         /// <summary>
         /// Command name.
+        /// This can be null if this is the default command.
         /// </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

@@ -10,11 +10,13 @@ namespace CliFx.Attributes
     {
         /// <summary>
         /// Option name.
+        /// Either <see cref="Name"/> or <see cref="ShortName"/> must be set.
         /// </summary>
-        public string Name { get; }
+        public string? Name { get; }
 
         /// <summary>
         /// Option short name.
+        /// Either <see cref="Name"/> or <see cref="ShortName"/> must be set.
         /// </summary>
         public char? ShortName { get; }
 
@@ -26,15 +28,20 @@ namespace CliFx.Attributes
         /// <summary>
         /// Option description, which is used in help text.
         /// </summary>
-        public string Description { get; set; }
+        public string? Description { get; set; }
+
+        /// <summary>
+        /// Optional environment variable name that will be used as fallback value 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
-            ShortName = shortName; // can be null
+            Name = name;
+            ShortName = shortName;
         }
 
         /// <summary>
@@ -57,7 +64,7 @@ namespace CliFx.Attributes
         /// Initializes an instance of <see cref="CommandOptionAttribute"/>.
         /// </summary>
         public CommandOptionAttribute(char shortName)
-            : this(null, shortName)
+            : this(null, (char?) shortName)
         {
         }
     }

CliFx/CliApplication.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using System.Threading.Tasks;
 using CliFx.Exceptions;
@@ -31,113 +32,197 @@ namespace CliFx
             IConsole console, ICommandInputParser commandInputParser, ICommandSchemaResolver commandSchemaResolver,
             ICommandFactory commandFactory, ICommandInitializer commandInitializer, IHelpTextRenderer helpTextRenderer)
         {
-            _metadata = metadata.GuardNotNull(nameof(metadata));
-            _configuration = configuration.GuardNotNull(nameof(configuration));
+            _metadata = metadata;
+            _configuration = configuration;
 
-            _console = console.GuardNotNull(nameof(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));
+            _console = console;
+            _commandInputParser = commandInputParser;
+            _commandSchemaResolver = commandSchemaResolver;
+            _commandFactory = commandFactory;
+            _commandInitializer = commandInitializer;
+            _helpTextRenderer = helpTextRenderer;
+        }
+
+        private async Task<int?> HandleDebugDirectiveAsync(CommandInput commandInput)
+        {
+            // Debug mode is enabled if it's allowed in the application and it was requested via corresponding directive
+            var isDebugMode = _configuration.IsDebugModeAllowed && commandInput.IsDebugDirectiveSpecified();
+
+            // If not in debug mode, pass execution to the next handler
+            if (!isDebugMode)
+                return null;
+
+            // Inform user which process they need to attach debugger to
+            _console.WithForegroundColor(ConsoleColor.Green,
+                () => _console.Output.WriteLine($"Attach debugger to PID {Process.GetCurrentProcess().Id} to continue."));
+
+            // Wait until debugger is attached
+            while (!Debugger.IsAttached)
+                await Task.Delay(100);
+
+            // Debug directive never short-circuits
+            return null;
+        }
+
+        private int? HandlePreviewDirective(CommandInput commandInput)
+        {
+            // Preview mode is enabled if it's allowed in the application and it was requested via corresponding directive
+            var isPreviewMode = _configuration.IsPreviewModeAllowed && commandInput.IsPreviewDirectiveSpecified();
+
+            // If not in preview mode, pass execution to the next handler
+            if (!isPreviewMode)
+                return null;
+
+            // Render command name
+            _console.Output.WriteLine($"Command name: {commandInput.CommandName}");
+            _console.Output.WriteLine();
+
+            // Render directives
+            _console.Output.WriteLine("Directives:");
+            foreach (var directive in commandInput.Directives)
+            {
+                _console.Output.Write(" ");
+                _console.Output.WriteLine(directive);
+            }
+
+            // Margin
+            _console.Output.WriteLine();
+
+            // Render options
+            _console.Output.WriteLine("Options:");
+            foreach (var option in commandInput.Options)
+            {
+                _console.Output.Write(" ");
+                _console.Output.WriteLine(option);
+            }
+
+            // Short-circuit with exit code 0
+            return 0;
+        }
+
+        private int? HandleVersionOption(CommandInput commandInput)
+        {
+            // Version should be rendered if it was requested on a default command
+            var shouldRenderVersion = !commandInput.IsCommandSpecified() && commandInput.IsVersionOptionSpecified();
+
+            // If shouldn't render version, pass execution to the next handler
+            if (!shouldRenderVersion)
+                return null;
+
+            // Render version text
+            _console.Output.WriteLine(_metadata.VersionText);
+
+            // Short-circuit with exit code 0
+            return 0;
+        }
+
+        private int? HandleHelpOption(CommandInput commandInput,
+            IReadOnlyList<CommandSchema> availableCommandSchemas, CommandSchema? targetCommandSchema)
+        {
+            // Help should be rendered if it was requested, or when executing a command which isn't defined
+            var shouldRenderHelp = commandInput.IsHelpOptionSpecified() || targetCommandSchema == null;
+
+            // If shouldn't render help, pass execution to the next handler
+            if (!shouldRenderHelp)
+                return null;
+
+            // Keep track whether there was an error in the input
+            var isError = false;
+
+            // If target command isn't defined, find its contextual replacement
+            if (targetCommandSchema == null)
+            {
+                // If command was specified, inform the user that it's not defined
+                if (commandInput.IsCommandSpecified())
+                {
+                    _console.WithForegroundColor(ConsoleColor.Red,
+                        () => _console.Error.WriteLine($"Specified command [{commandInput.CommandName}] is not defined."));
+
+                    isError = true;
+                }
+
+                // Replace target command with closest parent of specified command
+                targetCommandSchema = availableCommandSchemas.FindParent(commandInput.CommandName);
+
+                // If there's no parent, replace with stub default command
+                if (targetCommandSchema == null)
+                {
+                    targetCommandSchema = CommandSchema.StubDefaultCommand;
+                    availableCommandSchemas = availableCommandSchemas.Concat(CommandSchema.StubDefaultCommand).ToArray();
+                }
+            }
+
+            // Build help text source
+            var helpTextSource = new HelpTextSource(_metadata, availableCommandSchemas, targetCommandSchema);
+
+            // Render help text
+            _helpTextRenderer.RenderHelpText(_console, helpTextSource);
+
+            // Short-circuit with appropriate exit code
+            return isError ? -1 : 0;
+        }
+
+        private async Task<int> HandleCommandExecutionAsync(CommandInput commandInput, CommandSchema targetCommandSchema)
+        {
+            // 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);
+
+            // Finish the chain with exit code 0
+            return 0;
         }
 
         /// <inheritdoc />
         public async Task<int> RunAsync(IReadOnlyList<string> commandLineArguments)
         {
-            commandLineArguments.GuardNotNull(nameof(commandLineArguments));
-
             try
             {
-                // Get schemas for all available command types
-                var availableCommandSchemas = _commandSchemaResolver.GetCommandSchemas(_configuration.CommandTypes);
-
                 // Parse command input from arguments
                 var commandInput = _commandInputParser.ParseCommandInput(commandLineArguments);
 
+                // Get schemas for all available command types
+                var availableCommandSchemas = _commandSchemaResolver.GetCommandSchemas(_configuration.CommandTypes);
+
                 // Find command schema matching the name specified in the input
                 var targetCommandSchema = availableCommandSchemas.FindByName(commandInput.CommandName);
 
-                // Handle cases where requested command is not defined
-                if (targetCommandSchema == null)
-                {
-                    var isError = false;
-
-                    // If specified a command - show error
-                    if (commandInput.IsCommandSpecified())
-                    {
-                        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;
+                // Chain handlers until the first one that produces an exit code
+                return
+                    await HandleDebugDirectiveAsync(commandInput) ??
+                    HandlePreviewDirective(commandInput) ??
+                    HandleVersionOption(commandInput) ??
+                    HandleHelpOption(commandInput, availableCommandSchemas, targetCommandSchema) ??
+                    await HandleCommandExecutionAsync(commandInput, targetCommandSchema!);
             }
             catch (Exception ex)
             {
                 // We want to catch exceptions in order to print errors and return correct exit codes.
-                // Also, by doing this we get rid of the annoying Windows troubleshooting dialog that shows up on unhandled exceptions.
+                // Doing this also gets rid of the annoying Windows troubleshooting dialog that shows up on unhandled exceptions.
 
-                // 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.
+                // Prefer showing message without stack trace on exceptions coming from CliFx or on CommandException
+                if (!string.IsNullOrWhiteSpace(ex.Message) && (ex is CliFxException || ex is CommandException))
+                {
+                    _console.WithForegroundColor(ConsoleColor.Red, () => _console.Error.WriteLine(ex.Message));
+                }
+                else
+                {
+                    _console.WithForegroundColor(ConsoleColor.Red, () => _console.Error.WriteLine(ex));
+                }
 
-                // 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;
+                // Return exit code if it was specified via CommandException
+                if (ex is CommandException commandException)
+                {
+                    return commandException.ExitCode;
+                }
+                else
+                {
+                    return ex.HResult;
+                }
             }
         }
     }

CliFx/CliApplicationBuilder.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Reflection;
+using CliFx.Attributes;
 using CliFx.Internal;
 using CliFx.Models;
 using CliFx.Services;
@@ -16,18 +17,20 @@ namespace CliFx
     {
         private readonly HashSet<Type> _commandTypes = new HashSet<Type>();
 
-        private string _title;
-        private string _executableName;
-        private string _versionText;
-        private string _description;
-        private IConsole _console;
-        private ICommandFactory _commandFactory;
+        private bool _isDebugModeAllowed = true;
+        private bool _isPreviewModeAllowed = true;
+        private string? _title;
+        private string? _executableName;
+        private string? _versionText;
+        private string? _description;
+        private IConsole? _console;
+        private ICommandFactory? _commandFactory;
+        private ICommandOptionInputConverter? _commandOptionInputConverter;
+        private IEnvironmentVariablesProvider? _environmentVariablesProvider;
 
         /// <inheritdoc />
         public ICliApplicationBuilder AddCommand(Type commandType)
         {
-            commandType.GuardNotNull(nameof(commandType));
-
             _commandTypes.Add(commandType);
 
             return this;
@@ -36,9 +39,10 @@ namespace CliFx
         /// <inheritdoc />
         public ICliApplicationBuilder AddCommandsFrom(Assembly commandAssembly)
         {
-            commandAssembly.GuardNotNull(nameof(commandAssembly));
-
-            var commandTypes = commandAssembly.ExportedTypes.Where(t => t.Implements(typeof(ICommand)));
+            var commandTypes = commandAssembly.ExportedTypes
+                .Where(t => t.Implements(typeof(ICommand)))
+                .Where(t => t.IsDefined(typeof(CommandAttribute)))
+                .Where(t => !t.IsAbstract && !t.IsInterface);
 
             foreach (var commandType in commandTypes)
                 AddCommand(commandType);
@@ -46,112 +50,95 @@ namespace CliFx
             return this;
         }
 
+        /// <inheritdoc />
+        public ICliApplicationBuilder AllowDebugMode(bool isAllowed = true)
+        {
+            _isDebugModeAllowed = isAllowed;
+            return this;
+        }
+
+        /// <inheritdoc />
+        public ICliApplicationBuilder AllowPreviewMode(bool isAllowed = true)
+        {
+            _isPreviewModeAllowed = isAllowed;
+            return this;
+        }
+
         /// <inheritdoc />
         public ICliApplicationBuilder UseTitle(string title)
         {
-            _title = title.GuardNotNull(nameof(title));
+            _title = title;
             return this;
         }
 
         /// <inheritdoc />
         public ICliApplicationBuilder UseExecutableName(string executableName)
         {
-            _executableName = executableName.GuardNotNull(nameof(executableName));
+            _executableName = executableName;
             return this;
         }
 
         /// <inheritdoc />
         public ICliApplicationBuilder UseVersionText(string versionText)
         {
-            _versionText = versionText.GuardNotNull(nameof(versionText));
+            _versionText = versionText;
             return this;
         }
 
         /// <inheritdoc />
-        public ICliApplicationBuilder UseDescription(string description)
+        public ICliApplicationBuilder UseDescription(string? description)
         {
-            _description = description; // can be null
+            _description = description;
             return this;
         }
 
         /// <inheritdoc />
         public ICliApplicationBuilder UseConsole(IConsole console)
         {
-            _console = console.GuardNotNull(nameof(console));
+            _console = console;
             return this;
         }
 
         /// <inheritdoc />
         public ICliApplicationBuilder UseCommandFactory(ICommandFactory factory)
         {
-            _commandFactory = factory.GuardNotNull(nameof(factory));
+            _commandFactory = factory;
             return this;
         }
 
-        private void SetFallbackValues()
+        /// <inheritdoc />
+        public ICliApplicationBuilder UseCommandOptionInputConverter(ICommandOptionInputConverter converter)
         {
-            if (_title.IsNullOrWhiteSpace())
-            {
-                // Entry assembly is null in tests
-                UseTitle(EntryAssembly?.GetName().Name ?? "App");
-            }
+            _commandOptionInputConverter = converter;
+            return this;
+        }
 
-            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 />
+        public ICliApplicationBuilder UseEnvironmentVariablesProvider(IEnvironmentVariablesProvider environmentVariablesProvider)
+        {
+            _environmentVariablesProvider = environmentVariablesProvider;
+            return this;
         }
 
         /// <inheritdoc />
         public ICliApplication Build()
         {
             // Use defaults for required parameters that were not configured
-            SetFallbackValues();
+            _title ??= GetDefaultTitle() ?? "App";
+            _executableName ??= GetDefaultExecutableName() ?? "app";
+            _versionText ??= GetDefaultVersionText() ?? "v1.0";
+            _console ??= new SystemConsole();
+            _commandFactory ??= new CommandFactory();
+            _commandOptionInputConverter ??= new CommandOptionInputConverter();
+            _environmentVariablesProvider ??= new EnvironmentVariablesProvider();
 
             // 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,
-                _console, new CommandInputParser(), new CommandSchemaResolver(),
-                _commandFactory, new CommandInitializer(), new HelpTextRenderer());
+                _console, new CommandInputParser(_environmentVariablesProvider), new CommandSchemaResolver(),
+                _commandFactory, new CommandInitializer(_commandOptionInputConverter, new EnvironmentVariablesParser()), new HelpTextRenderer());
         }
     }
 
@@ -159,6 +146,25 @@ namespace CliFx
     {
         private static readonly Lazy<Assembly> LazyEntryAssembly = new Lazy<Assembly>(Assembly.GetEntryAssembly);
 
+        // 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;
+
+            // If it's a .dll assembly, prepend 'dotnet' and keep the file extension
+            if (string.Equals(Path.GetExtension(entryAssemblyLocation), ".dll", StringComparison.OrdinalIgnoreCase))
+            {
+                return "dotnet " + Path.GetFileName(entryAssemblyLocation);
+            }
+
+            // Otherwise just use assembly file name without extension
+            return Path.GetFileNameWithoutExtension(entryAssemblyLocation);
+        }
+
+        private static string GetDefaultVersionText() => EntryAssembly != null ? $"v{EntryAssembly.GetName().Version}" : "";
     }
 }

CliFx/CliFx.csproj

@@ -1,9 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net45;netstandard2.0</TargetFrameworks>
-    <LangVersion>latest</LangVersion>
-    <Version>0.0.2</Version>
+    <TargetFrameworks>net45;netstandard2.0;netstandard2.1</TargetFrameworks>
+    <Version>0.0.8</Version>
     <Company>Tyrrrz</Company>
     <Authors>$(Company)</Authors>
     <Copyright>Copyright (C) Alexey Golub</Copyright>
@@ -11,13 +10,27 @@
     <PackageTags>command line executable interface framework parser arguments net core</PackageTags>
     <PackageProjectUrl>https://github.com/Tyrrrz/CliFx</PackageProjectUrl>
     <PackageReleaseNotes>https://github.com/Tyrrrz/CliFx/blob/master/Changelog.md</PackageReleaseNotes>
-    <PackageIconUrl>https://raw.githubusercontent.com/Tyrrrz/CliFx/master/favicon.png</PackageIconUrl>
+    <PackageIcon>favicon.png</PackageIcon>
     <PackageLicenseExpression>LGPL-3.0-only</PackageLicenseExpression>
-    <RepositoryUrl>https://github.com/Tyrrrz/CliFx</RepositoryUrl>
-    <RepositoryType>git</RepositoryType>
     <PackageRequireLicenseAcceptance>True</PackageRequireLicenseAcceptance>
-    <GeneratePackageOnBuild>True</GeneratePackageOnBuild>
-    <DocumentationFile>bin/$(Configuration)/$(TargetFramework)/$(AssemblyName).xml</DocumentationFile>
+    <GenerateDocumentationFile>True</GenerateDocumentationFile>
+    <PublishRepositoryUrl>True</PublishRepositoryUrl>
+    <EmbedUntrackedSources>True</EmbedUntrackedSources>
+    <IncludeSymbols>True</IncludeSymbols>
+    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
-</Project>
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-19554-01" PrivateAssets="all" />
+    <PackageReference Include="Nullable" Version="1.1.1" PrivateAssets="all" />
+
+  </ItemGroup>
+
+  <ItemGroup>
+    <None Include="../favicon.png" Pack="True" PackagePath="" />
+  </ItemGroup>
+
+</Project>

CliFx/Exceptions/CliFxException.cs

@@ -5,12 +5,12 @@ namespace CliFx.Exceptions
     /// <summary>
     /// Domain exception thrown within CliFx.
     /// </summary>
-    public abstract class CliFxException : Exception
+    public class CliFxException : Exception
     {
         /// <summary>
         /// Initializes an instance of <see cref="CliFxException"/>.
         /// </summary>
-        protected CliFxException(string message)
+        public CliFxException(string? message)
             : base(message)
         {
         }
@@ -18,7 +18,7 @@ namespace CliFx.Exceptions
         /// <summary>
         /// Initializes an instance of <see cref="CliFxException"/>.
         /// </summary>
-        protected CliFxException(string message, Exception innerException)
+        public CliFxException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }

CliFx/Exceptions/CommandException.cs

@@ -1,5 +1,4 @@
 using System;
-using CliFx.Internal;
 
 namespace CliFx.Exceptions
 {
@@ -8,7 +7,7 @@ namespace CliFx.Exceptions
     /// Use this exception if you want to report an error that occured during execution of a command.
     /// This exception also allows specifying exit code which will be returned to the calling process.
     /// </summary>
-    public class CommandException : CliFxException
+    public class CommandException : Exception
     {
         private const int DefaultExitCode = -100;
 
@@ -20,16 +19,16 @@ namespace CliFx.Exceptions
         /// <summary>
         /// Initializes an instance of <see cref="CommandException"/>.
         /// </summary>
-        public CommandException(string message, Exception innerException, int exitCode = DefaultExitCode)
+        public CommandException(string? message, Exception? innerException, int exitCode = DefaultExitCode)
             : base(message, innerException)
         {
-            ExitCode = exitCode.GuardNotZero(nameof(exitCode));
+            ExitCode = exitCode != 0 ? exitCode : throw new ArgumentException("Exit code cannot be zero because that signifies success.");
         }
 
         /// <summary>
         /// Initializes an instance of <see cref="CommandException"/>.
         /// </summary>
-        public CommandException(string message, int exitCode = DefaultExitCode)
+        public CommandException(string? message, int exitCode = DefaultExitCode)
             : this(message, null, exitCode)
         {
         }

CliFx/Exceptions/InvalidCommandOptionInputException.cs

@@ -1,26 +0,0 @@
-using System;
-
-namespace CliFx.Exceptions
-{
-    /// <summary>
-    /// Thrown when a command option can't be converted to target type specified in its schema.
-    /// </summary>
-    public class InvalidCommandOptionInputException : CliFxException
-    {
-        /// <summary>
-        /// Initializes an instance of <see cref="InvalidCommandOptionInputException"/>.
-        /// </summary>
-        public InvalidCommandOptionInputException(string message)
-            : base(message)
-        {
-        }
-
-        /// <summary>
-        /// Initializes an instance of <see cref="InvalidCommandOptionInputException"/>.
-        /// </summary>
-        public InvalidCommandOptionInputException(string message, Exception innerException)
-            : base(message, innerException)
-        {
-        }
-    }
-}

CliFx/Exceptions/InvalidCommandSchemaException.cs

@@ -1,26 +0,0 @@
-using System;
-
-namespace CliFx.Exceptions
-{
-    /// <summary>
-    /// Thrown when a command schema fails validation.
-    /// </summary>
-    public class InvalidCommandSchemaException : CliFxException
-    {
-        /// <summary>
-        /// Initializes an instance of <see cref="InvalidCommandSchemaException"/>.
-        /// </summary>
-        public InvalidCommandSchemaException(string message)
-            : base(message)
-        {
-        }
-
-        /// <summary>
-        /// Initializes an instance of <see cref="InvalidCommandSchemaException"/>.
-        /// </summary>
-        public InvalidCommandSchemaException(string message, Exception innerException)
-            : base(message, innerException)
-        {
-        }
-    }
-}

CliFx/Exceptions/MissingCommandOptionInputException.cs

@@ -1,26 +0,0 @@
-using System;
-
-namespace CliFx.Exceptions
-{
-    /// <summary>
-    /// Thrown when a required command option was not set.
-    /// </summary>
-    public class MissingCommandOptionInputException : CliFxException
-    {
-        /// <summary>
-        /// Initializes an instance of <see cref="MissingCommandOptionInputException"/>.
-        /// </summary>
-        public MissingCommandOptionInputException(string message)
-            : base(message)
-        {
-        }
-
-        /// <summary>
-        /// Initializes an instance of <see cref="MissingCommandOptionInputException"/>.
-        /// </summary>
-        public MissingCommandOptionInputException(string message, Exception innerException)
-            : base(message, innerException)
-        {
-        }
-    }
-}

CliFx/Extensions.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.Reflection;
-using CliFx.Internal;
 using CliFx.Models;
 using CliFx.Services;
 
@@ -17,9 +16,6 @@ namespace CliFx
         /// </summary>
         public static ICliApplicationBuilder AddCommands(this ICliApplicationBuilder builder, IReadOnlyList<Type> commandTypes)
         {
-            builder.GuardNotNull(nameof(builder));
-            commandTypes.GuardNotNull(nameof(commandTypes));
-
             foreach (var commandType in commandTypes)
                 builder.AddCommand(commandType);
 
@@ -31,9 +27,6 @@ namespace CliFx
         /// </summary>
         public static ICliApplicationBuilder AddCommandsFrom(this ICliApplicationBuilder builder, IReadOnlyList<Assembly> commandAssemblies)
         {
-            builder.GuardNotNull(nameof(builder));
-            commandAssemblies.GuardNotNull(nameof(commandAssemblies));
-
             foreach (var commandAssembly in commandAssemblies)
                 builder.AddCommandsFrom(commandAssembly);
 
@@ -43,21 +36,13 @@ namespace CliFx
         /// <summary>
         /// Adds commands from calling assembly to the application.
         /// </summary>
-        public static ICliApplicationBuilder AddCommandsFromThisAssembly(this ICliApplicationBuilder builder)
-        {
-            builder.GuardNotNull(nameof(builder));
-            return builder.AddCommandsFrom(Assembly.GetCallingAssembly());
-        }
+        public static ICliApplicationBuilder AddCommandsFromThisAssembly(this ICliApplicationBuilder builder) =>
+            builder.AddCommandsFrom(Assembly.GetCallingAssembly());
 
         /// <summary>
         /// Configures application to use specified factory method for creating new instances of <see cref="ICommand"/>.
         /// </summary>
-        public static ICliApplicationBuilder UseCommandFactory(this ICliApplicationBuilder builder, Func<CommandSchema, ICommand> factoryMethod)
-        {
-            builder.GuardNotNull(nameof(builder));
-            factoryMethod.GuardNotNull(nameof(factoryMethod));
-
-            return builder.UseCommandFactory(new DelegateCommandFactory(factoryMethod));
-        }
+        public static ICliApplicationBuilder UseCommandFactory(this ICliApplicationBuilder builder, Func<CommandSchema, ICommand> factoryMethod) =>
+            builder.UseCommandFactory(new DelegateCommandFactory(factoryMethod));
     }
 }

CliFx/ICliApplicationBuilder.cs

@@ -19,6 +19,16 @@ namespace CliFx
         /// </summary>
         ICliApplicationBuilder AddCommandsFrom(Assembly commandAssembly);
 
+        /// <summary>
+        /// Specifies whether debug mode (enabled with [debug] directive) is allowed in the application.
+        /// </summary>
+        ICliApplicationBuilder AllowDebugMode(bool isAllowed = true);
+
+        /// <summary>
+        /// Specifies whether preview mode (enabled with [preview] directive) is allowed in the application.
+        /// </summary>
+        ICliApplicationBuilder AllowPreviewMode(bool isAllowed = true);
+
         /// <summary>
         /// Sets application title, which appears in the help text.
         /// </summary>
@@ -37,7 +47,7 @@ namespace CliFx
         /// <summary>
         /// Sets application description, which appears in the help text.
         /// </summary>
-        ICliApplicationBuilder UseDescription(string description);
+        ICliApplicationBuilder UseDescription(string? description);
 
         /// <summary>
         /// Configures application to use specified implementation of <see cref="IConsole"/>.
@@ -49,6 +59,16 @@ namespace CliFx
         /// </summary>
         ICliApplicationBuilder UseCommandFactory(ICommandFactory factory);
 
+        /// <summary>
+        /// Configures application to use specified implementation of <see cref="ICommandOptionInputConverter"/>.
+        /// </summary>
+        ICliApplicationBuilder UseCommandOptionInputConverter(ICommandOptionInputConverter converter);
+
+        /// <summary>
+        /// Configures application to use specified implementation of <see cref="IEnvironmentVariablesProvider"/>.
+        /// </summary>
+        ICliApplicationBuilder UseEnvironmentVariablesProvider(IEnvironmentVariablesProvider environmentVariablesProvider);
+
         /// <summary>
         /// Creates an instance of <see cref="ICliApplication"/> using configured parameters.
         /// Default values are used in place of parameters that were not specified.

CliFx/Internal/Extensions.cs

@@ -8,8 +8,6 @@ namespace CliFx.Internal
 {
     internal static class Extensions
     {
-        public static bool IsNullOrWhiteSpace(this string s) => string.IsNullOrWhiteSpace(s);
-
         public static string Repeat(this char c, int count) => new string(c, count);
 
         public static string AsString(this char c) => c.Repeat(1);
@@ -26,11 +24,6 @@ namespace CliFx.Internal
         public static StringBuilder AppendIfNotEmpty(this StringBuilder builder, char value) =>
             builder.Length > 0 ? builder.Append(value) : builder;
 
-        public static TValue GetValueOrDefault<TKey, TValue>(this IReadOnlyDictionary<TKey, TValue> dic, TKey key) =>
-            dic.TryGetValue(key, out var result) ? result : default;
-
-        public static IEnumerable<T> ExceptNull<T>(this IEnumerable<T> source) where T : class => source.Where(i => i != null);
-
         public static IEnumerable<T> Concat<T>(this IEnumerable<T> source, T value)
         {
             foreach (var i in source)
@@ -41,8 +34,13 @@ namespace CliFx.Internal
 
         public static bool Implements(this Type type, Type interfaceType) => type.GetInterfaces().Contains(interfaceType);
 
-        public static Type GetEnumerableUnderlyingType(this Type type)
+        public static Type? GetNullableUnderlyingType(this Type type) => Nullable.GetUnderlyingType(type);
+
+        public static Type? GetEnumerableUnderlyingType(this Type type)
         {
+            if (type.IsPrimitive)
+                return null;
+
             if (type == typeof(IEnumerable))
                 return typeof(object);
 
@@ -51,7 +49,7 @@ namespace CliFx.Internal
 
             return type.GetInterfaces()
                 .Select(GetEnumerableUnderlyingType)
-                .ExceptNull()
+                .Where(t => t != default)
                 .OrderByDescending(t => t != typeof(object)) // prioritize more specific types
                 .FirstOrDefault();
         }
@@ -65,5 +63,8 @@ namespace CliFx.Internal
 
             return array;
         }
+
+        public static bool IsCollection(this Type type) =>
+            type != typeof(string) && type.GetEnumerableUnderlyingType() != null;
     }
 }

CliFx/Internal/Guards.cs

@@ -1,13 +0,0 @@
-using System;
-
-namespace CliFx.Internal
-{
-    internal static class Guards
-    {
-        public static T GuardNotNull<T>(this T o, string argName = null) where T : class =>
-            o ?? throw new ArgumentNullException(argName);
-
-        public static int GuardNotZero(this int i, string argName = null) =>
-            i != 0 ? i : throw new ArgumentException("Cannot be zero.", argName);
-    }
-}

CliFx/Models/ApplicationConfiguration.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using CliFx.Internal;
 
 namespace CliFx.Models
 {
@@ -10,16 +9,29 @@ namespace CliFx.Models
     public class ApplicationConfiguration
     {
         /// <summary>
-        /// Command types defined in the application.
+        /// 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)
+        public ApplicationConfiguration(IReadOnlyList<Type> commandTypes,
+            bool isDebugModeAllowed, bool isPreviewModeAllowed)
         {
-            CommandTypes = commandTypes.GuardNotNull(nameof(commandTypes));
+            CommandTypes = commandTypes;
+            IsDebugModeAllowed = isDebugModeAllowed;
+            IsPreviewModeAllowed = isPreviewModeAllowed;
         }
     }
 }

CliFx/Models/ApplicationMetadata.cs

@@ -1,6 +1,4 @@
-using CliFx.Internal;
-
-namespace CliFx.Models
+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));
-            ExecutableName = executableName.GuardNotNull(nameof(executableName));
-            VersionText = versionText.GuardNotNull(nameof(versionText));
-            Description = description; // can be null
+            Title = title;
+            ExecutableName = executableName;
+            VersionText = versionText;
+            Description = description;
         }
     }
 }

CliFx/Models/CommandInput.cs

@@ -13,20 +13,57 @@ namespace CliFx.Models
         /// Specified command name.
         /// Can be null if command was not specified.
         /// </summary>
-        public string CommandName { get; }
+        public string? CommandName { get; }
+
+        /// <summary>
+        /// Specified directives.
+        /// </summary>
+        public IReadOnlyList<string> Directives { get; }
 
         /// <summary>
         /// Specified options.
         /// </summary>
         public IReadOnlyList<CommandOptionInput> Options { get; }
 
+        /// <summary>
+        /// Environment variables available when the command was parsed
+        /// </summary>
+        public IReadOnlyDictionary<string, string> EnvironmentVariables { get; }
+
         /// <summary>
         /// Initializes an instance of <see cref="CommandInput"/>.
         /// </summary>
-        public CommandInput(string commandName, IReadOnlyList<CommandOptionInput> options)
+        public CommandInput(string? commandName, IReadOnlyList<string> directives, IReadOnlyList<CommandOptionInput> options,
+            IReadOnlyDictionary<string, string> environmentVariables)
+        {
+            CommandName = commandName;
+            Directives = directives;
+            Options = options;
+            EnvironmentVariables = environmentVariables;
+        }
+
+        /// <summary>
+        /// Initializes an instance of <see cref="CommandInput"/>.
+        /// </summary>
+        public CommandInput(string? commandName, IReadOnlyList<string> directives, IReadOnlyList<CommandOptionInput> options)
+            : this(commandName, directives, options, EmptyEnvironmentVariables)
+        {
+        }
+
+        /// <summary>
+        /// Initializes an instance of <see cref="CommandInput"/>.
+        /// </summary>
+        public CommandInput(string? commandName, IReadOnlyList<CommandOptionInput> options, IReadOnlyDictionary<string, string> environmentVariables)
+            : this(commandName, EmptyDirectives, options, environmentVariables)
+        {
+        }
+
+        /// <summary>
+        /// Initializes an instance of <see cref="CommandInput"/>.
+        /// </summary>
+        public CommandInput(string? commandName, IReadOnlyList<CommandOptionInput> options)
+            : this(commandName, EmptyDirectives, options)
         {
-            CommandName = commandName; // can be null
-            Options = options.GuardNotNull(nameof(options));
         }
 
         /// <summary>
@@ -40,16 +77,8 @@ namespace CliFx.Models
         /// <summary>
         /// Initializes an instance of <see cref="CommandInput"/>.
         /// </summary>
-        public CommandInput(string commandName)
-            : this(commandName, new CommandOptionInput[0])
-        {
-        }
-
-        /// <summary>
-        /// Initializes an instance of <see cref="CommandInput"/>.
-        /// </summary>
-        public CommandInput()
-            : this(null, new CommandOptionInput[0])
+        public CommandInput(string? commandName)
+            : this(commandName, EmptyOptions)
         {
         }
 
@@ -58,9 +87,15 @@ namespace CliFx.Models
         {
             var buffer = new StringBuilder();
 
-            if (!CommandName.IsNullOrWhiteSpace())
+            if (!string.IsNullOrWhiteSpace(CommandName))
                 buffer.Append(CommandName);
 
+            foreach (var directive in Directives)
+            {
+                buffer.AppendIfNotEmpty(' ');
+                buffer.Append(directive);
+            }
+
             foreach (var option in Options)
             {
                 buffer.AppendIfNotEmpty(' ');
@@ -73,9 +108,13 @@ namespace CliFx.Models
 
     public partial class CommandInput
     {
+        private static readonly IReadOnlyList<string> EmptyDirectives = new string[0];
+        private static readonly IReadOnlyList<CommandOptionInput> EmptyOptions = new CommandOptionInput[0];
+        private static readonly IReadOnlyDictionary<string, string> EmptyEnvironmentVariables = new Dictionary<string, string>();
+
         /// <summary>
         /// Empty input.
         /// </summary>
-        public static CommandInput Empty { get; } = new CommandInput();
+        public static CommandInput Empty { get; } = new CommandInput(EmptyOptions);
     }
 }

CliFx/Models/CommandOptionInput.cs

@@ -7,7 +7,7 @@ namespace CliFx.Models
     /// <summary>
     /// Parsed option from command line input.
     /// </summary>
-    public class CommandOptionInput
+    public partial class CommandOptionInput
     {
         /// <summary>
         /// Specified option alias.
@@ -24,8 +24,8 @@ namespace CliFx.Models
         /// </summary>
         public CommandOptionInput(string alias, IReadOnlyList<string> values)
         {
-            Alias = alias.GuardNotNull(nameof(alias));
-            Values = values.GuardNotNull(nameof(values));
+            Alias = alias;
+            Values = values;
         }
 
         /// <summary>
@@ -40,7 +40,7 @@ namespace CliFx.Models
         /// Initializes an instance of <see cref="CommandOptionInput"/>.
         /// </summary>
         public CommandOptionInput(string alias)
-            : this(alias, new string[0])
+            : this(alias, EmptyValues)
         {
         }
 
@@ -70,4 +70,9 @@ namespace CliFx.Models
             return buffer.ToString();
         }
     }
+
+    public partial class CommandOptionInput
+    {
+        private static readonly IReadOnlyList<string> EmptyValues = new string[0];
+    }
 }

CliFx/Models/CommandOptionSchema.cs

@@ -1,6 +1,5 @@
 using System.Reflection;
 using System.Text;
-using CliFx.Internal;
 
 namespace CliFx.Models
 {
@@ -12,12 +11,12 @@ namespace CliFx.Models
         /// <summary>
         /// Underlying property.
         /// </summary>
-        public PropertyInfo Property { get; }
+        public PropertyInfo? Property { get; }
 
         /// <summary>
         /// Option name.
         /// </summary>
-        public string Name { get; }
+        public string? Name { get; }
 
         /// <summary>
         /// Option short name.
@@ -32,18 +31,24 @@ namespace CliFx.Models
         /// <summary>
         /// Option description.
         /// </summary>
-        public string Description { get; }
+        public string? Description { get; }
+
+        /// <summary>
+        /// Optional environment variable name that will be used as fallback value if no option value is specified.
+        /// </summary>
+        public string? EnvironmentVariableName { get; }
 
         /// <summary>
         /// Initializes an instance of <see cref="CommandOptionSchema"/>.
         /// </summary>
-        public CommandOptionSchema(PropertyInfo property, string name, char? shortName, bool isRequired, string description)
+        public CommandOptionSchema(PropertyInfo? property, string? name, char? shortName, bool isRequired, string? description, string? environmentVariableName)
         {
-            Property = property; // can be null
-            Name = name; // can be null
-            ShortName = shortName; // can be null
+            Property = property;
+            Name = name;
+            ShortName = shortName;
             IsRequired = isRequired;
-            Description = description; // can be null
+            Description = description;
+            EnvironmentVariableName = environmentVariableName;
         }
 
         /// <inheritdoc />
@@ -54,10 +59,10 @@ namespace CliFx.Models
             if (IsRequired)
                 buffer.Append('*');
 
-            if (!Name.IsNullOrWhiteSpace())
+            if (!string.IsNullOrWhiteSpace(Name))
                 buffer.Append(Name);
 
-            if (!Name.IsNullOrWhiteSpace() && ShortName != null)
+            if (!string.IsNullOrWhiteSpace(Name) && ShortName != null)
                 buffer.Append('|');
 
             if (ShortName != null)
@@ -75,9 +80,9 @@ namespace CliFx.Models
         // ...in CliApplication (when reading) and HelpTextRenderer (when writing).
 
         internal static CommandOptionSchema HelpOption { get; } =
-            new CommandOptionSchema(null, "help", 'h', false, "Shows help text.");
+            new CommandOptionSchema(null, "help", 'h', false, "Shows help text.", null);
 
         internal static CommandOptionSchema VersionOption { get; } =
-            new CommandOptionSchema(null, "version", null, false, "Shows version information.");
+            new CommandOptionSchema(null, "version", null, false, "Shows version information.", null);
     }
 }

CliFx/Models/CommandSchema.cs

@@ -13,17 +13,17 @@ namespace CliFx.Models
         /// <summary>
         /// Underlying type.
         /// </summary>
-        public Type Type { get; }
+        public Type? Type { get; }
 
         /// <summary>
         /// Command name.
         /// </summary>
-        public string Name { get; }
+        public string? Name { get; }
 
         /// <summary>
         /// Command description.
         /// </summary>
-        public string Description { get; }
+        public string? Description { get; }
 
         /// <summary>
         /// Command options.
@@ -33,12 +33,12 @@ namespace CliFx.Models
         /// <summary>
         /// Initializes an instance of <see cref="CommandSchema"/>.
         /// </summary>
-        public CommandSchema(Type type, string name, string description, IReadOnlyList<CommandOptionSchema> options)
+        public CommandSchema(Type? type, string? name, string? description, IReadOnlyList<CommandOptionSchema> options)
         {
-            Type = type; // can be null
-            Name = name; // can be null
-            Description = description; // can be null
-            Options = options.GuardNotNull(nameof(options));
+            Type = type;
+            Name = name;
+            Description = description;
+            Options = options;
         }
 
         /// <inheritdoc />
@@ -46,7 +46,7 @@ namespace CliFx.Models
         {
             var buffer = new StringBuilder();
 
-            if (!Name.IsNullOrWhiteSpace())
+            if (!string.IsNullOrWhiteSpace(Name))
                 buffer.Append(Name);
 
             foreach (var option in Options)

CliFx/Models/Extensions.cs

@@ -1,7 +1,7 @@
-using System;
+using CliFx.Internal;
+using System;
 using System.Collections.Generic;
 using System.Linq;
-using CliFx.Internal;
 
 namespace CliFx.Models
 {
@@ -13,13 +13,11 @@ namespace CliFx.Models
         /// <summary>
         /// Finds a command that has specified name, or null if not found.
         /// </summary>
-        public static CommandSchema FindByName(this IReadOnlyList<CommandSchema> commandSchemas, string commandName)
+        public static CommandSchema? FindByName(this IReadOnlyList<CommandSchema> commandSchemas, string? commandName)
         {
-            commandSchemas.GuardNotNull(nameof(commandSchemas));
-
             // If looking for default command, don't compare names directly
             // ...because null and empty are both valid names for default command
-            if (commandName.IsNullOrWhiteSpace())
+            if (string.IsNullOrWhiteSpace(commandName))
                 return commandSchemas.FirstOrDefault(c => c.IsDefault());
 
             return commandSchemas.FirstOrDefault(c => string.Equals(c.Name, commandName, StringComparison.OrdinalIgnoreCase));
@@ -28,12 +26,10 @@ namespace CliFx.Models
         /// <summary>
         /// Finds parent command to the command that has specified name, or null if not found.
         /// </summary>
-        public static CommandSchema FindParent(this IReadOnlyList<CommandSchema> commandSchemas, string commandName)
+        public static CommandSchema? FindParent(this IReadOnlyList<CommandSchema> commandSchemas, string? commandName)
         {
-            commandSchemas.GuardNotNull(nameof(commandSchemas));
-
             // If command has no name, it's the default command so it doesn't have a parent
-            if (commandName.IsNullOrWhiteSpace())
+            if (string.IsNullOrWhiteSpace(commandName))
                 return null;
 
             // Repeatedly cut off individual words from the name until we find a command with that name
@@ -56,12 +52,9 @@ namespace CliFx.Models
         /// </summary>
         public static bool MatchesAlias(this CommandOptionSchema optionSchema, string alias)
         {
-            optionSchema.GuardNotNull(nameof(optionSchema));
-            alias.GuardNotNull(nameof(alias));
-
             // Compare against name. Case is ignored.
             var matchesByName =
-                !optionSchema.Name.IsNullOrWhiteSpace() &&
+                !string.IsNullOrWhiteSpace(optionSchema.Name) &&
                 string.Equals(optionSchema.Name, alias, StringComparison.OrdinalIgnoreCase);
 
             // Compare against short name. Case is NOT ignored.
@@ -71,17 +64,12 @@ namespace CliFx.Models
 
             return matchesByName || matchesByShortName;
         }
-        
-        /// <summary>
-        /// Finds an option that matches specified alias, or null if not found.
-        /// </summary>
-        public static CommandOptionSchema FindByAlias(this IReadOnlyList<CommandOptionSchema> optionSchemas, string alias)
-        {
-            optionSchemas.GuardNotNull(nameof(optionSchemas));
-            alias.GuardNotNull(nameof(alias));
 
-            return optionSchemas.FirstOrDefault(o => o.MatchesAlias(alias));
-        }
+        /// <summary>
+        /// Finds an option input that matches the option schema specified, or null if not found.
+        /// </summary>
+        public static CommandOptionInput? FindByOptionSchema(this IReadOnlyList<CommandOptionInput> optionInputs, CommandOptionSchema optionSchema) =>
+            optionInputs.FirstOrDefault(o => optionSchema.MatchesAlias(o.Alias));
 
         /// <summary>
         /// Gets valid aliases for the option.
@@ -90,8 +78,8 @@ namespace CliFx.Models
         {
             var result = new List<string>(2);
 
-            if (!optionSchema.Name.IsNullOrWhiteSpace())
-                result.Add(optionSchema.Name);
+            if (!string.IsNullOrWhiteSpace(optionSchema.Name))
+                result.Add(optionSchema.Name!);
 
             if (optionSchema.ShortName != null)
                 result.Add(optionSchema.ShortName.Value.AsString());
@@ -102,43 +90,41 @@ namespace CliFx.Models
         /// <summary>
         /// Gets whether a command was specified in the input.
         /// </summary>
-        public static bool IsCommandSpecified(this CommandInput commandInput)
-        {
-            commandInput.GuardNotNull(nameof(commandInput));
-            return !commandInput.CommandName.IsNullOrWhiteSpace();
-        }
+        public static bool IsCommandSpecified(this CommandInput commandInput) => !string.IsNullOrWhiteSpace(commandInput.CommandName);
 
         /// <summary>
-        /// Gets whether help was requested in the input.
+        /// Gets whether debug directive was specified in the input.
         /// </summary>
-        public static bool IsHelpRequested(this CommandInput commandInput)
+        public static bool IsDebugDirectiveSpecified(this CommandInput commandInput) =>
+            commandInput.Directives.Contains("debug", StringComparer.OrdinalIgnoreCase);
+
+        /// <summary>
+        /// Gets whether preview directive was specified in the input.
+        /// </summary>
+        public static bool IsPreviewDirectiveSpecified(this CommandInput commandInput) =>
+            commandInput.Directives.Contains("preview", StringComparer.OrdinalIgnoreCase);
+
+        /// <summary>
+        /// Gets whether help option was specified in the input.
+        /// </summary>
+        public static bool IsHelpOptionSpecified(this CommandInput commandInput)
         {
-            commandInput.GuardNotNull(nameof(commandInput));
-
             var firstOption = commandInput.Options.FirstOrDefault();
-
             return firstOption != null && CommandOptionSchema.HelpOption.MatchesAlias(firstOption.Alias);
         }
 
         /// <summary>
-        /// Gets whether version information was requested in the input.
+        /// Gets whether version option was specified in the input.
         /// </summary>
-        public static bool IsVersionRequested(this CommandInput commandInput)
+        public static bool IsVersionOptionSpecified(this CommandInput commandInput)
         {
-            commandInput.GuardNotNull(nameof(commandInput));
-
             var firstOption = commandInput.Options.FirstOrDefault();
-
             return firstOption != null && CommandOptionSchema.VersionOption.MatchesAlias(firstOption.Alias);
         }
 
         /// <summary>
         /// Gets whether this command is the default command, i.e. without a name.
         /// </summary>
-        public static bool IsDefault(this CommandSchema commandSchema)
-        {
-            commandSchema.GuardNotNull(nameof(commandSchema));
-            return commandSchema.Name.IsNullOrWhiteSpace();
-        }
+        public static bool IsDefault(this CommandSchema commandSchema) => string.IsNullOrWhiteSpace(commandSchema.Name);
     }
 }

CliFx/Models/HelpTextSource.cs

@@ -1,5 +1,4 @@
 using System.Collections.Generic;
-using CliFx.Internal;
 
 namespace CliFx.Models
 {
@@ -30,9 +29,9 @@ namespace CliFx.Models
             IReadOnlyList<CommandSchema> availableCommandSchemas,
             CommandSchema targetCommandSchema)
         {
-            ApplicationMetadata = applicationMetadata.GuardNotNull(nameof(applicationMetadata));
-            AvailableCommandSchemas = availableCommandSchemas.GuardNotNull(nameof(availableCommandSchemas));
-            TargetCommandSchema = targetCommandSchema.GuardNotNull(nameof(targetCommandSchema));
+            ApplicationMetadata = applicationMetadata;
+            AvailableCommandSchemas = availableCommandSchemas;
+            TargetCommandSchema = targetCommandSchema;
         }
     }
 }

CliFx/Services/CommandFactory.cs

@@ -1,5 +1,4 @@
 using System;
-using CliFx.Internal;
 using CliFx.Models;
 
 namespace CliFx.Services
@@ -10,10 +9,6 @@ namespace CliFx.Services
     public class CommandFactory : ICommandFactory
     {
         /// <inheritdoc />
-        public ICommand CreateCommand(CommandSchema commandSchema)
-        {
-            commandSchema.GuardNotNull(nameof(commandSchema));
-            return (ICommand) Activator.CreateInstance(commandSchema.Type);
-        }
+        public ICommand CreateCommand(CommandSchema commandSchema) => (ICommand) Activator.CreateInstance(commandSchema.Type);
     }
 }

CliFx/Services/CommandInitializer.cs

@@ -11,42 +11,65 @@ namespace CliFx.Services
     public class CommandInitializer : ICommandInitializer
     {
         private readonly ICommandOptionInputConverter _commandOptionInputConverter;
+        private readonly IEnvironmentVariablesParser _environmentVariablesParser;
 
         /// <summary>
         /// Initializes an instance of <see cref="CommandInitializer"/>.
         /// </summary>
-        public CommandInitializer(ICommandOptionInputConverter commandOptionInputConverter)
+        public CommandInitializer(ICommandOptionInputConverter commandOptionInputConverter, IEnvironmentVariablesParser environmentVariablesParser)
+        {
+            _commandOptionInputConverter = commandOptionInputConverter;
+            _environmentVariablesParser = environmentVariablesParser;
+        }
+
+        /// <summary>
+        /// Initializes an instance of <see cref="CommandInitializer"/>.
+        /// </summary>
+        public CommandInitializer(IEnvironmentVariablesParser environmentVariablesParser)
+            : this(new CommandOptionInputConverter(), environmentVariablesParser)
         {
-            _commandOptionInputConverter = commandOptionInputConverter.GuardNotNull(nameof(commandOptionInputConverter));
         }
 
         /// <summary>
         /// Initializes an instance of <see cref="CommandInitializer"/>.
         /// </summary>
         public CommandInitializer()
-            : this(new CommandOptionInputConverter())
+            : this(new CommandOptionInputConverter(), new EnvironmentVariablesParser())
         {
         }
 
         /// <inheritdoc />
         public void InitializeCommand(ICommand command, CommandSchema commandSchema, CommandInput commandInput)
         {
-            command.GuardNotNull(nameof(command));
-            commandSchema.GuardNotNull(nameof(commandSchema));
-            commandInput.GuardNotNull(nameof(commandInput));
-
             // Keep track of unset required options to report an error at a later stage
             var unsetRequiredOptions = commandSchema.Options.Where(o => o.IsRequired).ToList();
 
-            // Set command options
-            foreach (var optionInput in commandInput.Options)
+            //Set command options
+            foreach (var optionSchema in commandSchema.Options)
             {
-                // Find matching option schema for this option input
-                var optionSchema = commandSchema.Options.FindByAlias(optionInput.Alias);
-                if (optionSchema == null)
+                // Ignore special options that are not backed by a property
+                if (optionSchema.Property == null)
+                    continue;
+
+                //Find matching option input
+                var optionInput = commandInput.Options.FindByOptionSchema(optionSchema);
+
+                //If no option input is available fall back to environment variable values
+                if (optionInput == null && !string.IsNullOrWhiteSpace(optionSchema.EnvironmentVariableName))
+                {
+                    var fallbackEnvironmentVariableExists = commandInput.EnvironmentVariables.ContainsKey(optionSchema.EnvironmentVariableName!);
+
+                    //If no environment variable is found or there is no valid value for this option skip it
+                    if (!fallbackEnvironmentVariableExists || string.IsNullOrWhiteSpace(commandInput.EnvironmentVariables[optionSchema.EnvironmentVariableName!]))
+                        continue;
+
+                    optionInput = _environmentVariablesParser.GetCommandOptionInputFromEnvironmentVariable(commandInput.EnvironmentVariables[optionSchema.EnvironmentVariableName!], optionSchema);
+                }
+
+                //No fallback available and no option input was specified, skip option
+                if (optionInput == null)
                     continue;
 
-                // Convert option to the type of the underlying property
                 var convertedValue = _commandOptionInputConverter.ConvertOptionInput(optionInput, optionSchema.Property.PropertyType);
 
                 // Set value of the underlying property
@@ -61,7 +84,7 @@ namespace CliFx.Services
             if (unsetRequiredOptions.Any())
             {
                 var unsetRequiredOptionNames = unsetRequiredOptions.Select(o => o.GetAliases().FirstOrDefault()).JoinToString(", ");
-                throw new MissingCommandOptionInputException($"One or more required options were not set: {unsetRequiredOptionNames}.");
+                throw new CliFxException($"One or more required options were not set: {unsetRequiredOptionNames}.");
             }
         }
     }

CliFx/Services/CommandInputParser.cs

@@ -12,14 +12,32 @@ namespace CliFx.Services
     /// </summary>
     public class CommandInputParser : ICommandInputParser
     {
+        private readonly IEnvironmentVariablesProvider _environmentVariablesProvider;
+
+        /// <summary>
+        /// Initializes an instance of <see cref="CommandInputParser"/>
+        /// </summary>
+        public CommandInputParser(IEnvironmentVariablesProvider environmentVariablesProvider)
+        {
+            _environmentVariablesProvider = environmentVariablesProvider;
+        }
+
+        /// <summary>
+        /// Initializes an instance of <see cref="CommandInputParser"/>
+        /// </summary>
+        public CommandInputParser()
+            : this(new EnvironmentVariablesProvider())
+        {
+        }
+
         /// <inheritdoc />
         public CommandInput ParseCommandInput(IReadOnlyList<string> commandLineArguments)
         {
-            commandLineArguments.GuardNotNull(nameof(commandLineArguments));
-
             var commandNameBuilder = new StringBuilder();
+            var directives = new List<string>();
             var optionsDic = new Dictionary<string, List<string>>();
 
+            // Option aliases and values are parsed in pairs so we need to keep track of last alias
             var lastOptionAlias = "";
 
             foreach (var commandLineArgument in commandLineArguments)
@@ -34,7 +52,7 @@ namespace CliFx.Services
                         optionsDic[lastOptionAlias] = new List<string>();
                 }
 
-                // Encountered short option name or multiple thereof
+                // Encountered short option name or multiple short option names
                 else if (commandLineArgument.StartsWith("-", StringComparison.OrdinalIgnoreCase))
                 {
                     // Handle stacked options
@@ -48,15 +66,26 @@ namespace CliFx.Services
                     }
                 }
 
-                // Encountered command name or part thereof
-                else if (lastOptionAlias.IsNullOrWhiteSpace())
+                // Encountered directive or (part of) command name
+                else if (string.IsNullOrWhiteSpace(lastOptionAlias))
                 {
-                    commandNameBuilder.AppendIfNotEmpty(' ');
-                    commandNameBuilder.Append(commandLineArgument);
+                    if (commandLineArgument.StartsWith("[", StringComparison.OrdinalIgnoreCase) &&
+                        commandLineArgument.EndsWith("]", StringComparison.OrdinalIgnoreCase))
+                    {
+                        // Extract directive
+                        var directive = commandLineArgument.Substring(1, commandLineArgument.Length - 2);
+
+                        directives.Add(directive);
+                    }
+                    else
+                    {
+                        commandNameBuilder.AppendIfNotEmpty(' ');
+                        commandNameBuilder.Append(commandLineArgument);
+                    }
                 }
 
                 // Encountered option value
-                else if (!lastOptionAlias.IsNullOrWhiteSpace())
+                else if (!string.IsNullOrWhiteSpace(lastOptionAlias))
                 {
                     optionsDic[lastOptionAlias].Add(commandLineArgument);
                 }
@@ -65,7 +94,9 @@ namespace CliFx.Services
             var commandName = commandNameBuilder.Length > 0 ? commandNameBuilder.ToString() : null;
             var options = optionsDic.Select(p => new CommandOptionInput(p.Key, p.Value)).ToArray();
 
-            return new CommandInput(commandName, options);
+            var environmentVariables = _environmentVariablesProvider.GetEnvironmentVariables();
+
+            return new CommandInput(commandName, directives, options, environmentVariables);
         }
     }
 }

CliFx/Services/CommandOptionInputConverter.cs

@@ -20,7 +20,7 @@ namespace CliFx.Services
         /// </summary>
         public CommandOptionInputConverter(IFormatProvider formatProvider)
         {
-            _formatProvider = formatProvider.GuardNotNull(nameof(formatProvider));
+            _formatProvider = formatProvider;
         }
 
         /// <summary>
@@ -31,7 +31,10 @@ namespace CliFx.Services
         {
         }
 
-        private object ConvertValue(string value, Type targetType)
+        /// <summary>
+        /// Converts a single string value to specified target type.
+        /// </summary>
+        protected virtual object? ConvertValue(string value, Type targetType)
         {
             try
             {
@@ -41,7 +44,7 @@ namespace CliFx.Services
 
                 // Bool
                 if (targetType == typeof(bool))
-                    return value.IsNullOrWhiteSpace() || bool.Parse(value);
+                    return string.IsNullOrWhiteSpace(value) || bool.Parse(value);
 
                 // Char
                 if (targetType == typeof(char))
@@ -108,9 +111,9 @@ namespace CliFx.Services
                     return Enum.Parse(targetType, value, true);
 
                 // Nullable
-                var nullableUnderlyingType = Nullable.GetUnderlyingType(targetType);
+                var nullableUnderlyingType = targetType.GetNullableUnderlyingType();
                 if (nullableUnderlyingType != null)
-                    return !value.IsNullOrWhiteSpace() ? ConvertValue(value, nullableUnderlyingType) : null;
+                    return !string.IsNullOrWhiteSpace(value) ? ConvertValue(value, nullableUnderlyingType) : null;
 
                 // Has a constructor that accepts a single string
                 var stringConstructor = GetStringConstructor(targetType);
@@ -126,48 +129,63 @@ namespace CliFx.Services
                 var parseMethod = GetStaticParseMethod(targetType);
                 if (parseMethod != null)
                     return parseMethod.Invoke(null, new object[] {value});
-
-                throw new InvalidCommandOptionInputException($"Can't convert value [{value}] to type [{targetType}].");
             }
             catch (Exception ex)
             {
-                throw new InvalidCommandOptionInputException($"Can't convert value [{value}] to type [{targetType}].", ex);
+                // Wrap and rethrow exceptions that occur when trying to convert the value
+                throw new CliFxException($"Can't convert value [{value}] to type [{targetType}].", ex);
             }
+
+            // Throw if we can't find a way to convert the value
+            throw new CliFxException($"Can't convert value [{value}] to type [{targetType}].");
         }
 
         /// <inheritdoc />
-        public object ConvertOptionInput(CommandOptionInput optionInput, Type targetType)
+        public virtual object? ConvertOptionInput(CommandOptionInput optionInput, Type targetType)
         {
-            optionInput.GuardNotNull(nameof(optionInput));
-            targetType.GuardNotNull(nameof(targetType));
+            // Get the underlying type of IEnumerable<T> if it's implemented by the target type.
+            // Ignore string type because it's IEnumerable<T> but we don't treat it as such.
+            var enumerableUnderlyingType = targetType != typeof(string) ? targetType.GetEnumerableUnderlyingType() : null;
 
-            // Single value
-            if (optionInput.Values.Count <= 1)
+            // Convert to a non-enumerable type
+            if (enumerableUnderlyingType == null)
             {
+                // Throw if provided with more than 1 value
+                if (optionInput.Values.Count > 1)
+                {
+                    throw new CliFxException(
+                        $"Can't convert a sequence of values [{optionInput.Values.JoinToString(", ")}] " +
+                        $"to non-enumerable type [{targetType}].");
+                }
+
+                // Retrieve a single value and convert
                 var value = optionInput.Values.SingleOrDefault();
                 return ConvertValue(value, targetType);
             }
-            // Multiple values
+            // Convert to an enumerable type
             else
             {
-                // Determine underlying type of elements inside the target collection type
-                var underlyingType = targetType.GetEnumerableUnderlyingType() ?? typeof(object);
+                // Convert values to the underlying enumerable type and cast it to dynamic array
+                var convertedValues = optionInput.Values
+                    .Select(v => ConvertValue(v, enumerableUnderlyingType))
+                    .ToNonGenericArray(enumerableUnderlyingType);
 
-                // Convert values to that type
-                var convertedValues = optionInput.Values.Select(v => ConvertValue(v, underlyingType)).ToNonGenericArray(underlyingType);
+                // Get the type of produced array
                 var convertedValuesType = convertedValues.GetType();
 
-                // Assignable from array of values (e.g. T[], IReadOnlyList<T>, IEnumerable<T>)
+                // Try to assign the array (works for T[], IReadOnlyList<T>, IEnumerable<T>, etc)
                 if (targetType.IsAssignableFrom(convertedValuesType))
                     return convertedValues;
 
-                // Has a constructor that accepts an array of values (e.g. HashSet<T>, List<T>)
+                // Try to inject the array into the constructor (works for HashSet<T>, List<T>, etc)
                 var arrayConstructor = targetType.GetConstructor(new[] {convertedValuesType});
                 if (arrayConstructor != null)
                     return arrayConstructor.Invoke(new object[] {convertedValues});
 
-                throw new InvalidCommandOptionInputException(
-                    $"Can't convert sequence of values [{optionInput.Values.JoinToString(", ")}] to type [{targetType}].");
+                // Throw if we can't find a way to convert the values
+                throw new CliFxException(
+                    $"Can't convert a sequence of values [{optionInput.Values.JoinToString(", ")}] " +
+                    $"to type [{targetType}].");
             }
         }
     }

CliFx/Services/CommandSchemaResolver.cs

@@ -14,114 +14,109 @@ namespace CliFx.Services
     /// </summary>
     public class CommandSchemaResolver : ICommandSchemaResolver
     {
-        private CommandOptionSchema GetCommandOptionSchema(PropertyInfo optionProperty)
+        private IReadOnlyList<CommandOptionSchema> GetCommandOptionSchemas(Type commandType)
         {
-            var attribute = optionProperty.GetCustomAttribute<CommandOptionAttribute>();
+            var result = new List<CommandOptionSchema>();
 
-            if (attribute == null)
-                return null;
+            foreach (var property in commandType.GetProperties())
+            {
+                var attribute = property.GetCustomAttribute<CommandOptionAttribute>();
 
-            return new CommandOptionSchema(optionProperty,
-                attribute.Name,
-                attribute.ShortName,
-                attribute.IsRequired,
-                attribute.Description);
-        }
+                // If an attribute is not set, then it's not an option so we just skip it
+                if (attribute == null)
+                    continue;
 
-        private CommandSchema GetCommandSchema(Type commandType)
-        {
-            // Attribute is optional for commands in order to reduce runtime rule complexity
-            var attribute = commandType.GetCustomAttribute<CommandAttribute>();
+                // Build option schema
+                var optionSchema = new CommandOptionSchema(property,
+                    attribute.Name,
+                    attribute.ShortName,
+                    attribute.IsRequired,
+                    attribute.Description,
+                    attribute.EnvironmentVariableName);
 
-            var options = commandType.GetProperties().Select(GetCommandOptionSchema).ExceptNull().ToArray();
+                // Make sure there are no other options with the same name
+                var existingOptionWithSameName = result
+                    .Where(o => !string.IsNullOrWhiteSpace(o.Name))
+                    .FirstOrDefault(o => string.Equals(o.Name, optionSchema.Name, StringComparison.OrdinalIgnoreCase));
 
-            return new CommandSchema(commandType,
-                attribute?.Name,
-                attribute?.Description,
-                options);
+                if (existingOptionWithSameName != null)
+                {
+                    throw new CliFxException(
+                        $"Command type [{commandType}] has options defined with the same name: " +
+                        $"[{existingOptionWithSameName.Property}] and [{optionSchema.Property}].");
+                }
+
+                // Make sure there are no other options with the same short name
+                var existingOptionWithSameShortName = result
+                    .Where(o => o.ShortName != null)
+                    .FirstOrDefault(o => o.ShortName == optionSchema.ShortName);
+
+                if (existingOptionWithSameShortName != null)
+                {
+                    throw new CliFxException(
+                        $"Command type [{commandType}] has options defined with the same short name: " +
+                        $"[{existingOptionWithSameShortName.Property}] and [{optionSchema.Property}].");
+                }
+
+                // Add schema to list
+                result.Add(optionSchema);
+            }
+
+            return result;
         }
 
         /// <inheritdoc />
         public IReadOnlyList<CommandSchema> GetCommandSchemas(IReadOnlyList<Type> commandTypes)
         {
-            commandTypes.GuardNotNull(nameof(commandTypes));
-
-            // Get command schemas
-            var commandSchemas = commandTypes.Select(GetCommandSchema).ToArray();
-
-            // Throw if there are no commands defined
-            if (!commandSchemas.Any())
+            // Make sure there's at least one command defined
+            if (!commandTypes.Any())
             {
-                throw new InvalidCommandSchemaException("There are no commands defined.");
+                throw new CliFxException("There are no commands defined.");
             }
 
-            // Throw if there are multiple commands with the same name
-            var nonUniqueCommandNames = commandSchemas
-                .Select(c => c.Name)
-                .GroupBy(i => i, StringComparer.OrdinalIgnoreCase)
-                .Where(g => g.Count() >= 2)
-                .SelectMany(g => g)
-                .Distinct(StringComparer.OrdinalIgnoreCase)
-                .ToArray();
+            var result = new List<CommandSchema>();
 
-            foreach (var commandName in nonUniqueCommandNames)
+            foreach (var commandType in commandTypes)
             {
-                throw new InvalidCommandSchemaException(!commandName.IsNullOrWhiteSpace()
-                    ? $"There are multiple commands defined with name [{commandName}]."
-                    : "There are multiple default commands defined.");
-            }
-
-            // Throw if there are commands that don't implement ICommand
-            var nonImplementedCommandNames = commandSchemas
-                .Where(c => !c.Type.Implements(typeof(ICommand)))
-                .Select(c => c.Name)
-                .Distinct(StringComparer.OrdinalIgnoreCase)
-                .ToArray();
-
-            foreach (var commandName in nonImplementedCommandNames)
-            {
-                throw new InvalidCommandSchemaException(!commandName.IsNullOrWhiteSpace()
-                    ? $"Command [{commandName}] doesn't implement ICommand."
-                    : "Default command doesn't implement ICommand.");
-            }
-
-            // Throw if there are multiple options with the same name inside the same command
-            foreach (var commandSchema in commandSchemas)
-            {
-                var nonUniqueOptionNames = commandSchema.Options
-                    .Where(o => !o.Name.IsNullOrWhiteSpace())
-                    .Select(o => o.Name)
-                    .GroupBy(i => i, StringComparer.OrdinalIgnoreCase)
-                    .Where(g => g.Count() >= 2)
-                    .SelectMany(g => g)
-                    .Distinct(StringComparer.OrdinalIgnoreCase)
-                    .ToArray();
-
-                foreach (var optionName in nonUniqueOptionNames)
+                // Make sure command type implements ICommand.
+                if (!commandType.Implements(typeof(ICommand)))
                 {
-                    throw new InvalidCommandSchemaException(!commandSchema.Name.IsNullOrWhiteSpace()
-                        ? $"There are multiple options defined with name [{optionName}] on command [{commandSchema.Name}]."
-                        : $"There are multiple options defined with name [{optionName}] on default command.");
+                    throw new CliFxException($"Command type [{commandType}] must implement {typeof(ICommand)}.");
                 }
 
-                var nonUniqueOptionShortNames = commandSchema.Options
-                    .Where(o => o.ShortName != null)
-                    .Select(o => o.ShortName.Value)
-                    .GroupBy(i => i)
-                    .Where(g => g.Count() >= 2)
-                    .SelectMany(g => g)
-                    .Distinct()
-                    .ToArray();
+                // Get attribute
+                var attribute = commandType.GetCustomAttribute<CommandAttribute>();
 
-                foreach (var optionShortName in nonUniqueOptionShortNames)
+                // Make sure attribute is set
+                if (attribute == null)
                 {
-                    throw new InvalidCommandSchemaException(!commandSchema.Name.IsNullOrWhiteSpace()
-                        ? $"There are multiple options defined with short name [{optionShortName}] on command [{commandSchema.Name}]."
-                        : $"There are multiple options defined with short name [{optionShortName}] on default command.");
+                    throw new CliFxException($"Command type [{commandType}] must be annotated with [{typeof(CommandAttribute)}].");
                 }
+
+                // Get option schemas
+                var optionSchemas = GetCommandOptionSchemas(commandType);
+
+                // Build command schema
+                var commandSchema = new CommandSchema(commandType,
+                    attribute.Name,
+                    attribute.Description,
+                    optionSchemas);
+
+                // Make sure there are no other commands with the same name
+                var existingCommandWithSameName = result
+                    .FirstOrDefault(c => string.Equals(c.Name, commandSchema.Name, StringComparison.OrdinalIgnoreCase));
+
+                if (existingCommandWithSameName != null)
+                {
+                    throw new CliFxException(
+                        $"Command type [{existingCommandWithSameName.Type}] has the same name as another command type [{commandType}].");
+                }
+
+                // Add schema to list
+                result.Add(commandSchema);
             }
 
-            return commandSchemas;
+            return result;
         }
     }
 }

CliFx/Services/DelegateCommandFactory.cs

@@ -1,5 +1,4 @@
 using System;
-using CliFx.Internal;
 using CliFx.Models;
 
 namespace CliFx.Services
@@ -16,14 +15,10 @@ namespace CliFx.Services
         /// </summary>
         public DelegateCommandFactory(Func<CommandSchema, ICommand> factoryMethod)
         {
-            _factoryMethod = factoryMethod.GuardNotNull(nameof(factoryMethod));
+            _factoryMethod = factoryMethod;
         }
 
         /// <inheritdoc />
-        public ICommand CreateCommand(CommandSchema commandSchema)
-        {
-            commandSchema.GuardNotNull(nameof(commandSchema));
-            return _factoryMethod(commandSchema);
-        }
+        public ICommand CreateCommand(CommandSchema commandSchema) => _factoryMethod(commandSchema);
     }
 }

CliFx/Services/EnvironmentVariablesParser.cs

@@ -0,0 +1,27 @@
+using System.IO;
+using System.Linq;
+using CliFx.Internal;
+using CliFx.Models;
+
+namespace CliFx.Services
+{
+    /// <inheritdoct />
+    public class EnvironmentVariablesParser : IEnvironmentVariablesParser
+    {
+        /// <inheritdoct />
+        public CommandOptionInput GetCommandOptionInputFromEnvironmentVariable(string environmentVariableValue, CommandOptionSchema targetOptionSchema)
+        {
+            //If the option is not a collection do not split environment variable values
+            var optionIsCollection = targetOptionSchema.Property != null && targetOptionSchema.Property.PropertyType.IsCollection();
+
+            if (!optionIsCollection) return new CommandOptionInput(targetOptionSchema.EnvironmentVariableName, environmentVariableValue);
+
+            //If the option is a collection split the values using System separator, empty values are discarded
+            var environmentVariableValues = environmentVariableValue.Split(Path.PathSeparator)
+                .Where(v => !string.IsNullOrWhiteSpace(v))
+                .ToList();
+
+            return new CommandOptionInput(targetOptionSchema.EnvironmentVariableName, environmentVariableValues);
+        }
+    }
+}

CliFx/Services/EnvironmentVariablesProvider.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Security;
+
+namespace CliFx.Services
+{
+    /// <inheritdoc />
+    public class EnvironmentVariablesProvider : IEnvironmentVariablesProvider
+    {
+        /// <inheritdoc />
+        public IReadOnlyDictionary<string, string> GetEnvironmentVariables()
+        {
+            try
+            {
+                var environmentVariables = Environment.GetEnvironmentVariables();
+
+                //Constructing the dictionary manually allows to specify a key comparer that ignores case
+                //This allows to ignore casing when looking for a fallback environment variable of an option
+                var environmentVariablesAsDictionary = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+
+                //Type DictionaryEntry must be explicitly used otherwise it will enumerate as a collection of objects
+                foreach (DictionaryEntry environmentVariable in environmentVariables)
+                {
+                    environmentVariablesAsDictionary.Add(environmentVariable.Key.ToString(), environmentVariable.Value.ToString());
+                }
+
+                return environmentVariablesAsDictionary;
+            }
+            catch (SecurityException)
+            {
+                return new Dictionary<string, string>();
+            }
+        }
+    }
+}

CliFx/Services/Extensions.cs

@@ -1,5 +1,4 @@
 using System;
-using CliFx.Internal;
 
 namespace CliFx.Services
 {
@@ -13,9 +12,6 @@ namespace CliFx.Services
         /// </summary>
         public static void WithForegroundColor(this IConsole console, ConsoleColor foregroundColor, Action action)
         {
-            console.GuardNotNull(nameof(console));
-            action.GuardNotNull(nameof(action));
-
             var lastColor = console.ForegroundColor;
             console.ForegroundColor = foregroundColor;
 
@@ -29,9 +25,6 @@ namespace CliFx.Services
         /// </summary>
         public static void WithBackgroundColor(this IConsole console, ConsoleColor backgroundColor, Action action)
         {
-            console.GuardNotNull(nameof(console));
-            action.GuardNotNull(nameof(action));
-
             var lastColor = console.BackgroundColor;
             console.BackgroundColor = backgroundColor;
 
@@ -43,12 +36,7 @@ namespace CliFx.Services
         /// <summary>
         /// Sets console foreground and background colors, executes specified action, and sets the colors back to the original values.
         /// </summary>
-        public static void WithColors(this IConsole console, ConsoleColor foregroundColor, ConsoleColor backgroundColor, Action action)
-        {
-            console.GuardNotNull(nameof(console));
-            action.GuardNotNull(nameof(action));
-
+        public static void WithColors(this IConsole console, ConsoleColor foregroundColor, ConsoleColor backgroundColor, Action action) =>
             console.WithForegroundColor(foregroundColor, () => console.WithBackgroundColor(backgroundColor, action));
-        }
     }
 }

CliFx/Services/HelpTextRenderer.cs

@@ -14,9 +14,6 @@ namespace CliFx.Services
         /// <inheritdoc />
         public void RenderHelpText(IConsole console, HelpTextSource source)
         {
-            console.GuardNotNull(nameof(console));
-            source.GuardNotNull(nameof(source));
-
             // Track position
             var column = 0;
             var row = 0;
@@ -105,7 +102,7 @@ namespace CliFx.Services
                 RenderNewLine();
 
                 // Description
-                if (!source.ApplicationMetadata.Description.IsNullOrWhiteSpace())
+                if (!string.IsNullOrWhiteSpace(source.ApplicationMetadata.Description))
                 {
                     Render(source.ApplicationMetadata.Description);
                     RenderNewLine();
@@ -114,7 +111,7 @@ namespace CliFx.Services
 
             void RenderDescription()
             {
-                if (source.TargetCommandSchema.Description.IsNullOrWhiteSpace())
+                if (string.IsNullOrWhiteSpace(source.TargetCommandSchema.Description))
                     return;
 
                 // Margin
@@ -142,7 +139,7 @@ namespace CliFx.Services
                 Render(source.ApplicationMetadata.ExecutableName);
 
                 // Command name
-                if (!source.TargetCommandSchema.IsDefault())
+                if (!string.IsNullOrWhiteSpace(source.TargetCommandSchema.Name))
                 {
                     Render(" ");
                     RenderWithColor(source.TargetCommandSchema.Name, ConsoleColor.Cyan);
@@ -169,8 +166,14 @@ namespace CliFx.Services
                 // Header
                 RenderHeader("Options");
 
+                // Order options and append built-in options
+                var allOptionSchemas = source.TargetCommandSchema.Options
+                    .OrderByDescending(o => o.IsRequired)
+                    .Concat(builtInOptionSchemas)
+                    .ToArray();
+
                 // Options
-                foreach (var optionSchema in source.TargetCommandSchema.Options.Concat(builtInOptionSchemas))
+                foreach (var optionSchema in allOptionSchemas)
                 {
                     // Is required
                     if (optionSchema.IsRequired)
@@ -189,19 +192,19 @@ namespace CliFx.Services
                     }
 
                     // Delimiter
-                    if (!optionSchema.Name.IsNullOrWhiteSpace() && optionSchema.ShortName != null)
+                    if (!string.IsNullOrWhiteSpace(optionSchema.Name) && optionSchema.ShortName != null)
                     {
                         Render("|");
                     }
 
                     // Name
-                    if (!optionSchema.Name.IsNullOrWhiteSpace())
+                    if (!string.IsNullOrWhiteSpace(optionSchema.Name))
                     {
                         RenderWithColor($"--{optionSchema.Name}", ConsoleColor.White);
                     }
 
                     // Description
-                    if (!optionSchema.Description.IsNullOrWhiteSpace())
+                    if (!string.IsNullOrWhiteSpace(optionSchema.Description))
                     {
                         RenderColumnIndent();
                         Render(optionSchema.Description);
@@ -225,14 +228,14 @@ namespace CliFx.Services
                 // Child commands
                 foreach (var childCommandSchema in childCommandSchemas)
                 {
-                    var relativeCommandName = GetRelativeCommandName(childCommandSchema, source.TargetCommandSchema);
+                    var relativeCommandName = GetRelativeCommandName(childCommandSchema, source.TargetCommandSchema)!;
 
                     // Name
                     RenderIndent();
                     RenderWithColor(relativeCommandName, ConsoleColor.Cyan);
 
                     // Description
-                    if (!childCommandSchema.Description.IsNullOrWhiteSpace())
+                    if (!string.IsNullOrWhiteSpace(childCommandSchema.Description))
                     {
                         RenderColumnIndent();
                         Render(childCommandSchema.Description);
@@ -248,7 +251,7 @@ namespace CliFx.Services
                 Render("You can run `");
                 Render(source.ApplicationMetadata.ExecutableName);
 
-                if (!source.TargetCommandSchema.IsDefault())
+                if (!string.IsNullOrWhiteSpace(source.TargetCommandSchema.Name))
                 {
                     Render(" ");
                     RenderWithColor(source.TargetCommandSchema.Name, ConsoleColor.Cyan);
@@ -279,8 +282,8 @@ namespace CliFx.Services
 
     public partial class HelpTextRenderer
     {
-        private static string GetRelativeCommandName(CommandSchema commandSchema, CommandSchema parentCommandSchema) =>
-            parentCommandSchema.Name.IsNullOrWhiteSpace()
+        private static string? GetRelativeCommandName(CommandSchema commandSchema, CommandSchema parentCommandSchema) =>
+            string.IsNullOrWhiteSpace(parentCommandSchema.Name) || string.IsNullOrWhiteSpace(commandSchema.Name)
                 ? commandSchema.Name
                 : commandSchema.Name.Substring(parentCommandSchema.Name.Length + 1);
     }