Update version

Changelog.md

@@ -1,3 +1,11 @@
+### v1.3 (23-May-2020)
+
+- Changed analyzers to report errors instead of warnings. If you find that some analyzer works incorrectly, please report it on GitHub. You can also configure inspection severity overrides in your project if you need to.
+- Improved help text by showing default values for non-required options. This only works on types that have a custom override for `ToString()` method. Additionally, if the type implements `IFormattable`, the overload with a format provider will be used instead. (Thanks [@Domn Werner](https://github.com/domn1995))
+- Changed default version text to only show 3 version components instead of 4, if the last component (revision) is not specified or is zero. This makes the default version text compliant with semantic versioning.
+- Fixed an issue where it was possible to define a command with an option that has the same name or short name as built-in help or version options. Previously it would lead to the user-defined option being ignored in favor of the built-in option. Now this will throw an exception instead.
+- Changed the underlying representation of `StreamReader`/`StreamWriter` objects used in `SystemConsole` and `VirtualConsole` to be thread-safe.
+
 ### v1.2 (11-May-2020)
 
 - Added built-in Roslyn analyzers that help catch incorrect usage of the library. Currently, all analyzers report issues as warnings so as to not prevent the project from building. In the future that may change.

CliFx.Analyzers/DiagnosticDescriptors.cs

@@ -8,67 +8,67 @@ namespace CliFx.Analyzers
             new DiagnosticDescriptor(nameof(CliFx0001),
                 "Type must implement the 'CliFx.ICommand' interface in order to be a valid command",
                 "Type must implement the 'CliFx.ICommand' interface in order to be a valid command",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0002 =
             new DiagnosticDescriptor(nameof(CliFx0002),
                 "Type must be annotated with the 'CliFx.Attributes.CommandAttribute' in order to be a valid command",
                 "Type must be annotated with the 'CliFx.Attributes.CommandAttribute' in order to be a valid command",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0021 =
             new DiagnosticDescriptor(nameof(CliFx0021),
                 "Parameter order must be unique within its command",
                 "Parameter order must be unique within its command",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0022 =
             new DiagnosticDescriptor(nameof(CliFx0022),
                 "Parameter order must have unique name within its command",
                 "Parameter order must have unique name within its command",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0023 =
             new DiagnosticDescriptor(nameof(CliFx0023),
                 "Only one non-scalar parameter per command is allowed",
                 "Only one non-scalar parameter per command is allowed",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0024 =
             new DiagnosticDescriptor(nameof(CliFx0024),
                 "Non-scalar parameter must be last in order",
                 "Non-scalar parameter must be last in order",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0041 =
             new DiagnosticDescriptor(nameof(CliFx0041),
                 "Option must have a name or short name specified",
                 "Option must have a name or short name specified",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0042 =
             new DiagnosticDescriptor(nameof(CliFx0042),
                 "Option name must be at least 2 characters long",
                 "Option name must be at least 2 characters long",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0043 =
             new DiagnosticDescriptor(nameof(CliFx0043),
                 "Option name must be unique within its command",
                 "Option name must be unique within its command",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0044 =
             new DiagnosticDescriptor(nameof(CliFx0044),
                 "Option short name must be unique within its command",
                 "Option short name must be unique within its command",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0045 =
             new DiagnosticDescriptor(nameof(CliFx0045),
                 "Option environment variable name must be unique within its command",
                 "Option environment variable name must be unique within its command",
-                "Usage", DiagnosticSeverity.Warning, true);
+                "Usage", DiagnosticSeverity.Error, true);
 
         public static readonly DiagnosticDescriptor CliFx0100 =
             new DiagnosticDescriptor(nameof(CliFx0100),

CliFx.Tests.Dummy/Commands/ShowErrorMessageThenHelpTextOnErrorCommand.cs

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

CliFx.Tests.Dummy/Commands/ShowHelpTextOnErrorCommand.cs

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

CliFx.Tests/ApplicationSpecs.Commands.cs

@@ -6,6 +6,18 @@ namespace CliFx.Tests
 {
     public partial class ApplicationSpecs
     {
+        [Command]
+        private class DefaultCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class AnotherDefaultCommand : ICommand
+        {
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
         [Command]
         private class NonImplementedCommand
         {
@@ -118,6 +130,24 @@ namespace CliFx.Tests
             public ValueTask ExecuteAsync(IConsole console) => default;
         }
 
+        [Command]
+        private class ConflictWithHelpOptionCommand : ICommand
+        {
+            [CommandOption("option-h", 'h')]
+            public string? OptionH { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
+        [Command]
+        private class ConflictWithVersionOptionCommand : ICommand
+        {
+            [CommandOption("version")]
+            public string? Version { get; set; }
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+
         [Command]
         private class DuplicateOptionEnvironmentVariableNamesCommand : ICommand
         {
@@ -130,12 +160,6 @@ namespace CliFx.Tests
             public ValueTask ExecuteAsync(IConsole console) => default;
         }
 
-        [Command]
-        private class ValidCommand : ICommand
-        {
-            public ValueTask ExecuteAsync(IConsole console) => default;
-        }
-
         [Command("hidden", Description = "Description")]
         private class HiddenPropertiesCommand : ICommand
         {

CliFx.Tests/ApplicationSpecs.cs

@@ -31,10 +31,10 @@ namespace CliFx.Tests
         {
             // Act
             var app = new CliApplicationBuilder()
-                .AddCommand(typeof(ValidCommand))
-                .AddCommandsFrom(typeof(ValidCommand).Assembly)
-                .AddCommands(new[] {typeof(ValidCommand)})
-                .AddCommandsFrom(new[] {typeof(ValidCommand).Assembly})
+                .AddCommand(typeof(DefaultCommand))
+                .AddCommandsFrom(typeof(DefaultCommand).Assembly)
+                .AddCommands(new[] {typeof(DefaultCommand)})
+                .AddCommandsFrom(new[] {typeof(DefaultCommand).Assembly})
                 .AddCommandsFromThisAssembly()
                 .AllowDebugMode()
                 .AllowPreviewMode()
@@ -57,7 +57,7 @@ namespace CliFx.Tests
             var commandTypes = Array.Empty<Type>();
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -68,7 +68,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(NonImplementedCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -79,7 +79,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(NonAnnotatedCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -90,7 +90,18 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(DuplicateNameCommandA), typeof(DuplicateNameCommandB)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_can_be_default_but_only_if_it_is_the_only_such_command()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(DefaultCommand), typeof(AnotherDefaultCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -101,7 +112,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(DuplicateParameterOrderCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -112,7 +123,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(DuplicateParameterNameCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -123,7 +134,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(MultipleNonScalarParametersCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -134,7 +145,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(NonLastNonScalarParameterCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -145,7 +156,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(EmptyOptionNameCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -156,7 +167,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(SingleCharacterOptionNameCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -167,7 +178,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(DuplicateOptionNamesCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -178,7 +189,29 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(DuplicateOptionShortNamesCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_options_must_not_have_conflicts_with_the_implicit_help_option()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(ConflictWithHelpOptionCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
+            _output.WriteLine(ex.Message);
+        }
+
+        [Fact]
+        public void Command_options_must_not_have_conflicts_with_the_implicit_version_option()
+        {
+            // Arrange
+            var commandTypes = new[] {typeof(ConflictWithVersionOptionCommand)};
+
+            // Act & assert
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -189,7 +222,7 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(DuplicateOptionEnvironmentVariableNamesCommand)};
 
             // Act & assert
-            var ex = Assert.Throws<CliFxException>(() => ApplicationSchema.Resolve(commandTypes));
+            var ex = Assert.Throws<CliFxException>(() => RootSchema.Resolve(commandTypes));
             _output.WriteLine(ex.Message);
         }
 
@@ -200,10 +233,10 @@ namespace CliFx.Tests
             var commandTypes = new[] {typeof(HiddenPropertiesCommand)};
 
             // Act
-            var schema = ApplicationSchema.Resolve(commandTypes);
+            var schema = RootSchema.Resolve(commandTypes);
 
             // Assert
-            schema.Should().BeEquivalentTo(new ApplicationSchema(new[]
+            schema.Should().BeEquivalentTo(new RootSchema(new[]
             {
                 new CommandSchema(
                     typeof(HiddenPropertiesCommand),
@@ -225,7 +258,8 @@ namespace CliFx.Tests
                             'o',
                             "ENV",
                             false,
-                            "Option description")
+                            "Option description"),
+                        CommandOptionSchema.HelpOption
                     })
             }));
 

CliFx.Tests/ArgumentSyntaxSpecs.cs

@@ -1,5 +1,7 @@
 using System;
+using System.Collections.Generic;
 using CliFx.Domain;
+using CliFx.Tests.Internal;
 using FluentAssertions;
 using Xunit;
 
@@ -11,13 +13,14 @@ namespace CliFx.Tests
         public void Input_is_empty_if_no_arguments_are_provided()
         {
             // Arrange
-            var args = Array.Empty<string>();
+            var arguments = Array.Empty<string>();
+            var commandNames = Array.Empty<string>();
 
             // Act
-            var input = CommandLineInput.Parse(args);
+            var input = CommandInput.Parse(arguments, commandNames);
 
             // Assert
-            input.Should().BeEquivalentTo(CommandLineInput.Empty);
+            input.Should().BeEquivalentTo(CommandInput.Empty);
         }
 
         public static object[][] DirectivesTestData => new[]
@@ -25,7 +28,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"[preview]"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddDirective("preview")
                     .Build()
             },
@@ -33,7 +36,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"[preview]", "[debug]"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddDirective("preview")
                     .AddDirective("debug")
                     .Build()
@@ -42,10 +45,13 @@ namespace CliFx.Tests
 
         [Theory]
         [MemberData(nameof(DirectivesTestData))]
-        internal void Directive_can_be_enabled_by_specifying_its_name_in_square_brackets(string[] arguments, CommandLineInput expectedInput)
+        internal void Directive_can_be_enabled_by_specifying_its_name_in_square_brackets(IReadOnlyList<string> arguments, CommandInput expectedInput)
         {
+            // Arrange
+            var commandNames = Array.Empty<string>();
+
             // Act
-            var input = CommandLineInput.Parse(arguments);
+            var input = CommandInput.Parse(arguments, commandNames);
 
             // Assert
             input.Should().BeEquivalentTo(expectedInput);
@@ -56,7 +62,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"--option"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("option")
                     .Build()
             },
@@ -64,7 +70,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"--option", "value"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("option", "value")
                     .Build()
             },
@@ -72,7 +78,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"--option", "value1", "value2"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("option", "value1", "value2")
                     .Build()
             },
@@ -80,7 +86,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"--option", "same value"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("option", "same value")
                     .Build()
             },
@@ -88,7 +94,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"--option1", "--option2"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("option1")
                     .AddOption("option2")
                     .Build()
@@ -97,7 +103,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"--option1", "value1", "--option2", "value2"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("option1", "value1")
                     .AddOption("option2", "value2")
                     .Build()
@@ -106,7 +112,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"--option1", "value1", "value2", "--option2", "value3", "value4"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("option1", "value1", "value2")
                     .AddOption("option2", "value3", "value4")
                     .Build()
@@ -115,7 +121,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"--option1", "value1", "value2", "--option2"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("option1", "value1", "value2")
                     .AddOption("option2")
                     .Build()
@@ -124,10 +130,13 @@ namespace CliFx.Tests
 
         [Theory]
         [MemberData(nameof(OptionsTestData))]
-        internal void Option_can_be_set_by_specifying_its_name_after_two_dashes(string[] arguments, CommandLineInput expectedInput)
+        internal void Option_can_be_set_by_specifying_its_name_after_two_dashes(IReadOnlyList<string> arguments, CommandInput expectedInput)
         {
+            // Arrange
+            var commandNames = Array.Empty<string>();
+
             // Act
-            var input = CommandLineInput.Parse(arguments);
+            var input = CommandInput.Parse(arguments, commandNames);
 
             // Assert
             input.Should().BeEquivalentTo(expectedInput);
@@ -138,7 +147,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-o"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("o")
                     .Build()
             },
@@ -146,7 +155,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-o", "value"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("o", "value")
                     .Build()
             },
@@ -154,7 +163,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-o", "value1", "value2"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("o", "value1", "value2")
                     .Build()
             },
@@ -162,7 +171,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-o", "same value"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("o", "same value")
                     .Build()
             },
@@ -170,7 +179,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-a", "-b"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("a")
                     .AddOption("b")
                     .Build()
@@ -179,7 +188,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-a", "value1", "-b", "value2"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("a", "value1")
                     .AddOption("b", "value2")
                     .Build()
@@ -188,7 +197,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-a", "value1", "value2", "-b", "value3", "value4"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("a", "value1", "value2")
                     .AddOption("b", "value3", "value4")
                     .Build()
@@ -197,7 +206,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-a", "value1", "value2", "-b"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("a", "value1", "value2")
                     .AddOption("b")
                     .Build()
@@ -206,7 +215,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-abc"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("a")
                     .AddOption("b")
                     .AddOption("c")
@@ -216,7 +225,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-abc", "value"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("a")
                     .AddOption("b")
                     .AddOption("c", "value")
@@ -226,7 +235,7 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"-abc", "value1", "value2"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddOption("a")
                     .AddOption("b")
                     .AddOption("c", "value1", "value2")
@@ -236,48 +245,51 @@ namespace CliFx.Tests
 
         [Theory]
         [MemberData(nameof(ShortOptionsTestData))]
-        internal void Option_can_be_set_by_specifying_its_short_name_after_a_single_dash(string[] arguments, CommandLineInput expectedInput)
+        internal void Option_can_be_set_by_specifying_its_short_name_after_a_single_dash(IReadOnlyList<string> arguments, CommandInput expectedInput)
         {
+            // Arrange
+            var commandNames = Array.Empty<string>();
+
             // Act
-            var input = CommandLineInput.Parse(arguments);
+            var input = CommandInput.Parse(arguments, commandNames);
 
             // Assert
             input.Should().BeEquivalentTo(expectedInput);
         }
 
-        public static object[][] UnboundArgumentsTestData => new[]
+        public static object[][] ParametersTestData => new[]
         {
             new object[]
             {
                 new[] {"foo"},
-                new CommandLineInputBuilder()
-                    .AddUnboundArgument("foo")
+                new CommandInputBuilder()
+                    .AddParameter("foo")
                     .Build()
             },
 
             new object[]
             {
                 new[] {"foo", "bar"},
-                new CommandLineInputBuilder()
-                    .AddUnboundArgument("foo")
-                    .AddUnboundArgument("bar")
+                new CommandInputBuilder()
+                    .AddParameter("foo")
+                    .AddParameter("bar")
                     .Build()
             },
 
             new object[]
             {
                 new[] {"[preview]", "foo"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddDirective("preview")
-                    .AddUnboundArgument("foo")
+                    .AddParameter("foo")
                     .Build()
             },
 
             new object[]
             {
                 new[] {"foo", "--option", "value", "-abc"},
-                new CommandLineInputBuilder()
-                    .AddUnboundArgument("foo")
+                new CommandInputBuilder()
+                    .AddParameter("foo")
                     .AddOption("option", "value")
                     .AddOption("a")
                     .AddOption("b")
@@ -288,11 +300,11 @@ namespace CliFx.Tests
             new object[]
             {
                 new[] {"[preview]", "[debug]", "foo", "bar", "--option", "value", "-abc"},
-                new CommandLineInputBuilder()
+                new CommandInputBuilder()
                     .AddDirective("preview")
                     .AddDirective("debug")
-                    .AddUnboundArgument("foo")
-                    .AddUnboundArgument("bar")
+                    .AddParameter("foo")
+                    .AddParameter("bar")
                     .AddOption("option", "value")
                     .AddOption("a")
                     .AddOption("b")
@@ -302,11 +314,62 @@ namespace CliFx.Tests
         };
 
         [Theory]
-        [MemberData(nameof(UnboundArgumentsTestData))]
-        internal void Any_remaining_arguments_are_treated_as_unbound_arguments(string[] arguments, CommandLineInput expectedInput)
+        [MemberData(nameof(ParametersTestData))]
+        internal void Parameter_can_be_set_by_specifying_the_value_directly(IReadOnlyList<string> arguments, CommandInput expectedInput)
+        {
+            // Arrange
+            var commandNames = Array.Empty<string>();
+
+            // Act
+            var input = CommandInput.Parse(arguments, commandNames);
+
+            // Assert
+            input.Should().BeEquivalentTo(expectedInput);
+        }
+
+        public static object[][] CommandNameTestData => new[]
+        {
+            new object[]
+            {
+                new[] {"cmd"},
+                new[] {"cmd"},
+                new CommandInputBuilder()
+                    .SetCommandName("cmd")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"cmd"},
+                new[] {"cmd", "foo", "bar", "-o", "value"},
+                new CommandInputBuilder()
+                    .SetCommandName("cmd")
+                    .AddParameter("foo")
+                    .AddParameter("bar")
+                    .AddOption("o", "value")
+                    .Build()
+            },
+
+            new object[]
+            {
+                new[] {"cmd", "cmd sub"},
+                new[] {"cmd", "sub", "foo"},
+                new CommandInputBuilder()
+                    .SetCommandName("cmd sub")
+                    .AddParameter("foo")
+                    .Build()
+            }
+        };
+
+        [Theory]
+        [MemberData(nameof(CommandNameTestData))]
+        internal void Command_name_is_matched_from_arguments_that_come_before_parameters(
+            IReadOnlyList<string> commandNames,
+            IReadOnlyList<string> arguments,
+            CommandInput expectedInput)
         {
             // Act
-            var input = CommandLineInput.Parse(arguments);
+            var input = CommandInput.Parse(arguments, commandNames);
 
             // Assert
             input.Should().BeEquivalentTo(expectedInput);

CliFx.Tests/CancellationSpecs.cs

@@ -13,6 +13,8 @@ namespace CliFx.Tests
         [Fact]
         public async Task Command_can_perform_additional_cleanup_if_cancellation_is_requested()
         {
+            // Can't test it with a real console because CliWrap can't send Ctrl+C
+
             // Arrange
             using var cts = new CancellationTokenSource();
 

CliFx.Tests/ConsoleSpecs.cs

@@ -51,6 +51,8 @@ namespace CliFx.Tests
             console.ResetColor();
             console.ForegroundColor = ConsoleColor.DarkMagenta;
             console.BackgroundColor = ConsoleColor.DarkMagenta;
+            console.CursorLeft = 42;
+            console.CursorTop = 24;
 
             // Assert
             stdInData.Should().Be("input");

CliFx.Tests/DirectivesSpecs.cs

@@ -1,6 +1,12 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.IO;
+using System.Text;
+using System.Threading;
 using System.Threading.Tasks;
+using CliFx.Tests.Internal;
+using CliWrap;
+using CliWrap.Buffered;
 using FluentAssertions;
 using Xunit;
 
@@ -9,7 +15,31 @@ namespace CliFx.Tests
     public partial class DirectivesSpecs
     {
         [Fact]
-        public async Task Preview_directive_can_be_enabled_to_print_provided_arguments_as_they_were_parsed()
+        public async Task Debug_directive_can_be_specified_to_have_the_application_wait_until_debugger_is_attached()
+        {
+            // We can't actually attach a debugger in tests, so instead just cancel execution after some time
+
+            // Arrange
+            using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(1));
+            var stdOut = new StringBuilder();
+
+            var command = Cli.Wrap("dotnet")
+                .WithArguments(a => a
+                    .Add(Dummy.Program.Location)
+                    .Add("[debug]"))
+                .WithEnvironmentVariables(e => e
+                    .Set("ENV_TARGET", "Mars")) | stdOut;
+
+            // Act
+            await command.ExecuteAsync(cts.Token).Task.IgnoreCancellation();
+            var stdOutData = stdOut.ToString();
+
+            // Assert
+            stdOutData.Should().Contain("Attach debugger to");
+        }
+
+        [Fact]
+        public async Task Preview_directive_can_be_specified_to_print_provided_arguments_as_they_were_parsed()
         {
             // Arrange
             await using var stdOut = new MemoryStream();
@@ -30,7 +60,7 @@ namespace CliFx.Tests
 
             // Assert
             exitCode.Should().Be(0);
-            stdOutData.Should().ContainAll("cmd", "<param>", "[-a]", "[-b]", "[-c]", "[--option foo]");
+            stdOutData.Should().ContainAll("cmd", "<param>", "[-a]", "[-b]", "[-c]", "[--option \"foo\"]");
         }
     }
 }

CliFx.Tests/EnvironmentVariablesSpecs.cs

@@ -2,6 +2,7 @@
 using System.IO;
 using System.Threading.Tasks;
 using CliFx.Domain;
+using CliFx.Tests.Internal;
 using CliWrap;
 using CliWrap.Buffered;
 using FluentAssertions;
@@ -53,19 +54,17 @@ namespace CliFx.Tests
         public void Option_of_non_scalar_type_can_take_multiple_separated_values_from_an_environment_variable()
         {
             // Arrange
-            var schema = ApplicationSchema.Resolve(new[] {typeof(EnvironmentVariableCollectionCommand)});
-
-            var input = CommandLineInput.Empty;
+            var input = CommandInput.Empty;
             var envVars = new Dictionary<string, string>
             {
                 ["ENV_OPT"] = $"foo{Path.PathSeparator}bar"
             };
 
             // Act
-            var command = schema.InitializeEntryPoint(input, envVars);
+            var instance = CommandHelper.ResolveCommand<EnvironmentVariableCollectionCommand>(input, envVars);
 
             // Assert
-            command.Should().BeEquivalentTo(new EnvironmentVariableCollectionCommand
+            instance.Should().BeEquivalentTo(new EnvironmentVariableCollectionCommand
             {
                 Option = new[] {"foo", "bar"}
             });
@@ -75,19 +74,17 @@ namespace CliFx.Tests
         public void Option_of_scalar_type_can_only_take_a_single_value_from_an_environment_variable_even_if_it_contains_separators()
         {
             // Arrange
-            var schema = ApplicationSchema.Resolve(new[] {typeof(EnvironmentVariableCommand)});
-
-            var input = CommandLineInput.Empty;
+            var input = CommandInput.Empty;
             var envVars = new Dictionary<string, string>
             {
                 ["ENV_OPT"] = $"foo{Path.PathSeparator}bar"
             };
 
             // Act
-            var command = schema.InitializeEntryPoint(input, envVars);
+            var instance = CommandHelper.ResolveCommand<EnvironmentVariableCommand>(input, envVars);
 
             // Assert
-            command.Should().BeEquivalentTo(new EnvironmentVariableCommand
+            instance.Should().BeEquivalentTo(new EnvironmentVariableCommand
             {
                 Option = $"foo{Path.PathSeparator}bar"
             });

CliFx.Tests/ErrorReportingSpecs.Commands.cs

@@ -25,53 +25,10 @@ namespace CliFx.Tests
             [CommandOption("msg", 'm')]
             public string? Message { get; set; }
 
-            public ValueTask ExecuteAsync(IConsole console) => throw new CommandException(Message, ExitCode);
-        }
+            [CommandOption("show-help")]
+            public bool ShowHelp { get; set; }
 
-        [Command("exc")]
-        private class ShowHelpTextOnlyCommand : ICommand
-        {
-            public ValueTask ExecuteAsync(IConsole console) => throw new CommandException(null, showHelp: true);
-        }
-
-        [Command("exc sub")]
-        private class ShowHelpTextOnlySubCommand : ICommand
-        {
-            public ValueTask ExecuteAsync(IConsole console) => default;
-        }
-
-        [Command("exc")]
-        private class ShowErrorMessageThenHelpTextCommand : ICommand
-        {
-            public ValueTask ExecuteAsync(IConsole console) =>
-                throw new CommandException("Error message.", showHelp: true);
-        }
-
-        [Command("exc sub")]
-        private class ShowErrorMessageThenHelpTextSubCommand : ICommand
-        {
-            public ValueTask ExecuteAsync(IConsole console) => default;
-        }
-
-        [Command("exc")]
-        private class StackTraceOnlyCommand : ICommand
-        {
-            [CommandOption("msg", 'm')]
-            public string? Message { get; set; }
-
-            public ValueTask ExecuteAsync(IConsole console) => throw new CommandException(null);
-        }
-
-        [Command("inv")]
-        private class InvalidUserInputCommand : ICommand
-        {
-            [CommandOption("required", 'r')]
-            public string? RequiredOption { get; }
-
-            public ValueTask ExecuteAsync(IConsole console)
-            {
-                throw new NotImplementedException();
-            }
+            public ValueTask ExecuteAsync(IConsole console) => throw new CommandException(Message, ExitCode, ShowHelp);
         }
     }
 }

CliFx.Tests/ErrorReportingSpecs.cs

@@ -82,104 +82,7 @@ namespace CliFx.Tests
 
             // Act
             var exitCode = await application.RunAsync(
-                new[] {"exc", "-m", "Kaput"},
-                new Dictionary<string, string>());
-
-            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
-
-            // Assert
-            exitCode.Should().NotBe(0);
-            stdErrData.Should().NotBeEmpty();
-
-            _output.WriteLine(stdErrData);
-        }
-
-        [Fact]
-        public async Task Command_may_throw_a_specialized_exception_which_shows_only_the_help_text()
-        {
-            // Arrange
-            await using var stdOut = new MemoryStream();
-            await using var stdErr = new MemoryStream();
-
-            var console = new VirtualConsole(output: stdOut);
-
-            var application = new CliApplicationBuilder()
-                .AddCommand(typeof(ShowHelpTextOnlyCommand))
-                .AddCommand(typeof(ShowHelpTextOnlySubCommand))
-                .UseConsole(console)
-                .Build();
-
-            // Act
-            await application.RunAsync(new[] {"exc"});
-            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
-            var stdErrData = console.Output.Encoding.GetString(stdErr.ToArray()).TrimEnd();
-
-            // Assert
-            stdErrData.Should().BeEmpty();
-            stdOutData.Should().ContainAll(
-                "Usage",
-                "[command]",
-                "Options",
-                "-h|--help", "Shows help text.",
-                "Commands",
-                "sub",
-                "You can run", "to show help on a specific command."
-            );
-
-            _output.WriteLine(stdOutData);
-            _output.WriteLine(stdErrData);
-        }
-
-        [Fact]
-        public async Task Command_may_throw_specialized_exception_which_shows_the_error_message_then_the_help_text()
-        {
-            // Arrange
-            await using var stdOut = new MemoryStream();
-            await using var stdErr = new MemoryStream();
-            var console = new VirtualConsole(output: stdOut, error: stdErr);
-
-            var application = new CliApplicationBuilder()
-                .AddCommand(typeof(ShowErrorMessageThenHelpTextCommand))
-                .AddCommand(typeof(ShowErrorMessageThenHelpTextSubCommand))
-                .UseConsole(console)
-                .Build();
-
-            // Act
-            await application.RunAsync(new[] {"exc"});
-            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
-            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
-
-            // Assert
-            stdErrData.Should().Be("Error message.");
-            stdOutData.Should().ContainAll(
-                "Usage",
-                "[command]",
-                "Options",
-                "-h|--help", "Shows help text.",
-                "Commands",
-                "sub",
-                "You can run", "to show help on a specific command."
-            );
-
-            _output.WriteLine(stdOutData);
-            _output.WriteLine(stdErrData);
-        }
-
-        [Fact]
-        public async Task Command_may_throw_a_specialized_exception_which_shows_only_a_stack_trace_and_no_help_text()
-        {
-            // Arrange
-            await using var stdErr = new MemoryStream();
-            var console = new VirtualConsole(error: stdErr);
-
-            var application = new CliApplicationBuilder()
-                .AddCommand(typeof(GenericExceptionCommand))
-                .UseConsole(console)
-                .Build();
-
-            // Act
-            var exitCode = await application.RunAsync(
-                new[] {"exc", "-m", "Kaput"},
+                new[] {"exc"},
                 new Dictionary<string, string>());
 
             var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
@@ -187,15 +90,51 @@ namespace CliFx.Tests
             // Assert
             exitCode.Should().NotBe(0);
             stdErrData.Should().ContainAll(
-                "System.Exception:",
-                "Kaput", "at",
+                "CliFx.Exceptions.CommandException:",
+                "at",
                 "CliFx.Tests");
 
             _output.WriteLine(stdErrData);
         }
 
         [Fact]
-        public async Task Command_shows_help_text_on_exceptions_related_to_invalid_user_input()
+        public async Task Command_may_throw_a_specialized_exception_which_exits_and_prints_help_text()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            await using var stdErr = new MemoryStream();
+
+            var console = new VirtualConsole(output: stdOut, error: stdErr);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(CommandExceptionCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            var exitCode = await application.RunAsync(
+                new[] {"exc", "-m", "Kaput", "--show-help"},
+                new Dictionary<string, string>());
+
+            var stdErrData = console.Error.Encoding.GetString(stdErr.ToArray()).TrimEnd();
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            exitCode.Should().NotBe(0);
+            stdErrData.Should().Be("Kaput");
+
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "Options",
+                "-h|--help", "Shows help text."
+            );
+
+            _output.WriteLine(stdErrData);
+            _output.WriteLine(stdOutData);
+        }
+
+        [Fact]
+        public async Task Command_shows_help_text_on_invalid_user_input()
         {
             // Arrange
             await using var stdOut = new MemoryStream();
@@ -203,7 +142,7 @@ namespace CliFx.Tests
             var console = new VirtualConsole(output: stdOut, error: stdErr);
 
             var application = new CliApplicationBuilder()
-                .AddCommand(typeof(InvalidUserInputCommand))
+                .AddCommand(typeof(CommandExceptionCommand))
                 .UseConsole(console)
                 .Build();
 
@@ -217,22 +156,17 @@ namespace CliFx.Tests
 
             // Assert
             exitCode.Should().NotBe(0);
-            stdErrData.Should().ContainAll(
-                "Can't find a command that matches the following arguments:",
-                "not-a-valid-command"
-            );
+            stdErrData.Should().NotBeNullOrWhiteSpace();
+
             stdOutData.Should().ContainAll(
                 "Usage",
                 "[command]",
                 "Options",
-                "-h|--help", "Shows help text.",
-                "Commands",
-                "inv",
-                "You can run", "to show help on a specific command."
+                "-h|--help", "Shows help text."
             );
 
-            _output.WriteLine(stdOutData);
             _output.WriteLine(stdErrData);
+            _output.WriteLine(stdOutData);
         }
     }
 }

CliFx.Tests/HelpTextSpecs.Commands.cs

@@ -1,7 +1,7 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Threading.Tasks;
 using CliFx.Attributes;
-using CliFx.Exceptions;
 
 namespace CliFx.Tests
 {
@@ -70,14 +70,14 @@ namespace CliFx.Tests
         [Command("cmd-with-req-opts")]
         private class RequiredOptionsCommand : ICommand
         {
-            [CommandOption("option-f", 'f', IsRequired = true)]
-            public string? OptionF { get; set; }
+            [CommandOption("option-a", 'a', IsRequired = true)]
+            public string? OptionA { get; set; }
 
-            [CommandOption("option-g", 'g', IsRequired = true)]
-            public IEnumerable<int>? OptionG { get; set; }
+            [CommandOption("option-b", 'b', IsRequired = true)]
+            public IEnumerable<int>? OptionB { get; set; }
 
-            [CommandOption("option-h", 'h')]
-            public string? OptionH { get; set; }
+            [CommandOption("option-c", 'c')]
+            public string? OptionC { get; set; }
 
             public ValueTask ExecuteAsync(IConsole console) => default;
         }
@@ -85,16 +85,16 @@ namespace CliFx.Tests
         [Command("cmd-with-enum-args")]
         private class EnumArgumentsCommand : ICommand
         {
-            public enum TestEnum { Value1, Value2, Value3 };
+            public enum CustomEnum { Value1, Value2, Value3 };
 
             [CommandParameter(0, Name = "value", Description = "Enum parameter.")]
-            public TestEnum ParamA { get; set; }
+            public CustomEnum ParamA { get; set; }
 
             [CommandOption("value", Description = "Enum option.", IsRequired = true)]
-            public TestEnum OptionA { get; set; } = TestEnum.Value1;
+            public CustomEnum OptionA { get; set; } = CustomEnum.Value1;
 
             [CommandOption("nullable-value", Description = "Nullable enum option.")]
-            public TestEnum? OptionB { get; set; }
+            public CustomEnum? OptionB { get; set; }
 
             public ValueTask ExecuteAsync(IConsole console) => default;
         }
@@ -110,5 +110,46 @@ namespace CliFx.Tests
 
             public ValueTask ExecuteAsync(IConsole console) => default;
         }
+
+        [Command("cmd-with-defaults")]
+        private class ArgumentsWithDefaultValuesCommand : ICommand
+        {
+            public enum CustomEnum { Value1, Value2, Value3 };
+
+            [CommandOption(nameof(Object))]
+            public object? Object { get; set; } = 42;
+
+            [CommandOption(nameof(String))]
+            public string? String { get; set; } = "foo";
+
+            [CommandOption(nameof(EmptyString))]
+            public string EmptyString { get; set; } = "";
+
+            [CommandOption(nameof(Bool))]
+            public bool Bool { get; set; } = true;
+
+            [CommandOption(nameof(Char))]
+            public char Char { get; set; } = 't';
+
+            [CommandOption(nameof(Int))]
+            public int Int { get; set; } = 1337;
+
+            [CommandOption(nameof(TimeSpan))]
+            public TimeSpan TimeSpan { get; set; } = TimeSpan.FromMinutes(123);
+
+            [CommandOption(nameof(Enum))]
+            public CustomEnum Enum { get; set; } = CustomEnum.Value2;
+
+            [CommandOption(nameof(IntNullable))]
+            public int? IntNullable { get; set; } = 1337;
+
+            [CommandOption(nameof(StringArray))]
+            public string[]? StringArray { get; set; } = { "foo", "bar", "baz" };
+
+            [CommandOption(nameof(IntArray))]
+            public int[]? IntArray { get; set; } = { 1, 2, 3 };
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
     }
 }

CliFx.Tests/HelpTextSpecs.cs

@@ -1,4 +1,6 @@
-using System.IO;
+using System;
+using System.Globalization;
+using System.IO;
 using System.Threading.Tasks;
 using FluentAssertions;
 using Xunit;
@@ -228,18 +230,18 @@ namespace CliFx.Tests
             // Assert
             stdOutData.Should().ContainAll(
                 "Usage",
-                "cmd-with-req-opts", "--option-f <value>", "--option-g <values...>", "[options]",
+                "cmd-with-req-opts", "--option-a <value>", "--option-b <values...>", "[options]",
                 "Options",
-                "* -f|--option-f",
-                "* -g|--option-g",
-                "-h|--option-h"
+                "* -a|--option-a",
+                "* -b|--option-b",
+                "-c|--option-c"
             );
 
             _output.WriteLine(stdOutData);
         }
 
         [Fact]
-        public async Task Help_text_shows_usage_format_which_lists_all_valid_values_for_enum_arguments()
+        public async Task Help_text_lists_all_valid_values_for_enum_arguments()
         {
             // Arrange
             await using var stdOut = new MemoryStream();
@@ -251,7 +253,7 @@ namespace CliFx.Tests
                 .Build();
 
             // Act
-            await application.RunAsync(new[] { "cmd-with-enum-args", "--help" });
+            await application.RunAsync(new[] {"cmd-with-enum-args", "--help"});
             var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
 
             // Assert
@@ -259,10 +261,10 @@ namespace CliFx.Tests
                 "Usage",
                 "cmd-with-enum-args", "[options]",
                 "Parameters",
-                "value", "Valid values: Value1, Value2, Value3.",
+                "value", "Valid values: \"Value1\", \"Value2\", \"Value3\".",
                 "Options",
-                "* --value", "Enum option.", "Valid values: Value1, Value2, Value3.",
-                "--nullable-value", "Nullable enum option.", "Valid values: Value1, Value2, Value3."
+                "* --value", "Enum option.", "Valid values: \"Value1\", \"Value2\", \"Value3\".",
+                "--nullable-value", "Nullable enum option.", "Valid values: \"Value1\", \"Value2\", \"Value3\"."
             );
 
             _output.WriteLine(stdOutData);
@@ -293,5 +295,42 @@ namespace CliFx.Tests
 
             _output.WriteLine(stdOutData);
         }
+
+        [Fact]
+        public async Task Help_text_shows_default_values_for_non_required_options()
+        {
+            // Arrange
+            await using var stdOut = new MemoryStream();
+            var console = new VirtualConsole(output: stdOut);
+
+            var application = new CliApplicationBuilder()
+                .AddCommand(typeof(ArgumentsWithDefaultValuesCommand))
+                .UseConsole(console)
+                .Build();
+
+            // Act
+            await application.RunAsync(new[] {"cmd-with-defaults", "--help"});
+            var stdOutData = console.Output.Encoding.GetString(stdOut.ToArray()).TrimEnd();
+
+            // Assert
+            stdOutData.Should().ContainAll(
+                "Usage",
+                "cmd-with-defaults", "[options]",
+                "Options",
+                "--Object", "Default: \"42\"",
+                "--String", "Default: \"foo\"",
+                "--EmptyString", "Default: \"\"",
+                "--Bool", "Default: \"True\"",
+                "--Char", "Default: \"t\"",
+                "--Int", "Default: \"1337\"",
+                "--TimeSpan", "Default: \"02:03:00\"",
+                "--Enum", "Default: \"Value2\"",
+                "--IntNullable", "Default: \"1337\"",
+                "--StringArray", "Default: \"foo\" \"bar\" \"baz\"",
+                "--IntArray", "Default: \"1\" \"2\" \"3\""
+            );
+
+            _output.WriteLine(stdOutData);
+        }
     }
 }

CliFx.Tests/Internal/CommandHelper.cs

@@ -0,0 +1,23 @@
+using System.Collections.Generic;
+using CliFx.Domain;
+
+namespace CliFx.Tests.Internal
+{
+    internal static class CommandHelper
+    {
+        public static TCommand ResolveCommand<TCommand>(CommandInput input, IReadOnlyDictionary<string, string> environmentVariables)
+            where TCommand : ICommand, new()
+        {
+            var schema = CommandSchema.TryResolve(typeof(TCommand))!;
+
+            var instance = new TCommand();
+            schema.Bind(instance, input, environmentVariables);
+
+            return instance;
+        }
+
+        public static TCommand ResolveCommand<TCommand>(CommandInput input)
+            where TCommand : ICommand, new() =>
+            ResolveCommand<TCommand>(input, new Dictionary<string, string>());
+    }
+}

CliFx.Tests/Internal/CommandInputBuilder.cs

@@ -0,0 +1,45 @@
+using System.Collections.Generic;
+using CliFx.Domain;
+
+namespace CliFx.Tests.Internal
+{
+    internal class CommandInputBuilder
+    {
+        private readonly List<CommandDirectiveInput> _directives = new List<CommandDirectiveInput>();
+        private readonly List<CommandParameterInput> _parameters = new List<CommandParameterInput>();
+        private readonly List<CommandOptionInput> _options = new List<CommandOptionInput>();
+
+        private string? _commandName;
+
+        public CommandInputBuilder SetCommandName(string commandName)
+        {
+            _commandName = commandName;
+            return this;
+        }
+
+        public CommandInputBuilder AddDirective(string directive)
+        {
+            _directives.Add(new CommandDirectiveInput(directive));
+            return this;
+        }
+
+        public CommandInputBuilder AddParameter(string parameter)
+        {
+            _parameters.Add(new CommandParameterInput(parameter));
+            return this;
+        }
+
+        public CommandInputBuilder AddOption(string alias, params string[] values)
+        {
+            _options.Add(new CommandOptionInput(alias, values));
+            return this;
+        }
+
+        public CommandInput Build() => new CommandInput(
+            _directives,
+            _commandName,
+            _parameters,
+            _options
+        );
+    }
+}

CliFx.Tests/Internal/TaskExtensions.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Threading.Tasks;
+
+namespace CliFx.Tests.Internal
+{
+    internal static class TaskExtensions
+    {
+        public static async Task IgnoreCancellation(this Task task)
+        {
+            try
+            {
+                await task;
+            }
+            catch (OperationCanceledException)
+            {
+            }
+        }
+    }
+}

CliFx.props

@@ -1,7 +1,7 @@
 <Project>
 
   <PropertyGroup>
-    <Version>1.2</Version>
+    <Version>1.3</Version>
     <Company>Tyrrrz</Company>
     <Copyright>Copyright (C) Alexey Golub</Copyright>
     <LangVersion>latest</LangVersion>

CliFx/CliApplication.cs

@@ -4,15 +4,17 @@ using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
 using System.Threading.Tasks;
+using CliFx.Attributes;
 using CliFx.Domain;
 using CliFx.Exceptions;
+using CliFx.Internal;
 
 namespace CliFx
 {
     /// <summary>
     /// Command line application facade.
     /// </summary>
-    public class CliApplication
+    public partial class CliApplication
     {
         private readonly ApplicationMetadata _metadata;
         private readonly ApplicationConfiguration _configuration;
@@ -36,42 +38,33 @@ namespace CliFx
             _helpTextWriter = new HelpTextWriter(metadata, console);
         }
 
-        private async ValueTask<int?> HandleDebugDirectiveAsync(CommandLineInput commandLineInput)
+        private void WriteError(string message) => _console.WithForegroundColor(ConsoleColor.Red, () =>
+            _console.Error.WriteLine(message));
+
+        private async ValueTask WaitForDebuggerAsync()
         {
-            var isDebugMode = _configuration.IsDebugModeAllowed && commandLineInput.IsDebugDirectiveSpecified;
-            if (!isDebugMode)
-                return null;
+            var processId = ProcessEx.GetCurrentProcessId();
 
             _console.WithForegroundColor(ConsoleColor.Green, () =>
-                _console.Output.WriteLine($"Attach debugger to PID {Process.GetCurrentProcess().Id} to continue."));
+                _console.Output.WriteLine($"Attach debugger to PID {processId} to continue."));
 
             while (!Debugger.IsAttached)
                 await Task.Delay(100);
-
-            return null;
         }
 
-        private int? HandlePreviewDirective(ApplicationSchema applicationSchema, CommandLineInput commandLineInput)
+        private void WriteCommandLineInput(CommandInput input)
         {
-            var isPreviewMode = _configuration.IsPreviewModeAllowed && commandLineInput.IsPreviewDirectiveSpecified;
-            if (!isPreviewMode)
-                return null;
-
-            var commandSchema = applicationSchema.TryFindCommand(commandLineInput, out var argumentOffset);
-
-            _console.Output.WriteLine("Parser preview:");
-
             // Command name
-            if (commandSchema != null && argumentOffset > 0)
+            if (!string.IsNullOrWhiteSpace(input.CommandName))
             {
                 _console.WithForegroundColor(ConsoleColor.Cyan, () =>
-                    _console.Output.Write(commandSchema.Name));
+                    _console.Output.Write(input.CommandName));
 
                 _console.Output.Write(' ');
             }
 
             // Parameters
-            foreach (var parameter in commandLineInput.UnboundArguments.Skip(argumentOffset))
+            foreach (var parameter in input.Parameters)
             {
                 _console.Output.Write('<');
 
@@ -83,123 +76,133 @@ namespace CliFx
             }
 
             // Options
-            foreach (var option in commandLineInput.Options)
+            foreach (var option in input.Options)
             {
                 _console.Output.Write('[');
 
                 _console.WithForegroundColor(ConsoleColor.White, () =>
-                    _console.Output.Write(option));
+                {
+                    // Alias
+                    _console.Output.Write(option.GetRawAlias());
+
+                    // Values
+                    if (option.Values.Any())
+                    {
+                        _console.Output.Write(' ');
+                        _console.Output.Write(option.GetRawValues());
+                    }
+                });
 
                 _console.Output.Write(']');
                 _console.Output.Write(' ');
             }
 
             _console.Output.WriteLine();
-
-            return 0;
         }
 
-        private int? HandleVersionOption(CommandLineInput commandLineInput)
-        {
-            // Version option is available only on the default command (i.e. when arguments are not specified)
-            var shouldRenderVersion = !commandLineInput.UnboundArguments.Any() && commandLineInput.IsVersionOptionSpecified;
-            if (!shouldRenderVersion)
-                return null;
-
-            _console.Output.WriteLine(_metadata.VersionText);
-
-            return 0;
-        }
-
-        private int? HandleHelpOption(ApplicationSchema applicationSchema, CommandLineInput commandLineInput)
-        {
-            // Help is rendered either when it's requested or when the user provides no arguments and there is no default command
-            var shouldRenderHelp =
-                commandLineInput.IsHelpOptionSpecified ||
-                !applicationSchema.Commands.Any(c => c.IsDefault) && !commandLineInput.UnboundArguments.Any() && !commandLineInput.Options.Any();
-
-            if (!shouldRenderHelp)
-                return null;
-
-            // Get the command schema that matches the input or use a dummy default command as a fallback
-            var commandSchema =
-                applicationSchema.TryFindCommand(commandLineInput) ??
-                CommandSchema.StubDefaultCommand;
-
-            _helpTextWriter.Write(applicationSchema, commandSchema);
-
-            return 0;
-        }
-
-        private async ValueTask<int> HandleCommandExecutionAsync(
-            ApplicationSchema applicationSchema,
-            CommandLineInput commandLineInput,
-            IReadOnlyDictionary<string, string> environmentVariables)
-        {
-            await applicationSchema
-                .InitializeEntryPoint(commandLineInput, environmentVariables, _typeActivator)
-                .ExecuteAsync(_console);
-
-            return 0;
-        }
-
-        /// <summary>
-        /// Handle <see cref="CommandException"/>s differently from the rest because we want to
-        /// display it different based on whether we are showing the help text or not.
-        /// </summary>
-        private int HandleCliFxException(IReadOnlyList<string> commandLineArguments, CliFxException cfe)
-        {
-            var showHelp = cfe.ShowHelp;
-
-            var errorMessage = cfe.HasMessage
-                ? cfe.Message
-                : cfe.ToString();
-               
-            _console.WithForegroundColor(ConsoleColor.Red, () => _console.Error.WriteLine(errorMessage));            
-
-            if (showHelp)
-            {
-                var applicationSchema = ApplicationSchema.Resolve(_configuration.CommandTypes);
-                var commandLineInput = CommandLineInput.Parse(commandLineArguments);
-                var commandSchema = applicationSchema.TryFindCommand(commandLineInput) ??
-                    CommandSchema.StubDefaultCommand;
-                _helpTextWriter.Write(applicationSchema, commandSchema);
-            }
-
-            return cfe.ExitCode;
-        }
+        private ICommand GetCommandInstance(CommandSchema command) =>
+            command != StubDefaultCommand.Schema
+                ? (ICommand) _typeActivator.CreateInstance(command.Type)
+                : new StubDefaultCommand();
 
         /// <summary>
         /// Runs the application with specified command line arguments and environment variables, and returns the exit code.
         /// </summary>
+        /// <remarks>
+        /// If a <see cref="CommandException"/> is thrown during command execution, it will be handled and routed to the console.
+        /// Additionally, if the debugger is not attached (i.e. the app is running in production), all other exceptions thrown within
+        /// this method will be handled and routed to the console as well.
+        /// </remarks>
         public async ValueTask<int> RunAsync(
             IReadOnlyList<string> commandLineArguments,
             IReadOnlyDictionary<string, string> environmentVariables)
         {
             try
             {
-                var applicationSchema = ApplicationSchema.Resolve(_configuration.CommandTypes);
-                var commandLineInput = CommandLineInput.Parse(commandLineArguments);
+                var root = RootSchema.Resolve(_configuration.CommandTypes);
+                var input = CommandInput.Parse(commandLineArguments, root.GetCommandNames());
 
-                return
-                    await HandleDebugDirectiveAsync(commandLineInput) ??
-                    HandlePreviewDirective(applicationSchema, commandLineInput) ??
-                    HandleVersionOption(commandLineInput) ??
-                    HandleHelpOption(applicationSchema, commandLineInput) ??
-                    await HandleCommandExecutionAsync(applicationSchema, commandLineInput, environmentVariables);
+                // Debug mode
+                if (_configuration.IsDebugModeAllowed && input.IsDebugDirectiveSpecified)
+                {
+                    // Ensure debugger is attached and continue
+                    await WaitForDebuggerAsync();
+                }
+
+                // Preview mode
+                if (_configuration.IsPreviewModeAllowed && input.IsPreviewDirectiveSpecified)
+                {
+                    WriteCommandLineInput(input);
+                    return ExitCode.Success;
+                }
+
+                // Try to get the command matching the input or fallback to default
+                var command =
+                    root.TryFindCommand(input.CommandName) ??
+                    root.TryFindDefaultCommand() ??
+                    StubDefaultCommand.Schema;
+
+                // Version option
+                if (command.IsVersionOptionAvailable && input.IsVersionOptionSpecified)
+                {
+                    _console.Output.WriteLine(_metadata.VersionText);
+                    return ExitCode.Success;
+                }
+
+                // Get command instance (also used in help text)
+                var instance = GetCommandInstance(command);
+
+                // To avoid instantiating the command twice, we need to get default values
+                // before the arguments are bound to the properties
+                var defaultValues = command.GetArgumentValues(instance);
+
+                // Help option
+                if (command.IsHelpOptionAvailable && input.IsHelpOptionSpecified ||
+                    command == StubDefaultCommand.Schema && !input.Parameters.Any() && !input.Options.Any())
+                {
+                    _helpTextWriter.Write(root, command, defaultValues);
+                    return ExitCode.Success;
+                }
+
+                // Bind arguments
+                try
+                {
+                    command.Bind(instance, input, environmentVariables);
+                }
+                // This may throw exceptions which are useful only to the end-user
+                catch (CliFxException ex)
+                {
+                    WriteError(ex.ToString());
+                    _helpTextWriter.Write(root, command, defaultValues);
+
+                    return ExitCode.FromException(ex);
+                }
+
+                // Execute the command
+                try
+                {
+                    await instance.ExecuteAsync(_console);
+                    return ExitCode.Success;
+                }
+                // Swallow command exceptions and route them to the console
+                catch (CommandException ex)
+                {
+                    WriteError(ex.ToString());
+
+                    if (ex.ShowHelp)
+                        _helpTextWriter.Write(root, command, defaultValues);
+
+                    return ex.ExitCode;
+                }
             }
-            catch (CliFxException cfe)
+            // To prevent the app from showing the annoying Windows troubleshooting dialog,
+            // we handle all exceptions and route them to the console nicely.
+            // However, we don't want to swallow unhandled exceptions when the debugger is attached,
+            // because we still want the IDE to show them to the developer.
+            catch (Exception ex) when (!Debugger.IsAttached)
             {
-                // We want to catch exceptions in order to print errors and return correct exit codes.
-                // Doing this also gets rid of the annoying Windows troubleshooting dialog that shows up on unhandled exceptions.
-                var exitCode = HandleCliFxException(commandLineArguments, cfe);
-                return exitCode;
-            }
-            catch (Exception ex)
-            {
-                // For all other errors, we just write the entire thing to stderr.
-                _console.WithForegroundColor(ConsoleColor.Red, () => _console.Error.WriteLine(ex.ToString()));
-                return ex.HResult;
+                WriteError(ex.ToString());
+                return ExitCode.FromException(ex);
             }
         }
 
@@ -207,6 +210,11 @@ namespace CliFx
         /// Runs the application with specified command line arguments and returns the exit code.
         /// Environment variables are retrieved automatically.
         /// </summary>
+        /// <remarks>
+        /// If a <see cref="CommandException"/> is thrown during command execution, it will be handled and routed to the console.
+        /// Additionally, if the debugger is not attached (i.e. the app is running in production), all other exceptions thrown within
+        /// this method will be handled and routed to the console as well.
+        /// </remarks>
         public async ValueTask<int> RunAsync(IReadOnlyList<string> commandLineArguments)
         {
             var environmentVariables = Environment.GetEnvironmentVariables()
@@ -220,6 +228,11 @@ namespace CliFx
         /// Runs the application and returns the exit code.
         /// Command line arguments and environment variables are retrieved automatically.
         /// </summary>
+        /// <remarks>
+        /// If a <see cref="CommandException"/> is thrown during command execution, it will be handled and routed to the console.
+        /// Additionally, if the debugger is not attached (i.e. the app is running in production), all other exceptions thrown within
+        /// this method will be handled and routed to the console as well.
+        /// </remarks>
         public async ValueTask<int> RunAsync()
         {
             var commandLineArguments = Environment.GetCommandLineArgs()
@@ -229,4 +242,25 @@ namespace CliFx
             return await RunAsync(commandLineArguments);
         }
     }
+
+    public partial class CliApplication
+    {
+        private static class ExitCode
+        {
+            public const int Success = 0;
+
+            public static int FromException(Exception ex) =>
+                ex is CommandException cmdEx
+                    ? cmdEx.ExitCode
+                    : ex.HResult;
+        }
+
+        [Command]
+        private class StubDefaultCommand : ICommand
+        {
+            public static CommandSchema Schema { get; } = CommandSchema.TryResolve(typeof(StubDefaultCommand))!;
+
+            public ValueTask ExecuteAsync(IConsole console) => default;
+        }
+    }
 }

CliFx/CliApplicationBuilder.cs

@@ -4,6 +4,7 @@ using System.IO;
 using System.Linq;
 using System.Reflection;
 using CliFx.Domain;
+using CliFx.Internal;
 
 namespace CliFx
 {
@@ -158,9 +159,9 @@ namespace CliFx
         /// </summary>
         public CliApplication Build()
         {
-            _title ??= GetDefaultTitle() ?? "App";
-            _executableName ??= GetDefaultExecutableName() ?? "app";
-            _versionText ??= GetDefaultVersionText() ?? "v1.0";
+            _title ??= TryGetDefaultTitle() ?? "App";
+            _executableName ??= TryGetDefaultExecutableName() ?? "app";
+            _versionText ??= TryGetDefaultVersionText() ?? "v1.0";
             _console ??= new SystemConsole();
             _typeActivator ??= new DefaultTypeActivator();
 
@@ -178,9 +179,9 @@ namespace CliFx
         // Entry assembly is null in tests
         private static Assembly? EntryAssembly => LazyEntryAssembly.Value;
 
-        private static string? GetDefaultTitle() => EntryAssembly?.GetName().Name;
+        private static string? TryGetDefaultTitle() => EntryAssembly?.GetName().Name;
 
-        private static string? GetDefaultExecutableName()
+        private static string? TryGetDefaultExecutableName()
         {
             var entryAssemblyLocation = EntryAssembly?.Location;
 
@@ -192,9 +193,9 @@ namespace CliFx
                 : Path.GetFileNameWithoutExtension(entryAssemblyLocation);
         }
 
-        private static string? GetDefaultVersionText() =>
+        private static string? TryGetDefaultVersionText() =>
             EntryAssembly != null
-                ? $"v{EntryAssembly.GetName().Version}"
+                ? $"v{EntryAssembly.GetName().Version.ToSemanticString()}"
                 : null;
     }
 }

CliFx/DefaultTypeActivator.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Text;
 using CliFx.Exceptions;
 
 namespace CliFx

CliFx/DelegateTypeActivator.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Text;
 using CliFx.Exceptions;
 
 namespace CliFx
@@ -18,6 +17,6 @@ namespace CliFx
 
         /// <inheritdoc />
         public object CreateInstance(Type type) =>
-            _func(type) ?? throw CliFxException.DelegateActivatorReceivedNull(type);
+            _func(type) ?? throw CliFxException.DelegateActivatorReturnedNull(type);
     }
 }

CliFx/Domain/ApplicationSchema.cs

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

CliFx/Domain/CommandArgumentSchema.cs

@@ -10,22 +10,21 @@ namespace CliFx.Domain
 {
     internal abstract partial class CommandArgumentSchema
     {
-        public PropertyInfo Property { get; }
+        // Property can be null on built-in arguments (help and version options)
+        public PropertyInfo? Property { get; }
 
         public string? Description { get; }
 
-        public abstract string DisplayName { get; }
-
         public bool IsScalar => TryGetEnumerableArgumentUnderlyingType() == null;
 
-        protected CommandArgumentSchema(PropertyInfo property, string? description)
+        protected CommandArgumentSchema(PropertyInfo? property, string? description)
         {
             Property = property;
             Description = description;
         }
 
         private Type? TryGetEnumerableArgumentUnderlyingType() =>
-            Property.PropertyType != typeof(string)
+            Property != null && Property.PropertyType != typeof(string)
                 ? Property.PropertyType.GetEnumerableUnderlyingType()
                 : null;
 
@@ -50,17 +49,17 @@ namespace CliFx.Domain
                         : null;
 
                 // String-constructable
-                var stringConstructor = GetStringConstructor(targetType);
+                var stringConstructor = targetType.GetConstructor(new[] {typeof(string)});
                 if (stringConstructor != null)
                     return stringConstructor.Invoke(new object[] {value!});
 
                 // String-parseable (with format provider)
-                var parseMethodWithFormatProvider = GetStaticParseMethodWithFormatProvider(targetType);
+                var parseMethodWithFormatProvider = targetType.GetStaticParseMethod(true);
                 if (parseMethodWithFormatProvider != null)
-                    return parseMethodWithFormatProvider.Invoke(null, new object[] {value!, ConversionFormatProvider});
+                    return parseMethodWithFormatProvider.Invoke(null, new object[] {value!, FormatProvider});
 
                 // String-parseable (without format provider)
-                var parseMethod = GetStaticParseMethod(targetType);
+                var parseMethod = targetType.GetStaticParseMethod();
                 if (parseMethod != null)
                     return parseMethod.Invoke(null, new object[] {value!});
             }
@@ -94,6 +93,10 @@ namespace CliFx.Domain
 
         private object? Convert(IReadOnlyList<string> values)
         {
+            // Short-circuit built-in arguments
+            if (Property == null)
+                return null;
+
             var targetType = Property.PropertyType;
             var enumerableUnderlyingType = TryGetEnumerableArgumentUnderlyingType();
 
@@ -111,34 +114,32 @@ namespace CliFx.Domain
             }
         }
 
-        public void Inject(ICommand command, IReadOnlyList<string> values) =>
-            Property.SetValue(command, Convert(values));
+        public void BindOn(ICommand command, IReadOnlyList<string> values) =>
+            Property?.SetValue(command, Convert(values));
 
-        public void Inject(ICommand command, params string[] values) =>
-            Inject(command, (IReadOnlyList<string>) values);
+        public void BindOn(ICommand command, params string[] values) =>
+            BindOn(command, (IReadOnlyList<string>) values);
 
         public IReadOnlyList<string> GetValidValues()
         {
-            var result = new List<string>();
-
-            // Some arguments may have this as null due to a hack that enables built-in options
             if (Property == null)
-                return result;
+                return Array.Empty<string>();
 
-            var underlyingPropertyType =
-                Property.PropertyType.GetNullableUnderlyingType() ?? Property.PropertyType;
+            var underlyingType =
+                Property.PropertyType.GetNullableUnderlyingType() ??
+                Property.PropertyType;
 
             // Enum
-            if (underlyingPropertyType.IsEnum)
-                result.AddRange(Enum.GetNames(underlyingPropertyType));
+            if (underlyingType.IsEnum)
+                return Enum.GetNames(underlyingType);
 
-            return result;
+            return Array.Empty<string>();
         }
     }
 
     internal partial class CommandArgumentSchema
     {
-        private static readonly IFormatProvider ConversionFormatProvider = CultureInfo.InvariantCulture;
+        private static readonly IFormatProvider FormatProvider = CultureInfo.InvariantCulture;
 
         private static readonly IReadOnlyDictionary<Type, Func<string?, object?>> PrimitiveConverters =
             new Dictionary<Type, Func<string?, object?>>
@@ -147,33 +148,20 @@ namespace CliFx.Domain
                 [typeof(string)] = v => v,
                 [typeof(bool)] = v => string.IsNullOrWhiteSpace(v) || bool.Parse(v),
                 [typeof(char)] = v => v.Single(),
-                [typeof(sbyte)] = v => sbyte.Parse(v, ConversionFormatProvider),
-                [typeof(byte)] = v => byte.Parse(v, ConversionFormatProvider),
-                [typeof(short)] = v => short.Parse(v, ConversionFormatProvider),
-                [typeof(ushort)] = v => ushort.Parse(v, ConversionFormatProvider),
-                [typeof(int)] = v => int.Parse(v, ConversionFormatProvider),
-                [typeof(uint)] = v => uint.Parse(v, ConversionFormatProvider),
-                [typeof(long)] = v => long.Parse(v, ConversionFormatProvider),
-                [typeof(ulong)] = v => ulong.Parse(v, ConversionFormatProvider),
-                [typeof(float)] = v => float.Parse(v, ConversionFormatProvider),
-                [typeof(double)] = v => double.Parse(v, ConversionFormatProvider),
-                [typeof(decimal)] = v => decimal.Parse(v, ConversionFormatProvider),
-                [typeof(DateTime)] = v => DateTime.Parse(v, ConversionFormatProvider),
-                [typeof(DateTimeOffset)] = v => DateTimeOffset.Parse(v, ConversionFormatProvider),
-                [typeof(TimeSpan)] = v => TimeSpan.Parse(v, ConversionFormatProvider),
+                [typeof(sbyte)] = v => sbyte.Parse(v, FormatProvider),
+                [typeof(byte)] = v => byte.Parse(v, FormatProvider),
+                [typeof(short)] = v => short.Parse(v, FormatProvider),
+                [typeof(ushort)] = v => ushort.Parse(v, FormatProvider),
+                [typeof(int)] = v => int.Parse(v, FormatProvider),
+                [typeof(uint)] = v => uint.Parse(v, FormatProvider),
+                [typeof(long)] = v => long.Parse(v, FormatProvider),
+                [typeof(ulong)] = v => ulong.Parse(v, FormatProvider),
+                [typeof(float)] = v => float.Parse(v, FormatProvider),
+                [typeof(double)] = v => double.Parse(v, FormatProvider),
+                [typeof(decimal)] = v => decimal.Parse(v, FormatProvider),
+                [typeof(DateTime)] = v => DateTime.Parse(v, FormatProvider),
+                [typeof(DateTimeOffset)] = v => DateTimeOffset.Parse(v, FormatProvider),
+                [typeof(TimeSpan)] = v => TimeSpan.Parse(v, FormatProvider),
             };
-
-        private static ConstructorInfo? GetStringConstructor(Type type) =>
-            type.GetConstructor(new[] {typeof(string)});
-
-        private static MethodInfo? GetStaticParseMethod(Type type) =>
-            type.GetMethod("Parse",
-                BindingFlags.Public | BindingFlags.Static,
-                null, new[] {typeof(string)}, null);
-
-        private static MethodInfo? GetStaticParseMethodWithFormatProvider(Type type) =>
-            type.GetMethod("Parse",
-                BindingFlags.Public | BindingFlags.Static,
-                null, new[] {typeof(string), typeof(IFormatProvider)}, null);
     }
 }

CliFx/Domain/CommandDirectiveInput.cs

@@ -10,10 +10,7 @@ namespace CliFx.Domain
 
         public bool IsPreviewDirective => string.Equals(Name, "preview", StringComparison.OrdinalIgnoreCase);
 
-        public CommandDirectiveInput(string name)
-        {
-            Name = name;
-        }
+        public CommandDirectiveInput(string name) => Name = name;
 
         public override string ToString() => $"[{Name}]";
     }

CliFx/Domain/CommandInput.cs

@@ -0,0 +1,238 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using CliFx.Internal;
+
+namespace CliFx.Domain
+{
+    internal partial class CommandInput
+    {
+        public IReadOnlyList<CommandDirectiveInput> Directives { get; }
+
+        public string? CommandName { get; }
+
+        public IReadOnlyList<CommandParameterInput> Parameters { get; }
+
+        public IReadOnlyList<CommandOptionInput> Options { get; }
+
+        public bool IsDebugDirectiveSpecified => Directives.Any(d => d.IsDebugDirective);
+
+        public bool IsPreviewDirectiveSpecified => Directives.Any(d => d.IsPreviewDirective);
+
+        public bool IsHelpOptionSpecified => Options.Any(o => o.IsHelpOption);
+
+        public bool IsVersionOptionSpecified => Options.Any(o => o.IsVersionOption);
+
+        public CommandInput(
+            IReadOnlyList<CommandDirectiveInput> directives,
+            string? commandName,
+            IReadOnlyList<CommandParameterInput> parameters,
+            IReadOnlyList<CommandOptionInput> options)
+        {
+            Directives = directives;
+            CommandName = commandName;
+            Parameters = parameters;
+            Options = options;
+        }
+
+        public override string ToString()
+        {
+            var buffer = new StringBuilder();
+
+            foreach (var directive in Directives)
+            {
+                buffer
+                    .AppendIfNotEmpty(' ')
+                    .Append(directive);
+            }
+
+            if (!string.IsNullOrWhiteSpace(CommandName))
+            {
+                buffer
+                    .AppendIfNotEmpty(' ')
+                    .Append(CommandName);
+            }
+
+            foreach (var parameter in Parameters)
+            {
+                buffer
+                    .AppendIfNotEmpty(' ')
+                    .Append(parameter);
+            }
+
+            foreach (var option in Options)
+            {
+                buffer
+                    .AppendIfNotEmpty(' ')
+                    .Append(option);
+            }
+
+            return buffer.ToString();
+        }
+    }
+
+    internal partial class CommandInput
+    {
+        private static IReadOnlyList<CommandDirectiveInput> ParseDirectives(
+            IReadOnlyList<string> commandLineArguments,
+            ref int index)
+        {
+            var result = new List<CommandDirectiveInput>();
+
+            for (; index < commandLineArguments.Count; index++)
+            {
+                var argument = commandLineArguments[index];
+
+                if (!argument.StartsWith('[') || !argument.EndsWith(']'))
+                    break;
+
+                var name = argument.Substring(1, argument.Length - 2);
+                result.Add(new CommandDirectiveInput(name));
+            }
+
+            return result;
+        }
+
+        private static string? ParseCommandName(
+            IReadOnlyList<string> commandLineArguments,
+            ISet<string> commandNames,
+            ref int index)
+        {
+            var buffer = new List<string>();
+
+            var commandName = default(string?);
+            var lastIndex = index;
+
+            // We need to look ahead to see if we can match as many consecutive arguments to a command name as possible
+            for (var i = index; i < commandLineArguments.Count; i++)
+            {
+                var argument = commandLineArguments[i];
+                buffer.Add(argument);
+
+                var potentialCommandName = buffer.JoinToString(" ");
+
+                if (commandNames.Contains(potentialCommandName))
+                {
+                    commandName = potentialCommandName;
+                    lastIndex = i;
+                }
+            }
+
+            // Update the index only if command name was found in the arguments
+            if (!string.IsNullOrWhiteSpace(commandName))
+                index = lastIndex + 1;
+
+            return commandName;
+        }
+
+        private static IReadOnlyList<CommandParameterInput> ParseParameters(
+            IReadOnlyList<string> commandLineArguments,
+            ref int index)
+        {
+            var result = new List<CommandParameterInput>();
+
+            for (; index < commandLineArguments.Count; index++)
+            {
+                var argument = commandLineArguments[index];
+
+                if (argument.StartsWith('-'))
+                    break;
+
+                result.Add(new CommandParameterInput(argument));
+            }
+
+            return result;
+        }
+
+        private static IReadOnlyList<CommandOptionInput> ParseOptions(
+            IReadOnlyList<string> commandLineArguments,
+            ref int index)
+        {
+            var result = new List<CommandOptionInput>();
+
+            var currentOptionAlias = default(string?);
+            var currentOptionValues = new List<string>();
+
+            for (; index < commandLineArguments.Count; index++)
+            {
+                var argument = commandLineArguments[index];
+
+                // Name
+                if (argument.StartsWith("--", StringComparison.Ordinal))
+                {
+                    // Flush previous
+                    if (!string.IsNullOrWhiteSpace(currentOptionAlias))
+                        result.Add(new CommandOptionInput(currentOptionAlias, currentOptionValues));
+
+                    currentOptionAlias = argument.Substring(2);
+                    currentOptionValues = new List<string>();
+                }
+                // Short name
+                else if (argument.StartsWith('-'))
+                {
+                    foreach (var alias in argument.Substring(1))
+                    {
+                        // Flush previous
+                        if (!string.IsNullOrWhiteSpace(currentOptionAlias))
+                            result.Add(new CommandOptionInput(currentOptionAlias, currentOptionValues));
+
+                        currentOptionAlias = alias.AsString();
+                        currentOptionValues = new List<string>();
+                    }
+                }
+                // Value
+                else if (!string.IsNullOrWhiteSpace(currentOptionAlias))
+                {
+                    currentOptionValues.Add(argument);
+                }
+            }
+
+            // Flush last option
+            if (!string.IsNullOrWhiteSpace(currentOptionAlias))
+                result.Add(new CommandOptionInput(currentOptionAlias, currentOptionValues));
+
+            return result;
+        }
+
+        public static CommandInput Parse(IReadOnlyList<string> commandLineArguments, IReadOnlyList<string> availableCommandNames)
+        {
+            var availableCommandNamesSet = availableCommandNames.ToHashSet(StringComparer.OrdinalIgnoreCase);
+
+            var index = 0;
+
+            var directives = ParseDirectives(
+                commandLineArguments,
+                ref index
+            );
+
+            var commandName = ParseCommandName(
+                commandLineArguments,
+                availableCommandNamesSet,
+                ref index
+            );
+
+            var parameters = ParseParameters(
+                commandLineArguments,
+                ref index
+            );
+
+            var options = ParseOptions(
+                commandLineArguments,
+                ref index
+            );
+
+            return new CommandInput(directives, commandName, parameters, options);
+        }
+    }
+
+    internal partial class CommandInput
+    {
+        public static CommandInput Empty { get; } = new CommandInput(
+            Array.Empty<CommandDirectiveInput>(),
+            null,
+            Array.Empty<CommandParameterInput>(),
+            Array.Empty<CommandOptionInput>()
+        );
+    }
+}

CliFx/Domain/CommandLineInput.cs

@@ -1,163 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using CliFx.Internal;
-
-namespace CliFx.Domain
-{
-    internal partial class CommandLineInput
-    {
-        public IReadOnlyList<CommandDirectiveInput> Directives { get; }
-
-        public IReadOnlyList<CommandUnboundArgumentInput> UnboundArguments { get; }
-
-        public IReadOnlyList<CommandOptionInput> Options { get; }
-
-        public bool IsDebugDirectiveSpecified => Directives.Any(d => d.IsDebugDirective);
-
-        public bool IsPreviewDirectiveSpecified => Directives.Any(d => d.IsPreviewDirective);
-
-        public bool IsHelpOptionSpecified => Options.Any(o => o.IsHelpOption);
-
-        public bool IsVersionOptionSpecified => Options.Any(o => o.IsVersionOption);
-
-        public CommandLineInput(
-            IReadOnlyList<CommandDirectiveInput> directives,
-            IReadOnlyList<CommandUnboundArgumentInput> unboundArguments,
-            IReadOnlyList<CommandOptionInput> options)
-        {
-            Directives = directives;
-            UnboundArguments = unboundArguments;
-            Options = options;
-        }
-
-        public override string ToString()
-        {
-            var buffer = new StringBuilder();
-
-            foreach (var directive in Directives)
-            {
-                buffer.AppendIfNotEmpty(' ');
-                buffer.Append(directive);
-            }
-
-            foreach (var argument in UnboundArguments)
-            {
-                buffer.AppendIfNotEmpty(' ');
-                buffer.Append(argument);
-            }
-
-            foreach (var option in Options)
-            {
-                buffer.AppendIfNotEmpty(' ');
-                buffer.Append(option);
-            }
-
-            return buffer.ToString();
-        }
-    }
-
-    internal partial class CommandLineInput
-    {
-        public static CommandLineInput Parse(IReadOnlyList<string> commandLineArguments)
-        {
-            var builder = new CommandLineInputBuilder();
-
-            var currentOptionAlias = "";
-            var currentOptionValues = new List<string>();
-
-            bool TryParseDirective(string argument)
-            {
-                if (!string.IsNullOrWhiteSpace(currentOptionAlias))
-                    return false;
-
-                if (!argument.StartsWith("[", StringComparison.OrdinalIgnoreCase) ||
-                    !argument.EndsWith("]", StringComparison.OrdinalIgnoreCase))
-                    return false;
-
-                var directive = argument.Substring(1, argument.Length - 2);
-                builder.AddDirective(directive);
-
-                return true;
-            }
-
-            bool TryParseArgument(string argument)
-            {
-                if (!string.IsNullOrWhiteSpace(currentOptionAlias))
-                    return false;
-
-                builder.AddUnboundArgument(argument);
-
-                return true;
-            }
-
-            bool TryParseOptionName(string argument)
-            {
-                if (!argument.StartsWith("--", StringComparison.OrdinalIgnoreCase))
-                    return false;
-
-                if (!string.IsNullOrWhiteSpace(currentOptionAlias))
-                    builder.AddOption(currentOptionAlias, currentOptionValues);
-
-                currentOptionAlias = argument.Substring(2);
-                currentOptionValues = new List<string>();
-
-                return true;
-            }
-
-            bool TryParseOptionShortName(string argument)
-            {
-                if (!argument.StartsWith("-", StringComparison.OrdinalIgnoreCase))
-                    return false;
-
-                foreach (var c in argument.Substring(1))
-                {
-                    if (!string.IsNullOrWhiteSpace(currentOptionAlias))
-                        builder.AddOption(currentOptionAlias, currentOptionValues);
-
-                    currentOptionAlias = c.AsString();
-                    currentOptionValues = new List<string>();
-                }
-
-                return true;
-            }
-
-            bool TryParseOptionValue(string argument)
-            {
-                if (string.IsNullOrWhiteSpace(currentOptionAlias))
-                    return false;
-
-                currentOptionValues.Add(argument);
-
-                return true;
-            }
-
-            foreach (var argument in commandLineArguments)
-            {
-                var _ =
-                    TryParseOptionName(argument) ||
-                    TryParseOptionShortName(argument) ||
-                    TryParseDirective(argument) ||
-                    TryParseArgument(argument) ||
-                    TryParseOptionValue(argument);
-            }
-
-            if (!string.IsNullOrWhiteSpace(currentOptionAlias))
-                builder.AddOption(currentOptionAlias, currentOptionValues);
-
-            return builder.Build();
-        }
-    }
-
-    internal partial class CommandLineInput
-    {
-        private static IReadOnlyList<CommandDirectiveInput> EmptyDirectives { get; } = new CommandDirectiveInput[0];
-
-        private static IReadOnlyList<CommandUnboundArgumentInput> EmptyUnboundArguments { get; } = new CommandUnboundArgumentInput[0];
-
-        private static IReadOnlyList<CommandOptionInput> EmptyOptions { get; } = new CommandOptionInput[0];
-
-        public static CommandLineInput Empty { get; } = new CommandLineInput(EmptyDirectives, EmptyUnboundArguments, EmptyOptions);
-    }
-}

CliFx/Domain/CommandLineInputBuilder.cs

@@ -1,43 +0,0 @@
-using System.Collections.Generic;
-
-namespace CliFx.Domain
-{
-    internal class CommandLineInputBuilder
-    {
-        private readonly List<CommandDirectiveInput> _directives = new List<CommandDirectiveInput>();
-        private readonly List<CommandUnboundArgumentInput> _unboundArguments = new List<CommandUnboundArgumentInput>();
-        private readonly List<CommandOptionInput> _options = new List<CommandOptionInput>();
-
-        public CommandLineInputBuilder AddDirective(CommandDirectiveInput directive)
-        {
-            _directives.Add(directive);
-            return this;
-        }
-
-        public CommandLineInputBuilder AddDirective(string directive) =>
-            AddDirective(new CommandDirectiveInput(directive));
-
-        public CommandLineInputBuilder AddUnboundArgument(CommandUnboundArgumentInput unboundArgument)
-        {
-            _unboundArguments.Add(unboundArgument);
-            return this;
-        }
-
-        public CommandLineInputBuilder AddUnboundArgument(string unboundArgument) =>
-            AddUnboundArgument(new CommandUnboundArgumentInput(unboundArgument));
-
-        public CommandLineInputBuilder AddOption(CommandOptionInput option)
-        {
-            _options.Add(option);
-            return this;
-        }
-
-        public CommandLineInputBuilder AddOption(string optionAlias, IReadOnlyList<string> values) =>
-            AddOption(new CommandOptionInput(optionAlias, values));
-
-        public CommandLineInputBuilder AddOption(string optionAlias, params string[] values) =>
-            AddOption(optionAlias, (IReadOnlyList<string>) values);
-
-        public CommandLineInput Build() => new CommandLineInput(_directives, _unboundArguments, _options);
-    }
-}

CliFx/Domain/CommandOptionInput.cs

@@ -1,5 +1,5 @@
 using System.Collections.Generic;
-using System.Text;
+using System.Linq;
 using CliFx.Internal;
 
 namespace CliFx.Domain
@@ -8,11 +8,6 @@ namespace CliFx.Domain
     {
         public string Alias { get; }
 
-        public string DisplayAlias =>
-            Alias.Length > 1
-                ? $"--{Alias}"
-                : $"-{Alias}";
-
         public IReadOnlyList<string> Values { get; }
 
         public bool IsHelpOption => CommandOptionSchema.HelpOption.MatchesNameOrShortName(Alias);
@@ -25,28 +20,15 @@ namespace CliFx.Domain
             Values = values;
         }
 
-        public override string ToString()
+        public string GetRawAlias() => Alias switch
         {
-            var buffer = new StringBuilder();
+            {Length: 0} => Alias,
+            {Length: 1} => $"-{Alias}",
+            _ => $"--{Alias}"
+        };
 
-            buffer.Append(DisplayAlias);
+        public string GetRawValues() => Values.Select(v => v.Quote()).JoinToString(" ");
 
-            foreach (var value in Values)
-            {
-                buffer.AppendIfNotEmpty(' ');
-
-                var isEscaped = value.Contains(" ");
-
-                if (isEscaped)
-                    buffer.Append('"');
-
-                buffer.Append(value);
-
-                if (isEscaped)
-                    buffer.Append('"');
-            }
-
-            return buffer.ToString();
-        }
+        public override string ToString() => $"{GetRawAlias()} {GetRawValues()}";
     }
 }

CliFx/Domain/CommandOptionSchema.cs

@@ -12,16 +12,12 @@ namespace CliFx.Domain
 
         public char? ShortName { get; }
 
-        public override string DisplayName => !string.IsNullOrWhiteSpace(Name)
-            ? $"--{Name}"
-            : $"-{ShortName}";
-
         public string? EnvironmentVariableName { get; }
 
         public bool IsRequired { get; }
 
         public CommandOptionSchema(
-            PropertyInfo property,
+            PropertyInfo? property,
             string? name,
             char? shortName,
             string? environmentVariableName,
@@ -51,27 +47,35 @@ namespace CliFx.Domain
             !string.IsNullOrWhiteSpace(EnvironmentVariableName) &&
             string.Equals(EnvironmentVariableName, environmentVariableName, StringComparison.OrdinalIgnoreCase);
 
-        public override string ToString()
+        public string GetUserFacingDisplayString()
         {
             var buffer = new StringBuilder();
 
             if (!string.IsNullOrWhiteSpace(Name))
             {
-                buffer.Append("--");
-                buffer.Append(Name);
+                buffer
+                    .Append("--")
+                    .Append(Name);
             }
 
             if (!string.IsNullOrWhiteSpace(Name) && ShortName != null)
+            {
                 buffer.Append('|');
+            }
 
             if (ShortName != null)
             {
-                buffer.Append('-');
-                buffer.Append(ShortName);
+                buffer
+                    .Append('-')
+                    .Append(ShortName);
             }
 
             return buffer.ToString();
         }
+
+        public string GetInternalDisplayString() => $"{Property?.Name ?? "<implicit>"} ('{GetUserFacingDisplayString()}')";
+
+        public override string ToString() => GetInternalDisplayString();
     }
 
     internal partial class CommandOptionSchema
@@ -82,9 +86,12 @@ namespace CliFx.Domain
             if (attribute == null)
                 return null;
 
+            // The user may mistakenly specify dashes, thinking it's required, so trim them
+            var name = attribute.Name?.TrimStart('-');
+
             return new CommandOptionSchema(
                 property,
-                attribute.Name,
+                name,
                 attribute.ShortName,
                 attribute.EnvironmentVariableName,
                 attribute.IsRequired,
@@ -96,9 +103,9 @@ namespace CliFx.Domain
     internal partial class CommandOptionSchema
     {
         public static CommandOptionSchema HelpOption { get; } =
-            new CommandOptionSchema(null!, "help", 'h', null, false, "Shows help text.");
+            new CommandOptionSchema(null, "help", 'h', null, false, "Shows help text.");
 
         public static CommandOptionSchema VersionOption { get; } =
-            new CommandOptionSchema(null!, "version", null, null, false, "Shows version information.");
+            new CommandOptionSchema(null, "version", null, null, false, "Shows version information.");
     }
 }

CliFx/Domain/CommandParameterInput.cs

@@ -0,0 +1,11 @@
+namespace CliFx.Domain
+{
+    internal class CommandParameterInput
+    {
+        public string Value { get; }
+
+        public CommandParameterInput(string value) => Value = value;
+
+        public override string ToString() => Value;
+    }
+}

CliFx/Domain/CommandParameterSchema.cs

@@ -8,31 +8,30 @@ namespace CliFx.Domain
     {
         public int Order { get; }
 
-        public string? Name { get; }
+        public string Name { get; }
 
-        public override string DisplayName =>
-            !string.IsNullOrWhiteSpace(Name)
-                ? Name
-                : Property.Name.ToLowerInvariant();
-
-        public CommandParameterSchema(PropertyInfo property, int order, string? name, string? description)
+        public CommandParameterSchema(PropertyInfo? property, int order, string name, string? description)
             : base(property, description)
         {
             Order = order;
             Name = name;
         }
 
-        public override string ToString()
+        public string GetUserFacingDisplayString()
         {
             var buffer = new StringBuilder();
 
             buffer
                 .Append('<')
-                .Append(DisplayName)
+                .Append(Name)
                 .Append('>');
 
             return buffer.ToString();
         }
+
+        public string GetInternalDisplayString() => $"{Property?.Name ?? "<implicit>"} ([{Order}] {GetUserFacingDisplayString()})";
+
+        public override string ToString() => GetInternalDisplayString();
     }
 
     internal partial class CommandParameterSchema
@@ -43,10 +42,12 @@ namespace CliFx.Domain
             if (attribute == null)
                 return null;
 
+            var name = attribute.Name ?? property.Name.ToLowerInvariant();
+
             return new CommandParameterSchema(
                 property,
                 attribute.Order,
-                attribute.Name,
+                name,
                 attribute.Description
             );
         }

CliFx/Domain/CommandSchema.cs

@@ -24,6 +24,10 @@ namespace CliFx.Domain
 
         public IReadOnlyList<CommandOptionSchema> Options { get; }
 
+        public bool IsHelpOptionAvailable => Options.Contains(CommandOptionSchema.HelpOption);
+
+        public bool IsVersionOptionAvailable => Options.Contains(CommandOptionSchema.VersionOption);
+
         public CommandSchema(
             Type type,
             string? name,
@@ -34,28 +38,42 @@ namespace CliFx.Domain
             Type = type;
             Name = name;
             Description = description;
-            Options = options;
             Parameters = parameters;
+            Options = options;
         }
 
-        public bool MatchesName(string? name) => string.Equals(name, Name, StringComparison.OrdinalIgnoreCase);
+        public bool MatchesName(string? name) =>
+            !string.IsNullOrWhiteSpace(Name)
+                ? string.Equals(name, Name, StringComparison.OrdinalIgnoreCase)
+                : string.IsNullOrWhiteSpace(name);
 
-        public IReadOnlyList<CommandOptionSchema> GetBuiltInOptions()
+        public IEnumerable<CommandArgumentSchema> GetArguments()
         {
-            var result = new List<CommandOptionSchema>(2);
+            foreach (var parameter in Parameters)
+                yield return parameter;
 
-            var helpOption = CommandOptionSchema.HelpOption;
-            var versionOption = CommandOptionSchema.VersionOption;
+            foreach (var option in Options)
+                yield return option;
+        }
 
-            result.Add(helpOption);
+        public IReadOnlyDictionary<CommandArgumentSchema, object?> GetArgumentValues(ICommand instance)
+        {
+            var result = new Dictionary<CommandArgumentSchema, object?>();
 
-            if (IsDefault)
-                result.Add(versionOption);
+            foreach (var argument in GetArguments())
+            {
+                // Skip built-in arguments
+                if (argument.Property == null)
+                    continue;
+
+                var value = argument.Property.GetValue(instance);
+                result[argument] = value;
+            }
 
             return result;
         }
 
-        private void InjectParameters(ICommand command, IReadOnlyList<CommandUnboundArgumentInput> parameterInputs)
+        private void BindParameters(ICommand instance, IReadOnlyList<CommandParameterInput> parameterInputs)
         {
             // All inputs must be bound
             var remainingParameterInputs = parameterInputs.ToList();
@@ -68,14 +86,14 @@ namespace CliFx.Domain
 
             for (var i = 0; i < scalarParameters.Length; i++)
             {
-                var scalarParameter = scalarParameters[i];
+                var parameter = scalarParameters[i];
 
-                var scalarParameterInput = i < parameterInputs.Count
+                var scalarInput = i < parameterInputs.Count
                     ? parameterInputs[i]
-                    : throw CliFxException.ParameterNotSet(scalarParameter);
+                    : throw CliFxException.ParameterNotSet(parameter);
 
-                scalarParameter.Inject(command, scalarParameterInput.Value);
-                remainingParameterInputs.Remove(scalarParameterInput);
+                parameter.BindOn(instance, scalarInput.Value);
+                remainingParameterInputs.Remove(scalarInput);
             }
 
             // Non-scalar parameter (only one is allowed)
@@ -85,21 +103,23 @@ namespace CliFx.Domain
 
             if (nonScalarParameter != null)
             {
-                var nonScalarParameterValues = parameterInputs.Skip(scalarParameters.Length).Select(i => i.Value).ToArray();
+                // TODO: Should it verify that at least one value is passed?
+                var nonScalarValues = parameterInputs
+                    .Skip(scalarParameters.Length)
+                    .Select(p => p.Value)
+                    .ToArray();
 
-                nonScalarParameter.Inject(command, nonScalarParameterValues);
+                nonScalarParameter.BindOn(instance, nonScalarValues);
                 remainingParameterInputs.Clear();
             }
 
             // Ensure all inputs were bound
             if (remainingParameterInputs.Any())
-            {
                 throw CliFxException.UnrecognizedParametersProvided(remainingParameterInputs);
-            }
         }
 
-        private void InjectOptions(
-            ICommand command,
+        private void BindOptions(
+            ICommand instance,
             IReadOnlyList<CommandOptionInput> optionInputs,
             IReadOnlyDictionary<string, string> environmentVariables)
         {
@@ -113,19 +133,17 @@ namespace CliFx.Domain
             foreach (var (name, value) in environmentVariables)
             {
                 var option = Options.FirstOrDefault(o => o.MatchesEnvironmentVariableName(name));
+                if (option == null)
+                    continue;
 
-                if (option != null)
-                {
-                    var values = option.IsScalar
-                        ? new[] {value}
-                        : value.Split(Path.PathSeparator);
+                var values = option.IsScalar
+                    ? new[] {value}
+                    : value.Split(Path.PathSeparator);
 
-                    option.Inject(command, values);
-                    unsetRequiredOptions.Remove(option);
-                }
+                option.BindOn(instance, values);
+                unsetRequiredOptions.Remove(option);
             }
 
-            // TODO: refactor this part? I wrote this while sick
             // Direct input
             foreach (var option in Options)
             {
@@ -133,67 +151,59 @@ namespace CliFx.Domain
                     .Where(i => option.MatchesNameOrShortName(i.Alias))
                     .ToArray();
 
-                if (inputs.Any())
-                {
-                    var inputValues = inputs.SelectMany(i => i.Values).ToArray();
-                    option.Inject(command, inputValues);
+                // Skip if the inputs weren't provided for this option
+                if (!inputs.Any())
+                    continue;
 
-                    foreach (var input in inputs)
-                        remainingOptionInputs.Remove(input);
+                var inputValues = inputs.SelectMany(i => i.Values).ToArray();
+                option.BindOn(instance, inputValues);
 
-                    if (inputValues.Any())
-                        unsetRequiredOptions.Remove(option);
-                }
-            }
+                remainingOptionInputs.RemoveRange(inputs);
 
-            // Ensure all required options were set
-            if (unsetRequiredOptions.Any())
-            {
-                throw CliFxException.RequiredOptionsNotSet(unsetRequiredOptions);
+                // Required option implies that the value has to be set and also be non-empty
+                if (inputValues.Any())
+                    unsetRequiredOptions.Remove(option);
             }
 
             // Ensure all inputs were bound
             if (remainingOptionInputs.Any())
-            {
                 throw CliFxException.UnrecognizedOptionsProvided(remainingOptionInputs);
-            }
+
+            // Ensure all required options were set
+            if (unsetRequiredOptions.Any())
+                throw CliFxException.RequiredOptionsNotSet(unsetRequiredOptions);
         }
 
-        public ICommand CreateInstance(
-            IReadOnlyList<CommandUnboundArgumentInput> parameterInputs,
-            IReadOnlyList<CommandOptionInput> optionInputs,
-            IReadOnlyDictionary<string, string> environmentVariables,
-            ITypeActivator activator)
+        public void Bind(
+            ICommand instance,
+            CommandInput input,
+            IReadOnlyDictionary<string, string> environmentVariables)
         {
-            var command = (ICommand) activator.CreateInstance(Type);
-
-            InjectParameters(command, parameterInputs);
-            InjectOptions(command, optionInputs, environmentVariables);
-
-            return command;
+            BindParameters(instance, input.Parameters);
+            BindOptions(instance, input.Options, environmentVariables);
         }
 
-        public override string ToString()
+        public string GetInternalDisplayString()
         {
             var buffer = new StringBuilder();
 
-            if (!string.IsNullOrWhiteSpace(Name))
-                buffer.Append(Name);
+            // Type
+            buffer.Append(Type.FullName);
 
-            foreach (var parameter in Parameters)
-            {
-                buffer.AppendIfNotEmpty(' ');
-                buffer.Append(parameter);
-            }
-
-            foreach (var option in Options)
-            {
-                buffer.AppendIfNotEmpty(' ');
-                buffer.Append(option);
-            }
+            // Name
+            buffer
+                .Append(' ')
+                .Append('(')
+                .Append(IsDefault
+                    ? "<default command>"
+                    : $"'{Name}'"
+                )
+                .Append(')');
 
             return buffer.ToString();
         }
+
+        public override string ToString() => GetInternalDisplayString();
     }
 
     internal partial class CommandSchema
@@ -211,6 +221,12 @@ namespace CliFx.Domain
 
             var attribute = type.GetCustomAttribute<CommandAttribute>();
 
+            var name = attribute?.Name;
+
+            var builtInOptions = string.IsNullOrWhiteSpace(name)
+                ? new[] {CommandOptionSchema.HelpOption, CommandOptionSchema.VersionOption}
+                : new[] {CommandOptionSchema.HelpOption};
+
             var parameters = type.GetProperties()
                 .Select(CommandParameterSchema.TryResolve)
                 .Where(p => p != null)
@@ -219,21 +235,16 @@ namespace CliFx.Domain
             var options = type.GetProperties()
                 .Select(CommandOptionSchema.TryResolve)
                 .Where(o => o != null)
+                .Concat(builtInOptions)
                 .ToArray();
 
             return new CommandSchema(
                 type,
-                attribute?.Name,
+                name,
                 attribute?.Description,
                 parameters!,
                 options!
             );
         }
     }
-
-    internal partial class CommandSchema
-    {
-        public static CommandSchema StubDefaultCommand { get; } =
-            new CommandSchema(null!, null, null, new CommandParameterSchema[0], new CommandOptionSchema[0]);
-    }
 }

CliFx/Domain/CommandUnboundArgumentInput.cs

@@ -1,14 +0,0 @@
-namespace CliFx.Domain
-{
-    internal class CommandUnboundArgumentInput
-    {
-        public string Value { get; }
-
-        public CommandUnboundArgumentInput(string value)
-        {
-            Value = value;
-        }
-
-        public override string ToString() => Value;
-    }
-}

CliFx/Domain/HelpTextWriter.cs

@@ -1,346 +1,388 @@
 using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using CliFx.Internal;
 
 namespace CliFx.Domain
 {
-    internal class HelpTextWriter
+    internal partial class HelpTextWriter
     {
         private readonly ApplicationMetadata _metadata;
         private readonly IConsole _console;
 
+        private int _column;
+        private int _row;
+
+        private bool IsEmpty => _column == 0 && _row == 0;
+
         public HelpTextWriter(ApplicationMetadata metadata, IConsole console)
         {
             _metadata = metadata;
             _console = console;
         }
 
-        public void Write(ApplicationSchema applicationSchema, CommandSchema command)
+        private void Write(char value)
         {
-            var column = 0;
-            var row = 0;
+            _console.Output.Write(value);
+            _column++;
+        }
 
-            var childCommands = applicationSchema.GetChildCommands(command.Name);
+        private void Write(string value)
+        {
+            _console.Output.Write(value);
+            _column += value.Length;
+        }
 
-            bool IsEmpty() => column == 0 && row == 0;
+        private void Write(ConsoleColor foregroundColor, string value)
+        {
+            _console.WithForegroundColor(foregroundColor, () => Write(value));
+        }
 
-            void Render(string text)
+        private void WriteLine()
+        {
+            _console.Output.WriteLine();
+            _column = 0;
+            _row++;
+        }
+
+        private void WriteVerticalMargin(int size = 1)
+        {
+            for (var i = 0; i < size; i++)
+                WriteLine();
+        }
+
+        private void WriteHorizontalMargin(int size = 2)
+        {
+            for (var i = 0; i < size; i++)
+                Write(' ');
+        }
+
+        private void WriteColumnMargin(int columnSize = 20, int offsetSize = 2)
+        {
+            if (_column + offsetSize < columnSize)
+                WriteHorizontalMargin(columnSize - _column);
+            else
+                WriteHorizontalMargin(offsetSize);
+        }
+
+        private void WriteHeader(string text)
+        {
+            Write(ConsoleColor.Magenta, text);
+            WriteLine();
+        }
+
+        private void WriteApplicationInfo()
+        {
+            // Title and version
+            Write(ConsoleColor.Yellow, _metadata.Title);
+            Write(' ');
+            Write(ConsoleColor.Yellow, _metadata.VersionText);
+            WriteLine();
+
+            // Description
+            if (!string.IsNullOrWhiteSpace(_metadata.Description))
             {
-                _console.Output.Write(text);
+                WriteHorizontalMargin();
+                Write(_metadata.Description);
+                WriteLine();
+            }
+        }
 
-                column += text.Length;
+        private void WriteCommandDescription(CommandSchema command)
+        {
+            if (string.IsNullOrWhiteSpace(command.Description))
+                return;
+
+            if (!IsEmpty)
+                WriteVerticalMargin();
+
+            WriteHeader("Description");
+
+            WriteHorizontalMargin();
+            Write(command.Description);
+            WriteLine();
+        }
+
+        private void WriteCommandUsage(CommandSchema command, IReadOnlyList<CommandSchema> childCommands)
+        {
+            if (!IsEmpty)
+                WriteVerticalMargin();
+
+            WriteHeader("Usage");
+
+            // Exe name
+            WriteHorizontalMargin();
+            Write(_metadata.ExecutableName);
+
+            // Command name
+            if (!string.IsNullOrWhiteSpace(command.Name))
+            {
+                Write(' ');
+                Write(ConsoleColor.Cyan, command.Name);
             }
 
-            void RenderNewLine()
+            // Child command placeholder
+            if (childCommands.Any())
             {
-                _console.Output.WriteLine();
-
-                column = 0;
-                row++;
+                Write(' ');
+                Write(ConsoleColor.Cyan, "[command]");
             }
 
-            void RenderMargin(int lines = 1)
+            // Parameters
+            foreach (var parameter in command.Parameters)
             {
-                if (!IsEmpty())
+                Write(' ');
+                Write(parameter.IsScalar
+                    ? $"<{parameter.Name}>"
+                    : $"<{parameter.Name}...>"
+                );
+            }
+
+            // Required options
+            foreach (var option in command.Options.Where(o => o.IsRequired))
+            {
+                Write(' ');
+                Write(ConsoleColor.White, !string.IsNullOrWhiteSpace(option.Name)
+                    ? $"--{option.Name}"
+                    : $"-{option.ShortName}"
+                );
+
+                Write(' ');
+                Write(option.IsScalar
+                    ? "<value>"
+                    : "<values...>"
+                );
+            }
+
+            // Options placeholder
+            Write(' ');
+            Write(ConsoleColor.White, "[options]");
+
+            WriteLine();
+        }
+
+        private void WriteCommandParameters(CommandSchema command)
+        {
+            if (!command.Parameters.Any())
+                return;
+
+            if (!IsEmpty)
+                WriteVerticalMargin();
+
+            WriteHeader("Parameters");
+
+            foreach (var parameter in command.Parameters.OrderBy(p => p.Order))
+            {
+                Write(ConsoleColor.Red, "* ");
+                Write(ConsoleColor.White, $"{parameter.Name}");
+
+                WriteColumnMargin();
+
+                // Description
+                if (!string.IsNullOrWhiteSpace(parameter.Description))
                 {
-                    for (var i = 0; i < lines; i++)
-                        RenderNewLine();
+                    Write(parameter.Description);
+                    Write(' ');
                 }
-            }
 
-            void RenderIndent(int spaces = 2)
-            {
-                Render(' '.Repeat(spaces));
-            }
-
-            void RenderColumnIndent(int spaces = 20, int margin = 2)
-            {
-                if (column + margin < spaces)
+                // Valid values
+                var validValues = parameter.GetValidValues();
+                if (validValues.Any())
                 {
-                    RenderIndent(spaces - column);
+                    Write($"Valid values: {FormatValidValues(validValues)}.");
+                }
+
+                WriteLine();
+            }
+        }
+
+        private void WriteCommandOptions(
+            CommandSchema command,
+            IReadOnlyDictionary<CommandArgumentSchema, object?> argumentDefaultValues)
+        {
+            if (!IsEmpty)
+                WriteVerticalMargin();
+
+            WriteHeader("Options");
+
+            foreach (var option in command.Options.OrderByDescending(o => o.IsRequired))
+            {
+                if (option.IsRequired)
+                {
+                    Write(ConsoleColor.Red, "* ");
                 }
                 else
                 {
-                    RenderIndent(margin);
+                    WriteHorizontalMargin();
                 }
-            }
 
-            void RenderWithColor(string text, ConsoleColor foregroundColor)
-            {
-                _console.WithForegroundColor(foregroundColor, () => Render(text));
-            }
+                // Short name
+                if (option.ShortName != null)
+                {
+                    Write(ConsoleColor.White, $"-{option.ShortName}");
+                }
 
-            void RenderHeader(string text)
-            {
-                RenderWithColor(text, ConsoleColor.Magenta);
-                RenderNewLine();
-            }
+                // Separator
+                if (!string.IsNullOrWhiteSpace(option.Name) && option.ShortName != null)
+                {
+                    Write('|');
+                }
 
-            void RenderApplicationInfo()
-            {
-                if (!command.IsDefault)
-                    return;
+                // Name
+                if (!string.IsNullOrWhiteSpace(option.Name))
+                {
+                    Write(ConsoleColor.White, $"--{option.Name}");
+                }
 
-                // Title and version
-                RenderWithColor(_metadata.Title, ConsoleColor.Yellow);
-                Render(" ");
-                RenderWithColor(_metadata.VersionText, ConsoleColor.Yellow);
-                RenderNewLine();
+                WriteColumnMargin();
 
                 // Description
-                if (!string.IsNullOrWhiteSpace(_metadata.Description))
+                if (!string.IsNullOrWhiteSpace(option.Description))
                 {
-                    Render(_metadata.Description);
-                    RenderNewLine();
+                    Write(option.Description);
+                    Write(' ');
                 }
-            }
 
-            void RenderDescription()
+                // Valid values
+                var validValues = option.GetValidValues();
+                if (validValues.Any())
+                {
+                    Write($"Valid values: {FormatValidValues(validValues)}.");
+                    Write(' ');
+                }
+
+                // Environment variable
+                if (!string.IsNullOrWhiteSpace(option.EnvironmentVariableName))
+                {
+                    Write($"Environment variable: \"{option.EnvironmentVariableName}\".");
+                    Write(' ');
+                }
+
+                // Default value
+                if (!option.IsRequired)
+                {
+                    var defaultValue = argumentDefaultValues.GetValueOrDefault(option);
+                    var defaultValueFormatted = FormatDefaultValue(defaultValue);
+                    if (defaultValueFormatted != null)
+                    {
+                        Write($"Default: {defaultValueFormatted}.");
+                    }
+                }
+
+                WriteLine();
+            }
+        }
+
+        private void WriteCommandChildren(
+            CommandSchema command,
+            IReadOnlyList<CommandSchema> childCommands)
+        {
+            if (!childCommands.Any())
+                return;
+
+            if (!IsEmpty)
+                WriteVerticalMargin();
+
+            WriteHeader("Commands");
+
+            foreach (var childCommand in childCommands)
             {
-                if (string.IsNullOrWhiteSpace(command.Description))
-                    return;
+                var relativeCommandName = !string.IsNullOrWhiteSpace(command.Name)
+                    ? childCommand.Name!.Substring(command.Name.Length).Trim()
+                    : childCommand.Name!;
 
-                RenderMargin();
-                RenderHeader("Description");
+                // Name
+                WriteHorizontalMargin();
+                Write(ConsoleColor.Cyan, relativeCommandName);
 
-                RenderIndent();
-                Render(command.Description);
-                RenderNewLine();
+                // Description
+                if (!string.IsNullOrWhiteSpace(childCommand.Description))
+                {
+                    WriteColumnMargin();
+                    Write(childCommand.Description);
+                }
+
+                WriteLine();
             }
 
-            void RenderUsage()
+            // Child command help tip
+            WriteVerticalMargin();
+            Write("You can run `");
+            Write(_metadata.ExecutableName);
+
+            if (!string.IsNullOrWhiteSpace(command.Name))
             {
-                RenderMargin();
-                RenderHeader("Usage");
-
-                // Exe name
-                RenderIndent();
-                Render(_metadata.ExecutableName);
-
-                // Command name
-                if (!string.IsNullOrWhiteSpace(command.Name))
-                {
-                    Render(" ");
-                    RenderWithColor(command.Name, ConsoleColor.Cyan);
-                }
-
-                // Child command placeholder
-                if (childCommands.Any())
-                {
-                    Render(" ");
-                    RenderWithColor("[command]", ConsoleColor.Cyan);
-                }
-
-                // Parameters
-                foreach (var parameter in command.Parameters)
-                {
-                    Render(" ");
-                    Render(parameter.IsScalar
-                        ? $"<{parameter.DisplayName}>"
-                        : $"<{parameter.DisplayName}...>");
-                }
-
-                // Required options
-                var requiredOptionSchemas = command.Options
-                    .Where(o => o.IsRequired)
-                    .ToArray();
-
-                foreach (var option in requiredOptionSchemas)
-                {
-                    Render(" ");
-                    if (!string.IsNullOrWhiteSpace(option.Name))
-                    {
-                        RenderWithColor($"--{option.Name}", ConsoleColor.White);
-                        Render(" ");
-                        Render(option.IsScalar
-                            ? "<value>"
-                            : "<values...>");
-                    }
-                    else
-                    {
-                        RenderWithColor($"-{option.ShortName}", ConsoleColor.White);
-                        Render(" ");
-                        Render(option.IsScalar
-                            ? "<value>"
-                            : "<values...>");
-                    }
-                }
-
-                // Options placeholder
-                if (command.Options.Count != requiredOptionSchemas.Length)
-                {
-                    Render(" ");
-                    RenderWithColor("[options]", ConsoleColor.White);
-                }
-
-                RenderNewLine();
+                Write(' ');
+                Write(ConsoleColor.Cyan, command.Name);
             }
 
-            void RenderParameters()
-            {
-                if (!command.Parameters.Any())
-                    return;
+            Write(' ');
+            Write(ConsoleColor.Cyan, "[command]");
 
-                RenderMargin();
-                RenderHeader("Parameters");
+            Write(' ');
+            Write(ConsoleColor.White, "--help");
 
-                var parameters = command.Parameters
-                    .OrderBy(p => p.Order)
-                    .ToArray();
+            Write("` to show help on a specific command.");
 
-                foreach (var parameter in parameters)
-                {
-                    RenderWithColor("* ", ConsoleColor.Red);
-                    RenderWithColor($"{parameter.DisplayName}", ConsoleColor.White);
+            WriteLine();
+        }
 
-                    RenderColumnIndent();
-
-                    // Description
-                    if (!string.IsNullOrWhiteSpace(parameter.Description))
-                    {
-                        Render(parameter.Description);
-                        Render(" ");
-                    }
-
-                    // Valid values
-                    var validValues = parameter.GetValidValues();
-                    if (validValues.Any())
-                    {
-                        Render($"Valid values: {string.Join(", ", validValues)}.");
-                        Render(" ");
-                    }
-
-                    RenderNewLine();
-                }
-            }
-
-            void RenderOptions()
-            {
-                RenderMargin();
-                RenderHeader("Options");
-
-                var options = command.Options
-                    .OrderByDescending(o => o.IsRequired)
-                    .Concat(command.GetBuiltInOptions())
-                    .ToArray();
-
-                foreach (var option in options)
-                {
-                    if (option.IsRequired)
-                    {
-                        RenderWithColor("* ", ConsoleColor.Red);
-                    }
-                    else
-                    {
-                        RenderIndent();
-                    }
-
-                    // Short name
-                    if (option.ShortName != null)
-                    {
-                        RenderWithColor($"-{option.ShortName}", ConsoleColor.White);
-                    }
-
-                    // Delimiter
-                    if (!string.IsNullOrWhiteSpace(option.Name) && option.ShortName != null)
-                    {
-                        Render("|");
-                    }
-
-                    // Name
-                    if (!string.IsNullOrWhiteSpace(option.Name))
-                    {
-                        RenderWithColor($"--{option.Name}", ConsoleColor.White);
-                    }
-
-                    RenderColumnIndent();
-
-                    // Description
-                    if (!string.IsNullOrWhiteSpace(option.Description))
-                    {
-                        Render(option.Description);
-                        Render(" ");
-                    }
-
-                    // Valid values
-                    var validValues = option.GetValidValues();
-                    if (validValues.Any())
-                    {
-                        Render($"Valid values: {string.Join(", ", validValues)}.");
-                        Render(" ");
-                    }
-
-                    // TODO: Render default value here.
-
-                    // Environment variable
-                    if (!string.IsNullOrWhiteSpace(option.EnvironmentVariableName))
-                    {
-                        Render($"Environment variable: {option.EnvironmentVariableName}");
-                    }
-
-                    RenderNewLine();
-                }
-            }
-
-            void RenderChildCommands()
-            {
-                if (!childCommands.Any())
-                    return;
-
-                RenderMargin();
-                RenderHeader("Commands");
-
-                foreach (var childCommand in childCommands)
-                {
-                    var relativeCommandName =
-                        !string.IsNullOrWhiteSpace(command.Name)
-                            ? childCommand.Name!.Substring(command.Name.Length + 1)
-                            : childCommand.Name!;
-
-                    // Name
-                    RenderIndent();
-                    RenderWithColor(relativeCommandName, ConsoleColor.Cyan);
-
-                    // Description
-                    if (!string.IsNullOrWhiteSpace(childCommand.Description))
-                    {
-                        RenderColumnIndent();
-                        Render(childCommand.Description);
-                    }
-
-                    RenderNewLine();
-                }
-
-                RenderMargin();
-
-                // Child command help tip
-                Render("You can run `");
-                Render(_metadata.ExecutableName);
-
-                if (!string.IsNullOrWhiteSpace(command.Name))
-                {
-                    Render(" ");
-                    RenderWithColor(command.Name, ConsoleColor.Cyan);
-                }
-
-                Render(" ");
-                RenderWithColor("[command]", ConsoleColor.Cyan);
-
-                Render(" ");
-                RenderWithColor("--help", ConsoleColor.White);
-
-                Render("` to show help on a specific command.");
-
-                RenderNewLine();
-            }
+        public void Write(
+            RootSchema root,
+            CommandSchema command,
+            IReadOnlyDictionary<CommandArgumentSchema, object?> defaultValues)
+        {
+            var childCommands = root.GetChildCommands(command.Name);
 
             _console.ResetColor();
-            RenderApplicationInfo();
-            RenderDescription();
-            RenderUsage();
-            RenderParameters();
-            RenderOptions();
-            RenderChildCommands();
+
+            if (command.IsDefault)
+                WriteApplicationInfo();
+
+            WriteCommandDescription(command);
+            WriteCommandUsage(command, childCommands);
+            WriteCommandParameters(command);
+            WriteCommandOptions(command, defaultValues);
+            WriteCommandChildren(command, childCommands);
+        }
+    }
+
+    internal partial class HelpTextWriter
+    {
+        private static string FormatValidValues(IReadOnlyList<string> values) =>
+            values.Select(v => v.Quote()).JoinToString(", ");
+
+        private static string? FormatDefaultValue(object? defaultValue)
+        {
+            if (defaultValue == null)
+                return null;
+
+            // Enumerable
+            if (!(defaultValue is string) && defaultValue is IEnumerable defaultValues)
+            {
+                var elementType = defaultValues.GetType().GetEnumerableUnderlyingType() ?? typeof(object);
+
+                // If the ToString() method is not overriden, the default value can't be formatted nicely
+                if (!elementType.IsToStringOverriden())
+                    return null;
+
+                return defaultValues
+                    .Cast<object?>()
+                    .Where(o => o != null)
+                    .Select(o => o!.ToFormattableString(CultureInfo.InvariantCulture).Quote())
+                    .JoinToString(" ");
+            }
+            // Non-enumerable
+            else
+            {
+                // If the ToString() method is not overriden, the default value can't be formatted nicely
+                if (!defaultValue.GetType().IsToStringOverriden())
+                    return null;
+
+                return defaultValue.ToFormattableString(CultureInfo.InvariantCulture).Quote();
+            }
         }
     }
 }

CliFx/Domain/RootSchema.cs

@@ -0,0 +1,232 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using CliFx.Exceptions;
+using CliFx.Internal;
+
+namespace CliFx.Domain
+{
+    internal partial class RootSchema
+    {
+        public IReadOnlyList<CommandSchema> Commands { get; }
+
+        public RootSchema(IReadOnlyList<CommandSchema> commands)
+        {
+            Commands = commands;
+        }
+
+        public IReadOnlyList<string> GetCommandNames() => Commands
+            .Select(c => c.Name)
+            .Where(n => !string.IsNullOrWhiteSpace(n))
+            .ToArray()!;
+
+        public CommandSchema? TryFindDefaultCommand() =>
+            Commands.FirstOrDefault(c => c.IsDefault);
+
+        public CommandSchema? TryFindCommand(string? commandName) =>
+            Commands.FirstOrDefault(c => c.MatchesName(commandName));
+
+        private IReadOnlyList<CommandSchema> GetDescendantCommands(
+            IReadOnlyList<CommandSchema> potentialParentCommands,
+            string? parentCommandName) =>
+            potentialParentCommands
+                // Default commands can't be children of anything
+                .Where(c => !string.IsNullOrWhiteSpace(c.Name))
+                // Command can't be its own child
+                .Where(c => !c.MatchesName(parentCommandName))
+                .Where(c =>
+                    string.IsNullOrWhiteSpace(parentCommandName) ||
+                    c.Name!.StartsWith(parentCommandName + ' ', StringComparison.OrdinalIgnoreCase))
+                .ToArray();
+
+        public IReadOnlyList<CommandSchema> GetDescendantCommands(string? parentCommandName) =>
+            GetDescendantCommands(Commands, parentCommandName);
+
+        public IReadOnlyList<CommandSchema> GetChildCommands(string? parentCommandName)
+        {
+            var descendants = GetDescendantCommands(parentCommandName);
+
+            // Filter out descendants of descendants, leave only children
+            var result = new List<CommandSchema>(descendants);
+
+            foreach (var descendant in descendants)
+            {
+                var descendantDescendants = GetDescendantCommands(descendants, descendant.Name);
+                result.RemoveRange(descendantDescendants);
+            }
+
+            return result;
+        }
+    }
+
+    internal partial class RootSchema
+    {
+        private static void ValidateParameters(CommandSchema command)
+        {
+            var duplicateOrderGroup = command.Parameters
+                .GroupBy(a => a.Order)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateOrderGroup != null)
+            {
+                throw CliFxException.ParametersWithSameOrder(
+                    command,
+                    duplicateOrderGroup.Key,
+                    duplicateOrderGroup.ToArray()
+                );
+            }
+
+            var duplicateNameGroup = command.Parameters
+                .Where(a => !string.IsNullOrWhiteSpace(a.Name))
+                .GroupBy(a => a.Name!, StringComparer.OrdinalIgnoreCase)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateNameGroup != null)
+            {
+                throw CliFxException.ParametersWithSameName(
+                    command,
+                    duplicateNameGroup.Key,
+                    duplicateNameGroup.ToArray()
+                );
+            }
+
+            var nonScalarParameters = command.Parameters
+                .Where(p => !p.IsScalar)
+                .ToArray();
+
+            if (nonScalarParameters.Length > 1)
+            {
+                throw CliFxException.TooManyNonScalarParameters(
+                    command,
+                    nonScalarParameters
+                );
+            }
+
+            var nonLastNonScalarParameter = command.Parameters
+                .OrderByDescending(a => a.Order)
+                .Skip(1)
+                .LastOrDefault(p => !p.IsScalar);
+
+            if (nonLastNonScalarParameter != null)
+            {
+                throw CliFxException.NonLastNonScalarParameter(
+                    command,
+                    nonLastNonScalarParameter
+                );
+            }
+        }
+
+        private static void ValidateOptions(CommandSchema command)
+        {
+            var noNameGroup = command.Options
+                .Where(o => o.ShortName == null && string.IsNullOrWhiteSpace(o.Name))
+                .ToArray();
+
+            if (noNameGroup.Any())
+            {
+                throw CliFxException.OptionsWithNoName(
+                    command,
+                    noNameGroup.ToArray()
+                );
+            }
+
+            var invalidLengthNameGroup = command.Options
+                .Where(o => !string.IsNullOrWhiteSpace(o.Name))
+                .Where(o => o.Name!.Length <= 1)
+                .ToArray();
+
+            if (invalidLengthNameGroup.Any())
+            {
+                throw CliFxException.OptionsWithInvalidLengthName(
+                    command,
+                    invalidLengthNameGroup
+                );
+            }
+
+            var duplicateNameGroup = command.Options
+                .Where(o => !string.IsNullOrWhiteSpace(o.Name))
+                .GroupBy(o => o.Name!, StringComparer.OrdinalIgnoreCase)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateNameGroup != null)
+            {
+                throw CliFxException.OptionsWithSameName(
+                    command,
+                    duplicateNameGroup.Key,
+                    duplicateNameGroup.ToArray()
+                );
+            }
+
+            var duplicateShortNameGroup = command.Options
+                .Where(o => o.ShortName != null)
+                .GroupBy(o => o.ShortName!.Value)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateShortNameGroup != null)
+            {
+                throw CliFxException.OptionsWithSameShortName(
+                    command,
+                    duplicateShortNameGroup.Key,
+                    duplicateShortNameGroup.ToArray()
+                );
+            }
+
+            var duplicateEnvironmentVariableNameGroup = command.Options
+                .Where(o => !string.IsNullOrWhiteSpace(o.EnvironmentVariableName))
+                .GroupBy(o => o.EnvironmentVariableName!, StringComparer.OrdinalIgnoreCase)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateEnvironmentVariableNameGroup != null)
+            {
+                throw CliFxException.OptionsWithSameEnvironmentVariableName(
+                    command,
+                    duplicateEnvironmentVariableNameGroup.Key,
+                    duplicateEnvironmentVariableNameGroup.ToArray()
+                );
+            }
+        }
+
+        private static void ValidateCommands(IReadOnlyList<CommandSchema> commands)
+        {
+            if (!commands.Any())
+            {
+                throw CliFxException.NoCommandsDefined();
+            }
+
+            var duplicateNameGroup = commands
+                .GroupBy(c => c.Name, StringComparer.OrdinalIgnoreCase)
+                .FirstOrDefault(g => g.Count() > 1);
+
+            if (duplicateNameGroup != null)
+            {
+                throw !string.IsNullOrWhiteSpace(duplicateNameGroup.Key)
+                    ? CliFxException.CommandsWithSameName(
+                        duplicateNameGroup.Key,
+                        duplicateNameGroup.ToArray()
+                    )
+                    : CliFxException.TooManyDefaultCommands(duplicateNameGroup.ToArray());
+            }
+        }
+
+        public static RootSchema Resolve(IReadOnlyList<Type> commandTypes)
+        {
+            var commands = new List<CommandSchema>();
+
+            foreach (var commandType in commandTypes)
+            {
+                var command =
+                    CommandSchema.TryResolve(commandType) ??
+                    throw CliFxException.InvalidCommandType(commandType);
+
+                ValidateParameters(command);
+                ValidateOptions(command);
+
+                commands.Add(command);
+            }
+
+            ValidateCommands(commands);
+
+            return new RootSchema(commands);
+        }
+    }
+}

CliFx/Exceptions/CliFxException.cs

@@ -3,6 +3,7 @@ using System.Collections.Generic;
 using System.Linq;
 using CliFx.Attributes;
 using CliFx.Domain;
+using CliFx.Internal;
 
 namespace CliFx.Exceptions
 {
@@ -11,53 +12,25 @@ namespace CliFx.Exceptions
     /// </summary>
     public partial class CliFxException : Exception
     {
-        /// <summary>
-        /// Represents the default exit code assigned to exceptions in CliFx.
-        /// </summary>
-        protected const int DefaultExitCode = -100;
-
-        /// <summary>
-        /// Whether to show the help text after handling this exception.
-        /// </summary>
-        public bool ShowHelp { get; }
-
-        /// <summary>
-        /// Whether this exception was constructed with a message.
-        /// </summary>
-        /// <remarks>
-        /// We cannot check against the 'Message' property because it will always return
-        /// a default message if it was constructed with a null value or is currently null.
-        /// </remarks>
-        public bool HasMessage { get; }
-
-        /// <summary>
-        /// Returns an exit code associated with this exception.
-        /// </summary>
-        public int ExitCode { get; }
+        private readonly bool _isMessageSet;
 
         /// <summary>
         /// Initializes an instance of <see cref="CliFxException"/>.
         /// </summary>
-        public CliFxException(string? message, bool showHelp = false)
-            : this(message, null, showHelp: showHelp)
-        {
-        }
-
-        /// <summary>
-        /// Initializes an instance of <see cref="CliFxException"/>.
-        /// </summary>
-        public CliFxException(string? message, Exception? innerException, int exitCode = DefaultExitCode, bool showHelp = false)
+        public CliFxException(string? message, Exception? innerException = null)
             : base(message, innerException)
         {
-            ExitCode = exitCode != 0
-                ? exitCode
-                : throw new ArgumentException("Exit code must not be zero in order to signify failure.");
-            HasMessage = !string.IsNullOrWhiteSpace(message);
-            ShowHelp = showHelp;
+            // Message property has a fallback so it's never empty, hence why we need this check
+            _isMessageSet = !string.IsNullOrWhiteSpace(message);
         }
+
+        /// <inheritdoc />
+        public override string ToString() => _isMessageSet
+            ? Message
+            : base.ToString();
     }
 
-    // Mid-user-facing exceptions
+    // Internal exceptions
     // Provide more diagnostic information here
     public partial class CliFxException
     {
@@ -75,13 +48,13 @@ Refer to the readme to learn how to integrate a dependency container of your cho
             return new CliFxException(message.Trim(), innerException);
         }
 
-        internal static CliFxException DelegateActivatorReceivedNull(Type type)
+        internal static CliFxException DelegateActivatorReturnedNull(Type type)
         {
             var message = $@"
 Failed to create an instance of type '{type.FullName}', received <null> instead.
 
 To fix this, ensure that the provided type activator was configured correctly, as it's not expected to return <null>.
-If you are using a dependency container, ensure this type is registered, because it may return <null> otherwise.";
+If you are using a dependency container, this error may signify that the type wasn't registered.";
 
             return new CliFxException(message.Trim());
         }
@@ -101,7 +74,7 @@ If you're experiencing problems, please refer to the readme for a quickstart exa
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandsNotRegistered()
+        internal static CliFxException NoCommandsDefined()
         {
             var message = $@"
 There are no commands configured in the application.
@@ -112,12 +85,11 @@ If you're experiencing problems, please refer to the readme for a quickstart exa
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandsTooManyDefaults(
-            IReadOnlyList<CommandSchema> invalidCommands)
+        internal static CliFxException TooManyDefaultCommands(IReadOnlyList<CommandSchema> invalidCommands)
         {
             var message = $@"
 Application configuration is invalid because there are {invalidCommands.Count} default commands:
-{string.Join(Environment.NewLine, invalidCommands.Select(p => p.Type.FullName))}
+{invalidCommands.JoinToString(Environment.NewLine)}
 
 There can only be one default command (i.e. command with no name) in an application.
 Other commands must have unique non-empty names that identify them.";
@@ -125,13 +97,13 @@ Other commands must have unique non-empty names that identify them.";
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandsDuplicateName(
+        internal static CliFxException CommandsWithSameName(
             string name,
             IReadOnlyList<CommandSchema> invalidCommands)
         {
             var message = $@"
 Application configuration is invalid because there are {invalidCommands.Count} commands with the same name ('{name}'):
-{string.Join(Environment.NewLine, invalidCommands.Select(p => p.Type.FullName))}
+{invalidCommands.JoinToString(Environment.NewLine)}
 
 Commands must have unique names.
 Names are not case-sensitive.";
@@ -139,28 +111,28 @@ Names are not case-sensitive.";
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandParametersDuplicateOrder(
+        internal static CliFxException ParametersWithSameOrder(
             CommandSchema command,
             int order,
             IReadOnlyList<CommandParameterSchema> invalidParameters)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains {invalidParameters.Count} parameters with the same order ({order}):
-{string.Join(Environment.NewLine, invalidParameters.Select(p => p.Property.Name))}
+{invalidParameters.JoinToString(Environment.NewLine)}
 
 Parameters must have unique order.";
 
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandParametersDuplicateName(
+        internal static CliFxException ParametersWithSameName(
             CommandSchema command,
             string name,
             IReadOnlyList<CommandParameterSchema> invalidParameters)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains {invalidParameters.Count} parameters with the same name ('{name}'):
-{string.Join(Environment.NewLine, invalidParameters.Select(p => p.Property.Name))}
+{invalidParameters.JoinToString(Environment.NewLine)}
 
 Parameters must have unique names to avoid potential confusion in the help text.
 Names are not case-sensitive.";
@@ -168,15 +140,15 @@ Names are not case-sensitive.";
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandParametersTooManyNonScalar(
+        internal static CliFxException TooManyNonScalarParameters(
             CommandSchema command,
             IReadOnlyList<CommandParameterSchema> invalidParameters)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains {invalidParameters.Count} non-scalar parameters:
-{string.Join(Environment.NewLine, invalidParameters.Select(p => p.Property.Name))}
+{invalidParameters.JoinToString(Environment.NewLine)}
 
-Non-scalar parameter is such that is bound from more than one value (e.g. array or a complex object).
+Non-scalar parameter is such that is bound from more than one value (e.g. array).
 Only one parameter in a command may be non-scalar and it must be the last one in order.
 
 If it's not feasible to fit into these constraints, consider using options instead as they don't have these limitations.";
@@ -184,15 +156,15 @@ If it's not feasible to fit into these constraints, consider using options inste
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandParametersNonLastNonScalar(
+        internal static CliFxException NonLastNonScalarParameter(
             CommandSchema command,
             CommandParameterSchema invalidParameter)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains a non-scalar parameter which is not the last in order:
-{invalidParameter.Property.Name}
+{invalidParameter}
 
-Non-scalar parameter is such that is bound from more than one value (e.g. array or a complex object).
+Non-scalar parameter is such that is bound from more than one value (e.g. array).
 Only one parameter in a command may be non-scalar and it must be the last one in order.
 
 If it's not feasible to fit into these constraints, consider using options instead as they don't have these limitations.";
@@ -200,26 +172,26 @@ If it's not feasible to fit into these constraints, consider using options inste
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandOptionsNoName(
+        internal static CliFxException OptionsWithNoName(
             CommandSchema command,
             IReadOnlyList<CommandOptionSchema> invalidOptions)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains one or more options without a name:
-{string.Join(Environment.NewLine, invalidOptions.Select(o => o.Property.Name))}
+{invalidOptions.JoinToString(Environment.NewLine)}
 
 Options must have either a name or a short name or both.";
 
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandOptionsInvalidLengthName(
+        internal static CliFxException OptionsWithInvalidLengthName(
             CommandSchema command,
             IReadOnlyList<CommandOptionSchema> invalidOptions)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains one or more options whose names are too short:
-{string.Join(Environment.NewLine, invalidOptions.Select(o => $"{o.Property.Name} ('{o.DisplayName}')"))}
+{invalidOptions.JoinToString(Environment.NewLine)}
 
 Option names must be at least 2 characters long to avoid confusion with short names.
 If you intended to set the short name instead, use the attribute overload that accepts a char.";
@@ -227,31 +199,29 @@ If you intended to set the short name instead, use the attribute overload that a
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandOptionsDuplicateName(
+        internal static CliFxException OptionsWithSameName(
             CommandSchema command,
             string name,
             IReadOnlyList<CommandOptionSchema> invalidOptions)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains {invalidOptions.Count} options with the same name ('{name}'):
-{string.Join(Environment.NewLine, invalidOptions.Select(o => o.Property.Name))}
+{invalidOptions.JoinToString(Environment.NewLine)}
 
-Options must have unique names, because that's what identifies them.
-Names are not case-sensitive.
-
-To fix this, ensure that all options have different names.";
+Options must have unique names.
+Names are not case-sensitive.";
 
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandOptionsDuplicateShortName(
+        internal static CliFxException OptionsWithSameShortName(
             CommandSchema command,
             char shortName,
             IReadOnlyList<CommandOptionSchema> invalidOptions)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains {invalidOptions.Count} options with the same short name ('{shortName}'):
-{string.Join(Environment.NewLine, invalidOptions.Select(o => o.Property.Name))}
+{invalidOptions.JoinToString(Environment.NewLine)}
 
 Options must have unique short names.
 Short names are case-sensitive (i.e. 'a' and 'A' are different short names).";
@@ -259,14 +229,14 @@ Short names are case-sensitive (i.e. 'a' and 'A' are different short names).";
             return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException CommandOptionsDuplicateEnvironmentVariableName(
+        internal static CliFxException OptionsWithSameEnvironmentVariableName(
             CommandSchema command,
             string environmentVariableName,
             IReadOnlyList<CommandOptionSchema> invalidOptions)
         {
             var message = $@"
 Command '{command.Type.FullName}' is invalid because it contains {invalidOptions.Count} options with the same fallback environment variable name ('{environmentVariableName}'):
-{string.Join(Environment.NewLine, invalidOptions.Select(o => o.Property.Name))}
+{invalidOptions.JoinToString(Environment.NewLine)}
 
 Options cannot share the same environment variable as a fallback.
 Environment variable names are not case-sensitive.";
@@ -279,98 +249,145 @@ Environment variable names are not case-sensitive.";
     // Avoid internal details and fix recommendations here
     public partial class CliFxException
     {
-        internal static CliFxException CannotFindMatchingCommand(CommandLineInput input)
+        internal static CliFxException CannotConvertMultipleValuesToNonScalar(
+            CommandParameterSchema parameter,
+            IReadOnlyList<string> values)
         {
             var message = $@"
-Can't find a command that matches the following arguments:
-{string.Join(" ", input.UnboundArguments.Select(a => a.Value))}";
+Parameter {parameter.GetUserFacingDisplayString()} expects a single value, but provided with multiple:
+{values.Select(v => v.Quote()).JoinToString(" ")}";
 
-            return new CliFxException(message.Trim(), showHelp: true);
+            return new CliFxException(message.Trim());
+        }
+
+        internal static CliFxException CannotConvertMultipleValuesToNonScalar(
+            CommandOptionSchema option,
+            IReadOnlyList<string> values)
+        {
+            var message = $@"
+Option {option.GetUserFacingDisplayString()} expects a single value, but provided with multiple:
+{values.Select(v => v.Quote()).JoinToString(" ")}";
+
+            return new CliFxException(message.Trim());
         }
 
         internal static CliFxException CannotConvertMultipleValuesToNonScalar(
             CommandArgumentSchema argument,
-            IReadOnlyList<string> values)
+            IReadOnlyList<string> values) => argument switch
         {
-            var argumentDisplayText = argument is CommandParameterSchema
-                ? $"Parameter <{argument.DisplayName}>"
-                : $"Option '{argument.DisplayName}'";
+            CommandParameterSchema parameter => CannotConvertMultipleValuesToNonScalar(parameter, values),
+            CommandOptionSchema option => CannotConvertMultipleValuesToNonScalar(option, values),
+            _ => throw new ArgumentOutOfRangeException(nameof(argument))
+        };
 
+        internal static CliFxException CannotConvertToType(
+            CommandParameterSchema parameter,
+            string? value,
+            Type type,
+            Exception? innerException = null)
+        {
             var message = $@"
-{argumentDisplayText} expects a single value, but provided with multiple:
-{string.Join(", ", values.Select(v => $"'{v}'"))}";
+Can't convert value ""{value ?? "<null>"}"" to type '{type.Name}' for parameter {parameter.GetUserFacingDisplayString()}.
+{innerException?.Message ?? "This type is not supported."}";
 
-            return new CliFxException(message.Trim(), showHelp: true);
+            return new CliFxException(message.Trim(), innerException);
+        }
+
+        internal static CliFxException CannotConvertToType(
+            CommandOptionSchema option,
+            string? value,
+            Type type,
+            Exception? innerException = null)
+        {
+            var message = $@"
+Can't convert value ""{value ?? "<null>"}"" to type '{type.Name}' for option {option.GetUserFacingDisplayString()}.
+{innerException?.Message ?? "This type is not supported."}";
+
+            return new CliFxException(message.Trim(), innerException);
         }
 
         internal static CliFxException CannotConvertToType(
             CommandArgumentSchema argument,
             string? value,
             Type type,
-            Exception? innerException = null)
+            Exception? innerException = null) => argument switch
         {
-            var argumentDisplayText = argument is CommandParameterSchema
-                ? $"parameter <{argument.DisplayName}>"
-                : $"option '{argument.DisplayName}'";
+            CommandParameterSchema parameter => CannotConvertToType(parameter, value, type, innerException),
+            CommandOptionSchema option => CannotConvertToType(option, value, type, innerException),
+            _ => throw new ArgumentOutOfRangeException(nameof(argument))
+        };
 
+        internal static CliFxException CannotConvertNonScalar(
+            CommandParameterSchema parameter,
+            IReadOnlyList<string> values,
+            Type type)
+        {
             var message = $@"
-Can't convert value '{value ?? "<null>"}' to type '{type.FullName}' for {argumentDisplayText}.
-{innerException?.Message ?? "This type is not supported."}";
+Can't convert provided values to type '{type.Name}' for parameter {parameter.GetUserFacingDisplayString()}:
+{values.Select(v => v.Quote()).JoinToString(" ")}
 
-            return new CliFxException(message.Trim(), innerException, showHelp: true);
+Target type is not assignable from array and doesn't have a public constructor that takes an array.";
+
+            return new CliFxException(message.Trim());
+        }
+
+        internal static CliFxException CannotConvertNonScalar(
+            CommandOptionSchema option,
+            IReadOnlyList<string> values,
+            Type type)
+        {
+            var message = $@"
+Can't convert provided values to type '{type.Name}' for option {option.GetUserFacingDisplayString()}:
+{values.Select(v => v.Quote()).JoinToString(" ")}
+
+Target type is not assignable from array and doesn't have a public constructor that takes an array.";
+
+            return new CliFxException(message.Trim());
         }
 
         internal static CliFxException CannotConvertNonScalar(
             CommandArgumentSchema argument,
             IReadOnlyList<string> values,
-            Type type)
+            Type type) => argument switch
         {
-            var argumentDisplayText = argument is CommandParameterSchema
-                ? $"parameter <{argument.DisplayName}>"
-                : $"option '{argument.DisplayName}'";
-
-            var message = $@"
-Can't convert provided values to type '{type.FullName}' for {argumentDisplayText}:
-{string.Join(", ", values.Select(v => $"'{v}'"))}
-
-Target type is not assignable from array and doesn't have a public constructor that takes an array.";
-
-            return new CliFxException(message.Trim(), showHelp: true);
-        }
+            CommandParameterSchema parameter => CannotConvertNonScalar(parameter, values, type),
+            CommandOptionSchema option => CannotConvertNonScalar(option, values, type),
+            _ => throw new ArgumentOutOfRangeException(nameof(argument))
+        };
 
         internal static CliFxException ParameterNotSet(CommandParameterSchema parameter)
         {
             var message = $@"
-Missing value for parameter <{parameter.DisplayName}>.";
+Missing value for parameter {parameter.GetUserFacingDisplayString()}.";
 
-            return new CliFxException(message.Trim(), showHelp: true);
+            return new CliFxException(message.Trim());
         }
 
         internal static CliFxException RequiredOptionsNotSet(IReadOnlyList<CommandOptionSchema> options)
         {
             var message = $@"
 Missing values for one or more required options:
-{string.Join(Environment.NewLine, options.Select(o => o.DisplayName))}";
+{options.Select(o => o.GetUserFacingDisplayString()).JoinToString(Environment.NewLine)}";
 
-            return new CliFxException(message.Trim(), showHelp: true);
+            return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException UnrecognizedParametersProvided(IReadOnlyList<CommandUnboundArgumentInput> inputs)
+        internal static CliFxException UnrecognizedParametersProvided(IReadOnlyList<CommandParameterInput> parameterInputs)
         {
             var message = $@"
 Unrecognized parameters provided:
-{string.Join(Environment.NewLine, inputs.Select(i => $"<{i.Value}>"))}";
+{parameterInputs.Select(p => p.Value).JoinToString(Environment.NewLine)}";
 
-            return new CliFxException(message.Trim(), showHelp: true);
+            return new CliFxException(message.Trim());
         }
 
-        internal static CliFxException UnrecognizedOptionsProvided(IReadOnlyList<CommandOptionInput> inputs)
+        internal static CliFxException UnrecognizedOptionsProvided(IReadOnlyList<CommandOptionInput> optionInputs)
         {
             var message = $@"
 Unrecognized options provided:
-{string.Join(Environment.NewLine, inputs.Select(i => i.DisplayAlias))}";
+{optionInputs.Select(o => o.GetRawAlias()).JoinToString(Environment.NewLine)}";
 
-            return new CliFxException(message.Trim(), showHelp: true);
+            return new CliFxException(message.Trim());
         }
     }
 }

CliFx/Exceptions/CommandException.cs

@@ -4,26 +4,43 @@ namespace CliFx.Exceptions
 {
     /// <summary>
     /// Thrown when a command cannot proceed with normal execution due to an error.
-    /// Use this exception if you want to report an error that occured during execution of a command.
+    /// Use this exception if you want to report an error that occured during the 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 = -1;
+
+        private readonly bool _isMessageSet;
+
+        /// <summary>
+        /// Returns an exit code associated with this exception.
+        /// </summary>
+        public int ExitCode { get; }
+
+        /// <summary>
+        /// Whether to show the help text after handling this exception.
+        /// </summary>
+        public bool ShowHelp { get; }
+
         /// <summary>
         /// Initializes an instance of <see cref="CommandException"/>.
         /// </summary>
-        public CommandException(string? message, Exception? innerException, 
-            int exitCode = DefaultExitCode, bool showHelp = false)
-                : base(message, innerException, exitCode, showHelp)
+        public CommandException(string? message, Exception? innerException, int exitCode = DefaultExitCode, bool showHelp = false)
+            : base(message, innerException)
         {
+            ExitCode = exitCode;
+            ShowHelp = showHelp;
 
+            // Message property has a fallback so it's never empty, hence why we need this check
+            _isMessageSet = !string.IsNullOrWhiteSpace(message);
         }
 
         /// <summary>
         /// Initializes an instance of <see cref="CommandException"/>.
         /// </summary>
         public CommandException(string? message, int exitCode = DefaultExitCode, bool showHelp = false)
-                : this(message, null, exitCode, showHelp)
+            : this(message, null, exitCode, showHelp)
         {
         }
 
@@ -34,5 +51,10 @@ namespace CliFx.Exceptions
             : this(null, exitCode, showHelp)
         {
         }
+
+        /// <inheritdoc />
+        public override string ToString() => _isMessageSet
+            ? Message
+            : base.ToString();
     }
 }

CliFx/ICommand.cs

@@ -9,7 +9,7 @@ namespace CliFx
     {
         /// <summary>
         /// Executes the command using the specified implementation of <see cref="IConsole"/>.
-        /// This is the method that's called when the command is invoked by a user through command line interface.
+        /// This is the method that's called when the command is invoked by a user through command line.
         /// </summary>
         /// <remarks>If the execution of the command is not asynchronous, simply end the method with <code>return default;</code></remarks>
         ValueTask ExecuteAsync(IConsole console);

CliFx/IConsole.cs

@@ -65,10 +65,15 @@ namespace CliFx
         int CursorTop { get; set; }
 
         /// <summary>
-        /// Provides a token that signals when application cancellation is requested.
-        /// Subsequent calls return the same token.
-        /// When working with system console, the user can request cancellation by issuing an interrupt signal (Ctrl+C).
+        /// Defers the application termination in case of a cancellation request and returns the token that represents it.
+        /// Subsequent calls to this method return the same token.
         /// </summary>
+        /// <remarks>
+        /// When working with <see cref="SystemConsole"/>:<br/>
+        /// - Cancellation can be requested by the user by pressing Ctrl+C.<br/>
+        /// - Cancellation can only be deferred once, subsequent requests to cancel by the user will result in instant termination.<br/>
+        /// - Any code executing prior to calling this method is not cancellation-aware and as such will terminate instantly when cancellation is requested.
+        /// </remarks>
         CancellationToken GetCancellationToken();
     }
 

CliFx/ITypeActivator.cs

@@ -3,12 +3,12 @@
 namespace CliFx
 {
     /// <summary>
-    /// Abstraction for a service can initialize objects at runtime.
+    /// Abstraction for a service that can initialize objects at runtime.
     /// </summary>
     public interface ITypeActivator
     {
         /// <summary>
-        /// Creates an instance of specified type.
+        /// Creates an instance of the specified type.
         /// </summary>
         object CreateInstance(Type type);
     }

CliFx/Internal/CollectionExtensions.cs

@@ -0,0 +1,13 @@
+using System.Collections.Generic;
+
+namespace CliFx.Internal
+{
+    internal static class CollectionExtensions
+    {
+        public static void RemoveRange<T>(this ICollection<T> source, IEnumerable<T> items)
+        {
+            foreach (var item in items)
+                source.Remove(item);
+        }
+    }
+}

CliFx/Internal/Polyfills.cs

@@ -3,6 +3,23 @@
 // Polyfills to bridge the missing APIs in older versions of the framework/standard.
 
 #if NETSTANDARD2_0
+namespace System
+{
+    using Linq;
+
+    internal static class Extensions
+    {
+        public static bool Contains(this string str, char c) =>
+            str.Any(i => i == c);
+
+        public static bool StartsWith(this string str, char c) =>
+            str.Length > 0 && str[0] == c;
+
+        public static bool EndsWith(this string str, char c) =>
+            str.Length > 0 && str[str.Length - 1] == c;
+    }
+}
+
 namespace System.Collections.Generic
 {
     internal static class Extensions
@@ -17,4 +34,15 @@ namespace System.Collections.Generic
             dic.TryGetValue(key, out var result) ? result! : default!;
     }
 }
+
+namespace System.Linq
+{
+    using Collections.Generic;
+
+    internal static class Extensions
+    {
+        public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source, IEqualityComparer<T> comparer) =>
+            new HashSet<T>(source, comparer);
+    }
+}
 #endif

CliFx/Internal/ProcessEx.cs

@@ -0,0 +1,13 @@
+using System.Diagnostics;
+
+namespace CliFx.Internal
+{
+    internal static class ProcessEx
+    {
+        public static int GetCurrentProcessId()
+        {
+            using var process = Process.GetCurrentProcess();
+            return process.Id;
+        }
+    }
+}

CliFx/Internal/StringExtensions.cs

@@ -1,4 +1,6 @@
-using System.Text;
+using System;
+using System.Collections.Generic;
+using System.Text;
 
 namespace CliFx.Internal
 {
@@ -8,7 +10,17 @@ namespace CliFx.Internal
 
         public static string AsString(this char c) => c.Repeat(1);
 
+        public static string Quote(this string str) => $"\"{str}\"";
+
+        public static string JoinToString<T>(this IEnumerable<T> source, string separator) => string.Join(separator, source);
+
         public static StringBuilder AppendIfNotEmpty(this StringBuilder builder, char value) =>
             builder.Length > 0 ? builder.Append(value) : builder;
+
+        public static string ToFormattableString(this object obj,
+            IFormatProvider? formatProvider = null, string? format = null) =>
+            obj is IFormattable formattable
+                ? formattable.ToString(format, formatProvider)
+                : obj.ToString();
     }
 }

CliFx/Internal/TypeExtensions.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 
 namespace CliFx.Internal
 {
@@ -22,13 +23,30 @@ namespace CliFx.Internal
             if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(IEnumerable<>))
                 return type.GetGenericArguments().FirstOrDefault();
 
-            return type.GetInterfaces()
+            return type
+                .GetInterfaces()
                 .Select(GetEnumerableUnderlyingType)
                 .Where(t => t != null)
                 .OrderByDescending(t => t != typeof(object)) // prioritize more specific types
                 .FirstOrDefault();
         }
 
+        public static MethodInfo GetToStringMethod(this Type type) => type.GetMethod(nameof(ToString), Type.EmptyTypes);
+
+        public static bool IsToStringOverriden(this Type type) => type.GetToStringMethod() != typeof(object).GetToStringMethod();
+
+        public static MethodInfo GetStaticParseMethod(this Type type, bool withFormatProvider = false)
+        {
+            var argumentTypes = withFormatProvider
+                ? new[] {typeof(string), typeof(IFormatProvider)}
+                : new[] {typeof(string)};
+
+            return type.GetMethod("Parse",
+                BindingFlags.Public | BindingFlags.Static,
+                null, argumentTypes, null
+            );
+        }
+
         public static Array ToNonGenericArray<T>(this IEnumerable<T> source, Type elementType)
         {
             var sourceAsCollection = source as ICollection ?? source.ToArray();

CliFx/Internal/VersionExtensions.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace CliFx.Internal
+{
+    internal static class VersionExtensions
+    {
+        public static string ToSemanticString(this Version version) =>
+            version.Revision <= 0 ? version.ToString(3) : version.ToString();
+    }
+}

CliFx/SystemConsole.cs

@@ -96,12 +96,12 @@ namespace CliFx
     {
         private static StreamReader WrapInput(Stream? stream) =>
             stream != null
-                ? new StreamReader(stream, Console.InputEncoding, false)
+                ? new StreamReader(Stream.Synchronized(stream), Console.InputEncoding, false)
                 : StreamReader.Null;
 
         private static StreamWriter WrapOutput(Stream? stream) =>
             stream != null
-                ? new StreamWriter(stream, Console.OutputEncoding) {AutoFlush = true}
+                ? new StreamWriter(Stream.Synchronized(stream), Console.OutputEncoding) {AutoFlush = true}
                 : StreamWriter.Null;
     }
 }

CliFx/VirtualConsole.cs

@@ -5,7 +5,7 @@ using System.Threading;
 namespace CliFx
 {
     /// <summary>
-    /// Implementation of <see cref="IConsole"/> that routes data to specified streams.
+    /// Implementation of <see cref="IConsole"/> that routes all data to preconfigured streams.
     /// Does not leak to system console in any way.
     /// Use this class as a substitute for system console when running tests.
     /// </summary>
@@ -94,12 +94,12 @@ namespace CliFx
     {
         private static StreamReader WrapInput(Stream? stream) =>
             stream != null
-                ? new StreamReader(stream, Console.InputEncoding, false)
+                ? new StreamReader(Stream.Synchronized(stream), Console.InputEncoding, false)
                 : StreamReader.Null;
 
         private static StreamWriter WrapOutput(Stream? stream) =>
             stream != null
-                ? new StreamWriter(stream, Console.OutputEncoding) {AutoFlush = true}
+                ? new StreamWriter(Stream.Synchronized(stream), Console.OutputEncoding) {AutoFlush = true}
                 : StreamWriter.Null;
     }
 }

Readme.md

@@ -27,7 +27,7 @@ An important property of CliFx, when compared to some other libraries, is that i
 - Provides comprehensive and colorful auto-generated help text
 - Highly testable and easy to debug
 - Comes with built-in analyzers to help catch common mistakes
-- Targets .NET Framework 4.5+ and .NET Standard 2.0+
+- Targets .NET Standard 2.0+
 - No external dependencies
 
 ## Screenshots
@@ -51,6 +51,8 @@ An important property of CliFx, when compared to some other libraries, is that i
 
 ### Quick start
 
+![quick start animated](https://i.imgur.com/uouNh2u.gif)
+
 To turn your application into a command line interface you need to change your program's `Main` method so that it delegates execution to `CliApplication`.
 
 The following code will create and run a default `CliApplication` that will resolve commands defined in the calling assembly. Using fluent interface provided by `CliApplicationBuilder` you can easily configure different aspects of your application.