Update version

Fixes #123

Changelog.md

@@ -1,3 +1,17 @@
+### v2.2.2 (30-Jan-2022)
+
+- Fixed an issue where `ConsoleWriter` and `ConsoleReader` were not properly thread-safe.
+- Fixed an issue where the analyzer failed to load under certain circumstances when running inside Visual Studio.
+
+### v2.2.1 (16-Jan-2022)
+
+- Fixed an issue which caused help text to not show default values for optional parameters. (Thanks [@AliReZa Sabouri](https://github.com/alirezanet))
+
+### v2.2 (11-Jan-2022)
+
+- Added support for optional parameters. A parameter can be marked as optional by setting `IsRequired = false` on the attribute. Only one parameter is allowed to be optional and such parameter must be the last in order. (Thanks [@AliReZa Sabouri](https://github.com/alirezanet))
+- Fixed an issue where parameters and options bound to properties implemented as default interface members were not working correctly. (Thanks [@AliReZa Sabouri](https://github.com/alirezanet))
+
 ### v2.1 (04-Jan-2022)
 
 - Added `IConsole.Clear()` with corresponding implementations in `SystemConsole`, `FakeConsole`, and `FakeInMemoryConsole`. (Thanks [@Alex Rosenfeld](https://github.com/alexrosenfeld10))

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

@@ -2,8 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
-    <IsPackable>false</IsPackable>
-    <IsTestProject>true</IsTestProject>
     <CollectCoverage>true</CollectCoverage>
     <CoverletOutputFormat>opencover</CoverletOutputFormat>
   </PropertyGroup>
@@ -15,7 +13,7 @@
   <ItemGroup>
     <PackageReference Include="Basic.Reference.Assemblies" Version="1.2.4" />
     <PackageReference Include="GitHubActionsTestLogger" Version="1.2.0" />
-    <PackageReference Include="FluentAssertions" Version="6.2.0" />
+    <PackageReference Include="FluentAssertions" Version="6.3.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.0.0" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.0.1" />
     <PackageReference Include="xunit" Version="2.4.1" />

CliFx.Analyzers.Tests/OptionMustHaveNameOrShortNameAnalyzerSpecs.cs

@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(null)]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -38,7 +38,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -57,7 +57,7 @@ public class MyCommand : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers.Tests/OptionMustHaveUniqueNameAnalyzerSpecs.cs

@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     [CommandOption(""foo"")]
     public string Bar { get; set; }
 
@@ -41,7 +41,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     [CommandOption(""bar"")]
     public string Bar { get; set; }
 
@@ -63,7 +63,7 @@ public class MyCommand : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers.Tests/OptionMustHaveUniqueShortNameAnalyzerSpecs.cs

@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     [CommandOption('f')]
     public string Bar { get; set; }
 
@@ -41,7 +41,7 @@ public class MyCommand : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     [CommandOption('b')]
     public string Bar { get; set; }
 
@@ -63,7 +63,7 @@ public class MyCommand : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     [CommandOption('F')]
     public string Bar { get; set; }
 
@@ -85,7 +85,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers.Tests/OptionMustHaveValidConverterAnalyzerSpecs.cs

@@ -24,7 +24,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"", Converter = typeof(MyConverter))]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -48,7 +48,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"", Converter = typeof(MyConverter))]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -67,7 +67,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers.Tests/OptionMustHaveValidNameAnalyzerSpecs.cs

@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""f"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -38,7 +38,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""1foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -57,7 +57,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -76,7 +76,7 @@ public class MyCommand : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers.Tests/OptionMustHaveValidShortNameAnalyzerSpecs.cs

@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandOption('1')]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -38,7 +38,7 @@ public class MyCommand : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -57,7 +57,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers.Tests/OptionMustHaveValidValidatorsAnalyzerSpecs.cs

@@ -24,7 +24,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"", Validators = new[] {typeof(MyValidator)})]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -48,7 +48,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"", Validators = new[] {typeof(MyValidator)})]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -67,7 +67,7 @@ public class MyCommand : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers.Tests/ParameterMustBeLastIfNonRequiredAnalyzerSpecs.cs

@@ -0,0 +1,94 @@
+using CliFx.Analyzers.Tests.Utils;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Xunit;
+
+namespace CliFx.Analyzers.Tests;
+
+public class ParameterMustBeLastIfNonRequiredAnalyzerSpecs
+{
+    private static DiagnosticAnalyzer Analyzer { get; } = new ParameterMustBeLastIfNonRequiredAnalyzer();
+
+    [Fact]
+    public void Analyzer_reports_an_error_if_a_non_required_parameter_is_not_the_last_in_order()
+    {
+        // Arrange
+        // language=cs
+        const string code = @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(0, Name = ""foo"", IsRequired = false)]
+    public string Foo { get; set; }
+
+    [CommandParameter(1, Name = ""bar"")]
+    public string Bar { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}";
+
+        // Act & assert
+        Analyzer.Should().ProduceDiagnostics(code);
+    }
+
+    [Fact]
+    public void Analyzer_does_not_report_an_error_if_a_non_required_parameter_is_the_last_in_order()
+    {
+        // Arrange
+        // language=cs
+        const string code = @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(0, Name = ""foo"")]
+    public string Foo { get; set; }
+
+    [CommandParameter(1, Name = ""bar"", IsRequired = false)]
+    public string Bar { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}";
+
+        // Act & assert
+        Analyzer.Should().NotProduceDiagnostics(code);
+    }
+
+    [Fact]
+    public void Analyzer_does_not_report_an_error_if_no_non_required_parameters_are_defined()
+    {
+        // Arrange
+        // language=cs
+        const string code = @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(0, Name = ""foo"")]
+    public string Foo { get; set; }
+
+    [CommandParameter(1, Name = ""bar"", IsRequired = true)]
+    public string Bar { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}";
+
+        // Act & assert
+        Analyzer.Should().NotProduceDiagnostics(code);
+    }
+
+    [Fact]
+    public void Analyzer_does_not_report_an_error_on_a_property_that_is_not_a_parameter()
+    {
+        // Arrange
+        // language=cs
+        const string code = @"
+[Command]
+public class MyCommand : ICommand
+{
+    public string Foo { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}";
+
+        // Act & assert
+        Analyzer.Should().NotProduceDiagnostics(code);
+    }
+}

CliFx.Analyzers.Tests/ParameterMustBeLastIfNonScalarAnalyzerSpecs.cs

@@ -9,7 +9,7 @@ public class ParameterMustBeLastIfNonScalarAnalyzerSpecs
     private static DiagnosticAnalyzer Analyzer { get; } = new ParameterMustBeLastIfNonScalarAnalyzer();
 
     [Fact]
-    public void Analyzer_reports_an_error_if_a_non_scalar_parameter_is_not_last_in_order()
+    public void Analyzer_reports_an_error_if_a_non_scalar_parameter_is_not_the_last_in_order()
     {
         // Arrange
         // language=cs
@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string[] Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string Bar { get; set; }
 
@@ -31,7 +31,7 @@ public class MyCommand : ICommand
     }
 
     [Fact]
-    public void Analyzer_does_not_report_an_error_if_a_non_scalar_parameter_is_last_in_order()
+    public void Analyzer_does_not_report_an_error_if_a_non_scalar_parameter_is_the_last_in_order()
     {
         // Arrange
         // language=cs
@@ -41,7 +41,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string[] Bar { get; set; }
 
@@ -63,7 +63,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string Bar { get; set; }
 

CliFx.Analyzers.Tests/ParameterMustBeSingleIfNonRequiredAnalyzerSpecs.cs

@@ -0,0 +1,94 @@
+using CliFx.Analyzers.Tests.Utils;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Xunit;
+
+namespace CliFx.Analyzers.Tests;
+
+public class ParameterMustBeSingleIfNonRequiredAnalyzerSpecs
+{
+    private static DiagnosticAnalyzer Analyzer { get; } = new ParameterMustBeSingleIfNonRequiredAnalyzer();
+
+    [Fact]
+    public void Analyzer_reports_an_error_if_more_than_one_non_required_parameters_are_defined()
+    {
+        // Arrange
+        // language=cs
+        const string code = @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(0, IsRequired = false)]
+    public string Foo { get; set; }
+
+    [CommandParameter(1, IsRequired = false)]
+    public string Bar { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}";
+
+        // Act & assert
+        Analyzer.Should().ProduceDiagnostics(code);
+    }
+
+    [Fact]
+    public void Analyzer_does_not_report_an_error_if_only_one_non_required_parameter_is_defined()
+    {
+        // Arrange
+        // language=cs
+        const string code = @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(0)]
+    public string Foo { get; set; }
+
+    [CommandParameter(1, IsRequired = false)]
+    public string Bar { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}";
+
+        // Act & assert
+        Analyzer.Should().NotProduceDiagnostics(code);
+    }
+
+    [Fact]
+    public void Analyzer_does_not_report_an_error_if_no_non_required_parameters_are_defined()
+    {
+        // Arrange
+        // language=cs
+        const string code = @"
+[Command]
+public class MyCommand : ICommand
+{
+    [CommandParameter(0)]
+    public string Foo { get; set; }
+
+    [CommandParameter(1, IsRequired = true)]
+    public string Bar { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}";
+
+        // Act & assert
+        Analyzer.Should().NotProduceDiagnostics(code);
+    }
+
+    [Fact]
+    public void Analyzer_does_not_report_an_error_on_a_property_that_is_not_a_parameter()
+    {
+        // Arrange
+        // language=cs
+        const string code = @"
+[Command]
+public class MyCommand : ICommand
+{
+    public string Foo { get; set; }
+
+    public ValueTask ExecuteAsync(IConsole console) => default;
+}";
+
+        // Act & assert
+        Analyzer.Should().NotProduceDiagnostics(code);
+    }
+}

CliFx.Analyzers.Tests/ParameterMustBeSingleIfNonScalarAnalyzerSpecs.cs

@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string[] Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string[] Bar { get; set; }
 
@@ -41,7 +41,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string[] Bar { get; set; }
 
@@ -63,7 +63,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string Bar { get; set; }
 

CliFx.Analyzers.Tests/ParameterMustHaveUniqueNameAnalyzerSpecs.cs

@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0, Name = ""foo"")]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1, Name = ""foo"")]
     public string Bar { get; set; }
 
@@ -41,7 +41,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0, Name = ""foo"")]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1, Name = ""bar"")]
     public string Bar { get; set; }
 

CliFx.Analyzers.Tests/ParameterMustHaveUniqueOrderAnalyzerSpecs.cs

@@ -19,7 +19,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(0)]
     public string Bar { get; set; }
 
@@ -41,7 +41,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string Bar { get; set; }
 

CliFx.Analyzers.Tests/ParameterMustHaveValidConverterAnalyzerSpecs.cs

@@ -24,7 +24,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0, Converter = typeof(MyConverter))]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -48,7 +48,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0, Converter = typeof(MyConverter))]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -67,7 +67,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers.Tests/ParameterMustHaveValidValidatorsAnalyzerSpecs.cs

@@ -24,7 +24,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0, Validators = new[] {typeof(MyValidator)})]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -48,7 +48,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0, Validators = new[] {typeof(MyValidator)})]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 
@@ -67,7 +67,7 @@ public class MyCommand : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }";
 

CliFx.Analyzers/CliFx.Analyzers.csproj

@@ -2,12 +2,13 @@
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
+    <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
     <Nullable>annotations</Nullable>
     <NoWarn>$(NoWarn);RS1025;RS1026</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="3.4.0" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.0.1" PrivateAssets="all" />
   </ItemGroup>
 
 </Project>

CliFx.Analyzers/ObjectModel/CommandOptionSymbol.cs

@@ -71,10 +71,9 @@ internal partial class CommandOptionSymbol
     {
         var attribute = TryGetOptionAttribute(property);
 
-        if (attribute is null)
+        return attribute is not null
-            return null;
+            ? FromAttribute(attribute)
-
+            : null;
-        return FromAttribute(attribute);
     }
 
     public static bool IsOptionProperty(IPropertySymbol property) =>

CliFx.Analyzers/ObjectModel/CommandParameterSymbol.cs

@@ -11,6 +11,8 @@ internal partial class CommandParameterSymbol
 
     public string? Name { get; }
 
+    public bool? IsRequired { get; }
+
     public ITypeSymbol? ConverterType { get; }
 
     public IReadOnlyList<ITypeSymbol> ValidatorTypes { get; }
@@ -18,11 +20,13 @@ internal partial class CommandParameterSymbol
     public CommandParameterSymbol(
         int order,
         string? name,
+        bool? isRequired,
         ITypeSymbol? converterType,
         IReadOnlyList<ITypeSymbol> validatorTypes)
     {
         Order = order;
         Name = name;
+        IsRequired = isRequired;
         ConverterType = converterType;
         ValidatorTypes = validatorTypes;
     }
@@ -37,7 +41,7 @@ internal partial class CommandParameterSymbol
 
     private static CommandParameterSymbol FromAttribute(AttributeData attribute)
     {
-        var order = (int) attribute
+        var order = (int)attribute
             .ConstructorArguments
             .Select(a => a.Value)
             .First()!;
@@ -48,6 +52,12 @@ internal partial class CommandParameterSymbol
             .Select(a => a.Value.Value)
             .FirstOrDefault() as string;
 
+        var isRequired = attribute
+            .NamedArguments
+            .Where(a => a.Key == "IsRequired")
+            .Select(a => a.Value.Value)
+            .FirstOrDefault() as bool?;
+
         var converter = attribute
             .NamedArguments
             .Where(a => a.Key == "Converter")
@@ -63,17 +73,16 @@ internal partial class CommandParameterSymbol
             .Cast<ITypeSymbol>()
             .ToArray();
 
-        return new CommandParameterSymbol(order, name, converter, validators);
+        return new CommandParameterSymbol(order, name, isRequired, converter, validators);
     }
 
     public static CommandParameterSymbol? TryResolve(IPropertySymbol property)
     {
         var attribute = TryGetParameterAttribute(property);
 
-        if (attribute is null)
+        return attribute is not null
-            return null;
+            ? FromAttribute(attribute)
-
+            : null;
-        return FromAttribute(attribute);
     }
 
     public static bool IsParameterProperty(IPropertySymbol property) =>

CliFx.Analyzers/ParameterMustBeLastIfNonRequiredAnalyzer.cs

@@ -0,0 +1,60 @@
+using System.Linq;
+using CliFx.Analyzers.ObjectModel;
+using CliFx.Analyzers.Utils.Extensions;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace CliFx.Analyzers;
+
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public class ParameterMustBeLastIfNonRequiredAnalyzer : AnalyzerBase
+{
+    public ParameterMustBeLastIfNonRequiredAnalyzer()
+        : base(
+            "Parameters marked as non-required must be the last in order",
+            "This parameter is non-required so it must be the last in order (its order must be highest within the command).")
+    {
+    }
+
+    private void Analyze(
+        SyntaxNodeAnalysisContext context,
+        PropertyDeclarationSyntax propertyDeclaration,
+        IPropertySymbol property)
+    {
+        if (property.ContainingType is null)
+            return;
+
+        var parameter = CommandParameterSymbol.TryResolve(property);
+        if (parameter is null)
+            return;
+
+        if (parameter.IsRequired != false)
+            return;
+
+        var otherProperties = property
+            .ContainingType
+            .GetMembers()
+            .OfType<IPropertySymbol>()
+            .Where(m => !m.Equals(property, SymbolEqualityComparer.Default))
+            .ToArray();
+
+        foreach (var otherProperty in otherProperties)
+        {
+            var otherParameter = CommandParameterSymbol.TryResolve(otherProperty);
+            if (otherParameter is null)
+                continue;
+
+            if (otherParameter.Order > parameter.Order)
+            {
+                context.ReportDiagnostic(CreateDiagnostic(propertyDeclaration.GetLocation()));
+            }
+        }
+    }
+
+    public override void Initialize(AnalysisContext context)
+    {
+        base.Initialize(context);
+        context.HandlePropertyDeclaration(Analyze);
+    }
+}

CliFx.Analyzers/ParameterMustBeLastIfNonScalarAnalyzer.cs

@@ -12,8 +12,8 @@ public class ParameterMustBeLastIfNonScalarAnalyzer : AnalyzerBase
 {
     public ParameterMustBeLastIfNonScalarAnalyzer()
         : base(
-            "Parameters of non-scalar types must be last in order",
+            "Parameters of non-scalar types must be the last in order",
-            "This parameter has a non-scalar type so it must be last in order (its order must be highest within the command).")
+            "This parameter has a non-scalar type so it must be the last in order (its order must be highest within the command).")
     {
     }
 

CliFx.Analyzers/ParameterMustBeSingleIfNonRequiredAnalyzer.cs

@@ -0,0 +1,60 @@
+using System.Linq;
+using CliFx.Analyzers.ObjectModel;
+using CliFx.Analyzers.Utils.Extensions;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace CliFx.Analyzers;
+
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public class ParameterMustBeSingleIfNonRequiredAnalyzer : AnalyzerBase
+{
+    public ParameterMustBeSingleIfNonRequiredAnalyzer()
+        : base(
+            "Parameters marked as non-required are limited to one per command",
+            "This parameter is non-required so it must be the only such parameter in the command.")
+    {
+    }
+
+    private void Analyze(
+        SyntaxNodeAnalysisContext context,
+        PropertyDeclarationSyntax propertyDeclaration,
+        IPropertySymbol property)
+    {
+        if (property.ContainingType is null)
+            return;
+
+        var parameter = CommandParameterSymbol.TryResolve(property);
+        if (parameter is null)
+            return;
+
+        if (parameter.IsRequired != false)
+            return;
+
+        var otherProperties = property
+            .ContainingType
+            .GetMembers()
+            .OfType<IPropertySymbol>()
+            .Where(m => !m.Equals(property, SymbolEqualityComparer.Default))
+            .ToArray();
+
+        foreach (var otherProperty in otherProperties)
+        {
+            var otherParameter = CommandParameterSymbol.TryResolve(otherProperty);
+            if (otherParameter is null)
+                continue;
+
+            if (otherParameter.IsRequired == false)
+            {
+                context.ReportDiagnostic(CreateDiagnostic(propertyDeclaration.GetLocation()));
+            }
+        }
+    }
+
+    public override void Initialize(AnalysisContext context)
+    {
+        base.Initialize(context);
+        context.HandlePropertyDeclaration(Analyze);
+    }
+}

CliFx.Benchmarks/CliFx.Benchmarks.csproj

@@ -10,7 +10,7 @@
     <PackageReference Include="clipr" Version="1.6.1" />
     <PackageReference Include="Cocona" Version="1.6.0" />
     <PackageReference Include="CommandLineParser" Version="2.8.0" />
-    <PackageReference Include="McMaster.Extensions.CommandLineUtils" Version="3.1.0" />
+    <PackageReference Include="McMaster.Extensions.CommandLineUtils" Version="4.0.0" />
     <PackageReference Include="PowerArgs" Version="3.6.0" />
     <PackageReference Include="System.CommandLine" Version="2.0.0-beta1.20574.7" />
   </ItemGroup>

CliFx.Tests/CliFx.Tests.csproj

@@ -2,8 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
-    <IsPackable>false</IsPackable>
-    <IsTestProject>true</IsTestProject>
     <CollectCoverage>true</CollectCoverage>
     <CoverletOutputFormat>opencover</CoverletOutputFormat>
   </PropertyGroup>
@@ -14,8 +12,8 @@
 
   <ItemGroup>
     <PackageReference Include="Basic.Reference.Assemblies" Version="1.2.4" />
-    <PackageReference Include="CliWrap" Version="3.3.3" />
+    <PackageReference Include="CliWrap" Version="3.4.0" />
-    <PackageReference Include="FluentAssertions" Version="6.2.0" />
+    <PackageReference Include="FluentAssertions" Version="6.3.0" />
     <PackageReference Include="GitHubActionsTestLogger" Version="1.2.0" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="4.0.1" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.0.0" />

CliFx.Tests/ConversionSpecs.cs

@@ -103,7 +103,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public bool Foo { get; set; }
-    
+
     [CommandOption('b')]
     public bool Bar { get; set; }
 
@@ -379,7 +379,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public int? Foo { get; set; }
-    
+
     [CommandOption('b')]
     public int? Bar { get; set; }
 
@@ -427,7 +427,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public CustomEnum? Foo { get; set; }
-    
+
     [CommandOption('b')]
     public CustomEnum? Bar { get; set; }
 
@@ -471,7 +471,7 @@ public class Command : ICommand
 public class CustomType
 {
     public string Value { get; }
-    
+
     public CustomType(string value) => Value = value;
 }
 
@@ -480,7 +480,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public CustomType Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo.Value);
@@ -516,9 +516,9 @@ public class Command : ICommand
 public class CustomTypeA
 {
     public string Value { get; }
-    
+
     private CustomTypeA(string value) => Value = value;
-    
+
     public static CustomTypeA Parse(string value) => 
         new CustomTypeA(value);
 }
@@ -526,9 +526,9 @@ public class CustomTypeA
 public class CustomTypeB
 {
     public string Value { get; }
-    
+
     private CustomTypeB(string value) => Value = value;
-    
+
     public static CustomTypeB Parse(string value, IFormatProvider formatProvider) =>
         new CustomTypeB(value);
 }
@@ -538,10 +538,10 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public CustomTypeA Foo { get; set; }
-    
+
     [CommandOption('b')]
     public CustomTypeB Bar { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(""Foo = "" + Foo.Value);
@@ -590,7 +590,7 @@ public class Command : ICommand
 {
     [CommandOption('f', Converter = typeof(CustomConverter))]
     public int Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo);
@@ -628,7 +628,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public string[] Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -672,7 +672,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public IReadOnlyList<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -716,7 +716,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public List<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -760,7 +760,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public int[] Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -804,7 +804,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public int Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -840,7 +840,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public CustomType Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -872,7 +872,7 @@ public class Command : ICommand
 public class CustomType : IEnumerable<object>
 {
     public IEnumerator<object> GetEnumerator() => Enumerable.Empty<object>().GetEnumerator();
-    
+
     IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 }
 
@@ -881,7 +881,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public CustomType Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -925,7 +925,7 @@ public class Command : ICommand
 {
     [CommandOption('f', Validators = new[] {typeof(ValidatorA), typeof(ValidatorB)})]
     public int Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -957,9 +957,9 @@ public class Command : ICommand
 public class CustomType
 {
     public string Value { get; }
-    
+
     private CustomType(string value) => Value = value;
-    
+
     public static CustomType Parse(string value) => throw new Exception(""Hello world"");
 }
 
@@ -968,7 +968,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public CustomType Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");

CliFx.Tests/EnvironmentSpecs.cs

@@ -31,7 +31,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", IsRequired = true, EnvironmentVariable = ""ENV_FOO"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo);
@@ -73,7 +73,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", EnvironmentVariable = ""ENV_FOO"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo);
@@ -115,12 +115,12 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", EnvironmentVariable = ""ENV_FOO"")]
     public IReadOnlyList<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
             console.Output.WriteLine(i);
-        
+
         return default;
     }
 }
@@ -162,7 +162,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", EnvironmentVariable = ""ENV_FOO"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo);
@@ -204,7 +204,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", EnvironmentVariable = ""ENV_FOO"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo);

CliFx.Tests/HelpTextSpecs.cs

@@ -339,13 +339,13 @@ public class Command : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string Bar { get; set; }
-    
+
     [CommandParameter(2)]
     public IReadOnlyList<string> Baz { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -396,7 +396,7 @@ public class Command : CommandBase
 
     [CommandParameter(1)]
     public string Bar { get; set; }
-    
+
     public override ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -434,13 +434,13 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", IsRequired = true)]
     public string Foo { get; set; }
-    
+
     [CommandOption(""bar"")]
     public string Bar { get; set; }
-    
+
     [CommandOption(""baz"", IsRequired = true)]
     public IReadOnlyList<string> Baz { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -478,10 +478,10 @@ public class Command : ICommand
 {
     [CommandParameter(0, Name = ""foo"", Description = ""Description of foo."")]
     public string Foo { get; set; }
-    
+
     [CommandOption(""bar"", Description = ""Description of bar."")]
     public string Bar { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -597,10 +597,10 @@ public class Command : ICommand
 {
     [CommandParameter(0)]
     public CustomEnum Foo { get; set; }
-    
+
     [CommandOption(""bar"")]
     public CustomEnum Bar { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -732,10 +732,10 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", EnvironmentVariable = ""ENV_FOO"")]
     public CustomEnum Foo { get; set; }
-    
+
     [CommandOption(""bar"", EnvironmentVariable = ""ENV_BAR"")]
     public CustomEnum Bar { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");
@@ -794,10 +794,10 @@ public class Command : ICommand
 
     [CommandOption(""lol"")]
     public CustomEnum Lol { get; set; } = CustomEnum.Two;
-    
+
     [CommandOption(""hmm"", IsRequired = true)]
     public string Hmm { get; set; } = ""not printed"";
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }
 ");

CliFx.Tests/OptionBindingSpecs.cs

@@ -28,7 +28,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"")]
     public bool Foo { get; set; }
-       
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo);
@@ -66,7 +66,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public bool Foo { get; set; }
-       
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo);
@@ -104,10 +104,10 @@ public class Command : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     [CommandOption(""bar"")]
     public string Bar { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(""Foo = "" + Foo);
@@ -150,10 +150,10 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     [CommandOption('b')]
     public string Bar { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(""Foo = "" + Foo);
@@ -196,10 +196,10 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public string Foo { get; set; }
-    
+
     [CommandOption('b')]
     public string Bar { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(""Foo = "" + Foo);
@@ -242,7 +242,7 @@ public class Command : ICommand
 {
     [CommandOption(""Foo"")]
     public IReadOnlyList<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -286,7 +286,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public IReadOnlyList<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -330,7 +330,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"")]
     public IReadOnlyList<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -374,7 +374,7 @@ public class Command : ICommand
 {
     [CommandOption('f')]
     public IReadOnlyList<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -418,7 +418,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", 'f')]
     public IReadOnlyList<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         foreach (var i in Foo)
@@ -462,10 +462,10 @@ public class Command : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     [CommandOption(""bar"")]
     public string Bar { get; set; } = ""hello"";
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(""Foo = "" + Foo);
@@ -496,6 +496,83 @@ public class Command : ICommand
         );
     }
 
+    [Fact]
+    public async Task Option_binding_supports_multiple_inheritance_through_default_interface_members()
+    {
+        // Arrange
+        var commandType = DynamicCommandBuilder.Compile(
+            // language=cs
+            @"
+public static class SharedContext
+{
+    public static int Foo { get; set; }
+
+    public static bool Bar { get; set; }
+}
+
+public interface IHasFoo : ICommand
+{
+    [CommandOption(""foo"")]
+    public int Foo
+    {
+        get => SharedContext.Foo;
+        set => SharedContext.Foo = value;
+    }
+}
+
+public interface IHasBar : ICommand
+{
+    [CommandOption(""bar"")]
+    public bool Bar
+    {
+        get => SharedContext.Bar;
+        set => SharedContext.Bar = value;
+    }
+}
+
+public interface IHasBaz : ICommand
+{
+    public string Baz { get; set; }
+}
+
+[Command]
+public class Command : IHasFoo, IHasBar, IHasBaz
+{
+    [CommandOption(""baz"")]
+    public string Baz { get; set; }
+
+	public ValueTask ExecuteAsync(IConsole console)
+	{
+        console.Output.WriteLine(""Foo = "" + SharedContext.Foo);
+        console.Output.WriteLine(""Bar = "" + SharedContext.Bar);
+        console.Output.WriteLine(""Baz = "" + Baz);
+
+        return default;
+    }
+}
+");
+
+        var application = new CliApplicationBuilder()
+            .AddCommand(commandType)
+            .UseConsole(FakeConsole)
+            .Build();
+
+        // Act
+        var exitCode = await application.RunAsync(
+            new[] { "--foo", "42", "--bar", "--baz", "xyz" }
+        );
+
+        var stdOut = FakeConsole.ReadOutputString();
+
+        // Assert
+        exitCode.Should().Be(0);
+        stdOut.Should().ConsistOfLines(
+            "Foo = 42",
+            "Bar = True",
+            "Baz = xyz"
+        );
+    }
+
     [Fact]
     public async Task Option_binding_does_not_consider_a_negative_number_as_an_option_name_or_short_name()
     {
@@ -508,7 +585,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console)
     {
         console.Output.WriteLine(Foo);
@@ -547,7 +624,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", IsRequired = true)]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }");
 
@@ -581,7 +658,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", IsRequired = true)]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }");
 
@@ -615,7 +692,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"", IsRequired = true)]
     public IReadOnlyList<string> Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }");
 
@@ -649,7 +726,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }");
 
@@ -683,7 +760,7 @@ public class Command : ICommand
 {
     [CommandOption(""foo"")]
     public string Foo { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }");
 

CliFx.Tests/ParameterBindingSpecs.cs

@@ -27,7 +27,7 @@ public class Command : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string Bar { get; set; }
 
@@ -35,7 +35,7 @@ public class Command : ICommand
     {
         console.Output.WriteLine(""Foo = "" + Foo);
         console.Output.WriteLine(""Bar = "" + Bar);
-        
+
         return default;
     }
 }");
@@ -73,13 +73,13 @@ public class Command : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string Bar { get; set; }
-    
+
     [CommandParameter(2)]
     public IReadOnlyList<string> Baz { get; set; }
-    
+
     [CommandOption(""boo"")]
     public string Boo { get; set; }
 
@@ -87,10 +87,10 @@ public class Command : ICommand
     {
         console.Output.WriteLine(""Foo = "" + Foo);
         console.Output.WriteLine(""Bar = "" + Bar);
-        
+
         foreach (var i in Baz)
             console.Output.WriteLine(""Baz = "" + i);
-        
+
         return default;
     }
 }");
@@ -120,7 +120,7 @@ public class Command : ICommand
     }
 
     [Fact]
-    public async Task Parameter_binding_fails_if_one_of_the_parameters_has_not_been_provided()
+    public async Task Parameter_is_not_bound_if_there_are_no_arguments_matching_its_order()
     {
         // Arrange
         var commandType = DynamicCommandBuilder.Compile(
@@ -131,7 +131,53 @@ public class Command : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
+    [CommandParameter(1, IsRequired = false)]
+    public string Bar { get; set; } = ""xyz"";
+
+    public ValueTask ExecuteAsync(IConsole console)
+    {
+        console.Output.WriteLine(""Foo = "" + Foo);
+        console.Output.WriteLine(""Bar = "" + Bar);
+
+        return default;
+    }
+}");
+
+        var application = new CliApplicationBuilder()
+            .AddCommand(commandType)
+            .UseConsole(FakeConsole)
+            .Build();
+
+        // Act
+        var exitCode = await application.RunAsync(
+            new[] {"abc"},
+            new Dictionary<string, string>()
+        );
+
+        var stdOut = FakeConsole.ReadOutputString();
+
+        // Assert
+        exitCode.Should().Be(0);
+        stdOut.Should().ConsistOfLines(
+            "Foo = abc",
+            "Bar = xyz"
+        );
+    }
+
+    [Fact]
+    public async Task Parameter_binding_fails_if_a_required_parameter_has_not_been_provided()
+    {
+        // Arrange
+        var commandType = DynamicCommandBuilder.Compile(
+            // language=cs
+            @"
+[Command]
+public class Command : ICommand
+{
+    [CommandParameter(0)]
+    public string Foo { get; set; }
+
     [CommandParameter(1)]
     public string Bar { get; set; }
 
@@ -153,7 +199,7 @@ public class Command : ICommand
 
         // Assert
         exitCode.Should().NotBe(0);
-        stdErr.Should().Contain("Missing parameter(s)");
+        stdErr.Should().Contain("Missing required parameter(s)");
     }
 
     [Fact]
@@ -168,10 +214,10 @@ public class Command : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public IReadOnlyList<string> Bar { get; set; }
-    
+
     public ValueTask ExecuteAsync(IConsole console) => default;
 }");
 
@@ -190,7 +236,7 @@ public class Command : ICommand
 
         // Assert
         exitCode.Should().NotBe(0);
-        stdErr.Should().Contain("Missing parameter(s)");
+        stdErr.Should().Contain("Missing required parameter(s)");
     }
 
     [Fact]
@@ -205,7 +251,7 @@ public class Command : ICommand
 {
     [CommandParameter(0)]
     public string Foo { get; set; }
-    
+
     [CommandParameter(1)]
     public string Bar { get; set; }
 

CliFx/Attributes/CommandOptionAttribute.cs

@@ -29,7 +29,7 @@ public sealed class CommandOptionAttribute : Attribute
     public char? ShortName { get; }
 
     /// <summary>
-    /// Whether this option is required.
+    /// Whether this option is required (default: <c>false</c>).
     /// If an option is required, the user will get an error if they don't set it.
     /// </summary>
     public bool IsRequired { get; set; }

CliFx/Attributes/CommandParameterAttribute.cs

@@ -11,18 +11,26 @@ public sealed class CommandParameterAttribute : Attribute
 {
     /// <summary>
     /// Parameter order.
-    /// </summary>
-    /// <remarks>
     /// Higher order means the parameter appears later, lower order means
     /// it appears earlier.
-    ///
+    /// </summary>
+    /// <remarks>
     /// All parameters in a command must have unique order.
-    ///
     /// Parameter whose type is a non-scalar (e.g. array), must always be the last in order.
     /// Only one non-scalar parameter is allowed in a command.
     /// </remarks>
     public int Order { get; }
 
+    /// <summary>
+    /// Whether this parameter is required (default: <c>true</c>).
+    /// If a parameter is required, the user will get an error if they don't set it.
+    /// </summary>
+    /// <remarks>
+    /// Parameter marked as non-required must always be the last in order.
+    /// Only one non-required parameter is allowed in a command.
+    /// </remarks>
+    public bool IsRequired { get; set; } = true;
+
     /// <summary>
     /// Parameter name.
     /// This is shown to the user in the help text.

CliFx/CliFx.csproj

@@ -4,6 +4,7 @@
     <TargetFrameworks>netstandard2.1;netstandard2.0</TargetFrameworks>
     <Authors>$(Company)</Authors>
     <Description>Declarative framework for building command line applications</Description>
+    <IsPackable>true</IsPackable>
     <PackageTags>command line executable interface framework parser arguments cli app application net core</PackageTags>
     <PackageProjectUrl>https://github.com/Tyrrrz/CliFx</PackageProjectUrl>
     <PackageReleaseNotes>https://github.com/Tyrrrz/CliFx/blob/master/Changelog.md</PackageReleaseNotes>
@@ -34,6 +35,16 @@
   <ItemGroup>
     <ProjectReference Include="../CliFx.Analyzers/CliFx.Analyzers.csproj" ReferenceOutputAssembly="false" OutputItemType="Analyzer" />
     <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/CliFx.Analyzers.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/Microsoft.CodeAnalysis.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/Microsoft.CodeAnalysis.CSharp.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/System.Buffers.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/System.Collections.Immutable.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/System.Memory.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/System.Numerics.Vectors.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/System.Reflection.Metadata.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/System.Runtime.CompilerServices.Unsafe.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/System.Text.Encoding.CodePages.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
+    <None Include="../CliFx.Analyzers/bin/$(Configuration)/netstandard2.0/System.Threading.Tasks.Extensions.dll" Pack="true" PackagePath="analyzers/dotnet/cs" Visible="false" />
   </ItemGroup>
 
 </Project>

CliFx/CommandBinder.cs

@@ -43,12 +43,6 @@ internal class CommandBinder
             return string.IsNullOrWhiteSpace(rawValue) || bool.Parse(rawValue);
         }
 
-        // IConvertible primitives (int, double, char, etc)
-        if (targetType.IsConvertible())
-        {
-            return Convert.ChangeType(rawValue, targetType, _formatProvider);
-        }
-
         // Special case for DateTimeOffset
         if (targetType == typeof(DateTimeOffset))
         {
@@ -68,6 +62,12 @@ internal class CommandBinder
             return Enum.Parse(targetType, rawValue!, true);
         }
 
+        // Convertible primitives (int, double, char, etc)
+        if (targetType.Implements(typeof(IConvertible)))
+        {
+            return Convert.ChangeType(rawValue, targetType, _formatProvider);
+        }
+
         // Nullable<T>
         var nullableUnderlyingType = targetType.TryGetNullableUnderlyingType();
         if (nullableUnderlyingType is not null)
@@ -223,7 +223,7 @@ internal class CommandBinder
     {
         // Ensure there are no unexpected parameters and that all parameters are provided
         var remainingParameterInputs = commandInput.Parameters.ToList();
-        var remainingParameterSchemas = commandSchema.Parameters.ToList();
+        var remainingRequiredParameterSchemas = commandSchema.Parameters.Where(p => p.IsRequired).ToList();
 
         var position = 0;
 
@@ -258,7 +258,7 @@ internal class CommandBinder
                 remainingParameterInputs.RemoveRange(parameterInputs);
             }
 
-            remainingParameterSchemas.Remove(parameterSchema);
+            remainingRequiredParameterSchemas.Remove(parameterSchema);
         }
 
         if (remainingParameterInputs.Any())
@@ -272,12 +272,12 @@ internal class CommandBinder
             );
         }
 
-        if (remainingParameterSchemas.Any())
+        if (remainingRequiredParameterSchemas.Any())
         {
             throw CliFxException.UserError(
-                "Missing parameter(s):" +
+                "Missing required parameter(s):" +
                 Environment.NewLine +
-                remainingParameterSchemas
+                remainingRequiredParameterSchemas
                     .Select(o => o.GetFormattedIdentifier())
                     .JoinToString(" ")
             );

CliFx/Formatting/HelpConsoleFormatter.cs

@@ -157,7 +157,15 @@ internal class HelpConsoleFormatter : ConsoleFormatter
 
         foreach (var parameterSchema in _context.CommandSchema.Parameters.OrderBy(p => p.Order))
         {
-            Write(ConsoleColor.Red, "* ");
+            if (parameterSchema.IsRequired)
+            {
+                Write(ConsoleColor.Red, "* ");
+            }
+            else
+            {
+                WriteHorizontalMargin();
+            }
+
             Write(ConsoleColor.DarkCyan, $"{parameterSchema.Name}");
 
             WriteColumnMargin();
@@ -198,6 +206,12 @@ internal class HelpConsoleFormatter : ConsoleFormatter
                 Write('.');
                 Write(' ');
             }
+            
+            // Default value
+            if (!parameterSchema.IsRequired)
+            {
+                WriteDefaultValue(parameterSchema);
+            }
 
             WriteLine();
         }
@@ -290,60 +304,65 @@ internal class HelpConsoleFormatter : ConsoleFormatter
             // Default value
             if (!optionSchema.IsRequired)
             {
-                var defaultValue = _context.CommandDefaultValues.GetValueOrDefault(optionSchema);
+                WriteDefaultValue(optionSchema);
-                if (defaultValue is not null)
-                {
-                    // Non-Scalar
-                    if (defaultValue is not string && defaultValue is IEnumerable defaultValues)
-                    {
-                        var elementType =
-                            defaultValues.GetType().TryGetEnumerableUnderlyingType() ??
-                            typeof(object);
-
-                        if (elementType.IsToStringOverriden())
-                        {
-                            Write(ConsoleColor.White, "Default: ");
-
-                            var isFirst = true;
-
-                            foreach (var element in defaultValues)
-                            {
-                                if (isFirst)
-                                {
-                                    isFirst = false;
-                                }
-                                else
-                                {
-                                    Write(", ");
-                                }
-
-                                Write('"');
-                                Write(element.ToString(CultureInfo.InvariantCulture));
-                                Write('"');
-                            }
-
-                            Write('.');
-                        }
-                    }
-                    else
-                    {
-                        if (defaultValue.GetType().IsToStringOverriden())
-                        {
-                            Write(ConsoleColor.White, "Default: ");
-
-                            Write('"');
-                            Write(defaultValue.ToString(CultureInfo.InvariantCulture));
-                            Write('"');
-                            Write('.');
-                        }
-                    }
-                }
             }
 
             WriteLine();
         }
     }
 
+    private void WriteDefaultValue(IMemberSchema schema)
+    {
+        var defaultValue = _context.CommandDefaultValues.GetValueOrDefault(schema);
+        if (defaultValue is not null)
+        {
+            // Non-Scalar
+            if (defaultValue is not string && defaultValue is IEnumerable defaultValues)
+            {
+                var elementType =
+                    defaultValues.GetType().TryGetEnumerableUnderlyingType() ??
+                    typeof(object);
+
+                if (elementType.IsToStringOverriden())
+                {
+                    Write(ConsoleColor.White, "Default: ");
+
+                    var isFirst = true;
+
+                    foreach (var element in defaultValues)
+                    {
+                        if (isFirst)
+                        {
+                            isFirst = false;
+                        }
+                        else
+                        {
+                            Write(", ");
+                        }
+
+                        Write('"');
+                        Write(element.ToString(CultureInfo.InvariantCulture));
+                        Write('"');
+                    }
+
+                    Write('.');
+                }
+            }
+            else
+            {
+                if (defaultValue.GetType().IsToStringOverriden())
+                {
+                    Write(ConsoleColor.White, "Default: ");
+
+                    Write('"');
+                    Write(defaultValue.ToString(CultureInfo.InvariantCulture));
+                    Write('"');
+                    Write('.');
+                }
+            }
+        }
+    }
+
     private void WriteCommandChildren()
     {
         var childCommandSchemas = _context

CliFx/Infrastructure/ConsoleReader.cs

@@ -1,11 +1,16 @@
-using System.IO;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Runtime.CompilerServices;
 using System.Text;
+using System.Threading.Tasks;
 
 namespace CliFx.Infrastructure;
 
 /// <summary>
 /// Implements a <see cref="TextReader"/> for reading characters from a console stream.
 /// </summary>
+// Both the underlying stream AND the stream reader must be synchronized!
+// https://github.com/Tyrrrz/CliFx/issues/123
 public partial class ConsoleReader : StreamReader
 {
     /// <summary>
@@ -29,6 +34,77 @@ public partial class ConsoleReader : StreamReader
         : this(console, stream, System.Console.InputEncoding)
     {
     }
+
+    // The following overrides are required to establish thread-safe behavior
+    // in methods deriving from StreamReader.
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override int Peek() => base.Peek();
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override int Read() => base.Read();
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override int Read(char[] buffer, int index, int count) =>
+        base.Read(buffer, index, count);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override int ReadBlock(char[] buffer, int index, int count)
+    {
+        return base.ReadBlock(buffer, index, count);
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override string? ReadLine() => base.ReadLine();
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override string ReadToEnd() => base.ReadToEnd();
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task<int> ReadAsync(char[] buffer, int index, int count)
+    {
+        // Must be non-async to work with locks
+        return Task.FromResult(Read(buffer, index, count));
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task<int> ReadBlockAsync(char[] buffer, int index, int count)
+    {
+        // Must be non-async to work with locks
+        return Task.FromResult(ReadBlock(buffer, index, count));
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task<string?> ReadLineAsync()
+    {
+        // Must be non-async to work with locks
+        return Task.FromResult(ReadLine());
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task<string> ReadToEndAsync()
+    {
+        // Must be non-async to work with locks
+        return Task.FromResult(ReadToEnd());
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Close() => base.Close();
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    protected override void Dispose(bool disposing) => base.Dispose(disposing);
 }
 
 public partial class ConsoleReader

CliFx/Infrastructure/ConsoleWriter.cs

@@ -1,5 +1,8 @@
-using System.IO;
+using System.Diagnostics.CodeAnalysis;
+using System.IO;
+using System.Runtime.CompilerServices;
 using System.Text;
+using System.Threading.Tasks;
 using CliFx.Utils;
 
 namespace CliFx.Infrastructure;
@@ -7,6 +10,8 @@ namespace CliFx.Infrastructure;
 /// <summary>
 /// Implements a <see cref="TextWriter"/> for writing characters to a console stream.
 /// </summary>
+// Both the underlying stream AND the stream writer must be synchronized!
+// https://github.com/Tyrrrz/CliFx/issues/123
 public partial class ConsoleWriter : StreamWriter
 {
     /// <summary>
@@ -30,6 +35,239 @@ public partial class ConsoleWriter : StreamWriter
         : this(console, stream, System.Console.OutputEncoding)
     {
     }
+
+    // The following overrides are required to establish thread-safe behavior
+    // in methods deriving from StreamWriter.
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(char[] buffer, int index, int count) => base.Write(buffer, index, count);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(char[] buffer) => base.Write(buffer);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(char value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(string? value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(string format, object? arg0) => base.Write(format, arg0);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(string format, object? arg0, object? arg1) =>
+        base.Write(format, arg0, arg1);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(string format, object? arg0, object? arg1, object? arg2) =>
+        base.Write(format, arg0, arg1, arg2);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(string format, params object?[] arg) => base.Write(format, arg);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(bool value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(int value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(long value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(uint value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(ulong value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(float value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(double value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(decimal value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Write(object? value) => base.Write(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task WriteAsync(char[] buffer, int index, int count)
+    {
+        // Must be non-async to work with locks
+        Write(buffer, index, count);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task WriteAsync(char value)
+    {
+        // Must be non-async to work with locks
+        Write(value);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task WriteAsync(string? value)
+    {
+        // Must be non-async to work with locks
+        Write(value);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine() => base.WriteLine();
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(char[] buffer, int index, int count) =>
+        base.WriteLine(buffer, index, count);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(char[] buffer) => base.WriteLine(buffer);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(char value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(string? value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(string format, object? arg0) => base.WriteLine(format, arg0);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(string format, object? arg0, object? arg1) =>
+        base.WriteLine(format, arg0, arg1);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(string format, object? arg0, object? arg1, object? arg2) =>
+        base.WriteLine(format, arg0, arg1, arg2);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(string format, params object?[] arg) =>
+        base.WriteLine(format, arg);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(bool value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(int value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(long value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(uint value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(ulong value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(float value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(double value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(decimal value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void WriteLine(object? value) => base.WriteLine(value);
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task WriteLineAsync()
+    {
+        // Must be non-async to work with locks
+        WriteLine();
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task WriteLineAsync(char value)
+    {
+        // Must be non-async to work with locks
+        WriteLine(value);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task WriteLineAsync(char[] buffer, int index, int count)
+    {
+        // Must be non-async to work with locks
+        WriteLine(buffer, index, count);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task WriteLineAsync(string? value)
+    {
+        // Must be non-async to work with locks
+        WriteLine(value);
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Flush() => base.Flush();
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override Task FlushAsync()
+    {
+        // Must be non-async to work with locks
+        Flush();
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    public override void Close() => base.Close();
+
+    /// <inheritdoc />
+    [ExcludeFromCodeCoverage, MethodImpl(MethodImplOptions.Synchronized)]
+    protected override void Dispose(bool disposing) => base.Dispose(disposing);
 }
 
 public partial class ConsoleWriter

CliFx/Infrastructure/IConsole.cs

@@ -82,7 +82,7 @@ public interface IConsole
     /// </para>
     /// </remarks>
     CancellationToken RegisterCancellationHandler();
-        
+
     /// <summary>
     /// Clears the console buffer and corresponding console window of display information.
     /// </summary>

CliFx/Schema/BindablePropertyDescriptor.cs

@@ -33,7 +33,7 @@ internal class BindablePropertyDescriptor : IPropertyDescriptor
 
             return type;
         }
-            
+
         var underlyingType = GetUnderlyingType(Type);
 
         // We can only get valid values for enums

CliFx/Schema/CommandSchema.cs

@@ -87,12 +87,27 @@ internal partial class CommandSchema
             ? new[] {OptionSchema.HelpOption, OptionSchema.VersionOption}
             : new[] {OptionSchema.HelpOption};
 
-        var parameterSchemas = type.GetProperties()
+        var properties = type
+            // Get properties directly on command type
+            .GetProperties()
+            // Get non-abstract properties on interfaces (to support default interfaces members)
+            .Union(type
+                .GetInterfaces()
+                // Only interfaces implementing ICommand for explicitness
+                .Where(i => typeof(ICommand).IsAssignableFrom(i) && i != typeof(ICommand))
+                .SelectMany(i => i
+                    .GetProperties()
+                    .Where(p => !p.GetMethod.IsAbstract && !p.SetMethod.IsAbstract)
+                )
+            )
+            .ToArray();
+
+        var parameterSchemas = properties
             .Select(ParameterSchema.TryResolve)
             .WhereNotNull()
             .ToArray();
 
-        var optionSchemas = type.GetProperties()
+        var optionSchemas = properties
             .Select(OptionSchema.TryResolve)
             .WhereNotNull()
             .Concat(implicitOptionSchemas)

CliFx/Schema/ParameterSchema.cs

@@ -13,6 +13,8 @@ internal partial class ParameterSchema : IMemberSchema
 
     public string Name { get; }
 
+    public bool IsRequired { get; }
+
     public string? Description { get; }
 
     public Type? ConverterType { get; }
@@ -23,6 +25,7 @@ internal partial class ParameterSchema : IMemberSchema
         IPropertyDescriptor property,
         int order,
         string name,
+        bool isRequired,
         string? description,
         Type? converterType,
         IReadOnlyList<Type> validatorTypes)
@@ -30,6 +33,7 @@ internal partial class ParameterSchema : IMemberSchema
         Property = property;
         Order = order;
         Name = name;
+        IsRequired = isRequired;
         Description = description;
         ConverterType = converterType;
         ValidatorTypes = validatorTypes;
@@ -55,6 +59,7 @@ internal partial class ParameterSchema
             new BindablePropertyDescriptor(property),
             attribute.Order,
             name,
+            attribute.IsRequired,
             description,
             attribute.Converter,
             attribute.Validators

CliFx/Utils/Extensions/TypeExtensions.cs

@@ -59,27 +59,4 @@ internal static class TypeExtensions
         var toStringMethod = type.GetMethod(nameof(ToString), Type.EmptyTypes);
         return toStringMethod?.GetBaseDefinition()?.DeclaringType != toStringMethod?.DeclaringType;
     }
-
-    // Types supported by `Convert.ChangeType(...)`
-    private static readonly HashSet<Type> ConvertibleTypes = new()
-    {
-        typeof(bool),
-        typeof(char),
-        typeof(sbyte),
-        typeof(byte),
-        typeof(short),
-        typeof(ushort),
-        typeof(int),
-        typeof(uint),
-        typeof(long),
-        typeof(ulong),
-        typeof(float),
-        typeof(double),
-        typeof(decimal),
-        typeof(DateTime),
-        typeof(string),
-        typeof(object)
-    };
-
-    public static bool IsConvertible(this Type type) => ConvertibleTypes.Contains(type);
 }

CliFx/Utils/NoPreambleEncoding.cs

@@ -8,6 +8,8 @@ namespace CliFx.Utils;
 // https://github.com/dotnet/runtime/blob/01b7e73cd378145264a7cb7a09365b41ed42b240/src/libraries/Common/src/System/Text/ConsoleEncoding.cs
 // Also see:
 // https://source.dot.net/#System.Console/ConsoleEncoding.cs,5eedd083a4a4f4a2
+// Majority of overrides are just proxy calls to avoid potentially more expensive base behavior.
+// The important part is the GetPreamble() method that has been overriden to return an empty array.
 internal class NoPreambleEncoding : Encoding
 {
     private readonly Encoding _underlyingEncoding;
@@ -72,20 +74,20 @@ internal class NoPreambleEncoding : Encoding
     public override int GetBytes(char[] chars, int charIndex, int charCount, byte[] bytes, int byteIndex) =>
         _underlyingEncoding.GetBytes(chars, charIndex, charCount, bytes, byteIndex);
 
-    [ExcludeFromCodeCoverage]
-    public override byte[] GetBytes(char[] chars) => _underlyingEncoding.GetBytes(chars);
-
     [ExcludeFromCodeCoverage]
     public override byte[] GetBytes(char[] chars, int index, int count) =>
         _underlyingEncoding.GetBytes(chars, index, count);
 
     [ExcludeFromCodeCoverage]
-    public override byte[] GetBytes(string s) => _underlyingEncoding.GetBytes(s);
+    public override byte[] GetBytes(char[] chars) => _underlyingEncoding.GetBytes(chars);
 
     [ExcludeFromCodeCoverage]
     public override int GetBytes(string s, int charIndex, int charCount, byte[] bytes, int byteIndex) =>
         _underlyingEncoding.GetBytes(s, charIndex, charCount, bytes, byteIndex);
 
+    [ExcludeFromCodeCoverage]
+    public override byte[] GetBytes(string s) => _underlyingEncoding.GetBytes(s);
+
     [ExcludeFromCodeCoverage]
     public override int GetCharCount(byte[] bytes, int index, int count) =>
         _underlyingEncoding.GetCharCount(bytes, index, count);

CliFx/Utils/Polyfills.cs

@@ -32,7 +32,6 @@ internal static partial class PolyfillExtensions
         stream.Write(buffer, 0, buffer.Length);
 }
 
-
 namespace System.Linq
 {
     internal static class PolyfillExtensions

Directory.Build.props

@@ -1,12 +1,13 @@
 <Project>
 
   <PropertyGroup>
-    <Version>2.1</Version>
+    <Version>2.2.2</Version>
     <Company>Tyrrrz</Company>
-    <Copyright>Copyright (C) Alexey Golub</Copyright>
+    <Copyright>Copyright (C) Oleksii Holub</Copyright>
     <LangVersion>latest</LangVersion>
     <Nullable>enable</Nullable>
     <WarningsAsErrors>nullable</WarningsAsErrors>
+    <IsPackable>false</IsPackable>
   </PropertyGroup>
 
   <!-- Disable nullability warnings on older frameworks because there is no nullability info for BCL -->

License.txt

@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2019-2022 Alexey Golub
+Copyright (c) 2019-2022 Oleksii Holub
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

Readme.md

@@ -168,7 +168,7 @@ In case the user forgets to specify the `value` parameter, the application will
 ```sh
 > dotnet myapp.dll -b 10
 
-Missing parameter(s):
+Missing required parameter(s):
 <value>
 ```
 
@@ -193,11 +193,12 @@ OPTIONS
 
 Overall, parameters and options are both used to consume input from the command line, but they differ in a few important ways:
 
-- Parameters are identified by their relative order. Options are identified by their name or a single-character short name.
+|                    | Parameters                                                                                            | Options                                                                                         |
-- Parameters technically also have a name, but it's only used in the help text.
+|--------------------|-------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------|
-- Parameters are always required. Options are normally optional, but can also be configured to require a value.
+| **Identification** | Positional (by relative order).                                                                       | Named (by name or short name).                                                                  |
-- Options can be configured to use an environment variable as a fallback.
+| **Requiredness**   | Required by default. Only the last parameter can be configured to be optional.                        | Optional by default. Any option can be configured to be required without limitations.           |
-- Both parameters and options can take multiple values, but there can only be one such parameter in a command and it must be the last in order. Options are not limited in this regard.
+| **Arity**          | Depends on the property type. Only the last parameter can be bound to a non-scalar type (i.e. array). | Depends on the property type. Any option can be bound to a non-scalar type without limitations. |
+| **Fallback**       | —                                                                                                     | Can be configured to use an environment variable as fallback, in case the option isn't set.     |
 
 As a general guideline, it's recommended to use parameters for required inputs that the command can't function without.
 Use options for all other non-required inputs or when specifying the name explicitly makes the usage clearer.
@@ -226,7 +227,7 @@ Similarly, unseparated arguments in the form of `myapp -ofile` will be treated a
 
 Because of these rules, order of arguments is semantically important and must always follow this pattern:
 
-```ini
+```txt
 [directives] [command name] [parameters] [options]
 ```
 
@@ -596,7 +597,7 @@ public async Task ConcatCommand_executes_successfully()
 
     // Act
     await command.ExecuteAsync(console);
-    
+
     var stdOut = console.ReadOutputString();
 
     // Assert
@@ -624,7 +625,7 @@ public async Task ConcatCommand_executes_successfully()
 
     // Act
     await app.RunAsync(args, envVars);
-    
+
     var stdOut = console.ReadOutputString();
 
     // Assert