Dotnet System Commandline

Out of scope

• CLI application architecture patterns (layered design, exit codes, stdin/stdout/stderr) -- see [skill:dotnet-cli-architecture]
• Native AOT compilation -- see [skill:dotnet-native-aot]
• CLI distribution strategy -- see [skill:dotnet-cli-distribution]
• General CI/CD patterns -- see [skill:dotnet-gha-patterns] and [skill:dotnet-ado-patterns]
• DI container mechanics -- see [skill:dotnet-csharp-dependency-injection]
• General coding standards -- see [skill:dotnet-csharp-coding-standards]
• CLI packaging for Homebrew, apt, winget -- see [skill:dotnet-cli-packaging]

Cross-references: [skill:dotnet-cli-architecture] for CLI design patterns, [skill:dotnet-native-aot] for AOT publishing CLI tools, [skill:dotnet-csharp-dependency-injection] for DI fundamentals, [skill:dotnet-csharp-configuration] for configuration integration, [skill:dotnet-csharp-coding-standards] for naming and style conventions.

Package Reference

<ItemGroup>
  <PackageReference Include="System.CommandLine" Version="2.0.*" />
</ItemGroup>

System.CommandLine 2.0 targets .NET 8+ and .NET Standard 2.0. A single package provides all functionality -- the separate System.CommandLine.Hosting, System.CommandLine.NamingConventionBinder, and System.CommandLine.Rendering packages from the beta era are discontinued.

RootCommand and Command Hierarchy

Basic Command Structure

using System.CommandLine;

// Root command -- the entry point
var rootCommand = new RootCommand("My CLI tool description");

// Add a subcommand via mutable collection
var listCommand = new Command("list", "List all items");
rootCommand.Subcommands.Add(listCommand);

// Nested subcommands: mycli migrate up
var migrateCommand = new Command("migrate", "Database migrations");
var upCommand = new Command("up", "Apply pending migrations");
var downCommand = new Command("down", "Revert last migration");
migrateCommand.Subcommands.Add(upCommand);
migrateCommand.Subcommands.Add(downCommand);
rootCommand.Subcommands.Add(migrateCommand);

Collection Initializer Syntax

// Fluent collection initializer (commands, options, arguments)
RootCommand rootCommand = new("My CLI tool")
{
    new Option<string>("--output", "-o") { Description = "Output file path" },
    new Argument<FileInfo>("file") { Description = "Input file" },
    new Command("list", "List all items")
    {
        new Option<int>("--limit") { Description = "Max items to return" }
    }
};

Options and Arguments

Option<T> -- Named Parameters

// Option<T> -- named parameter (--output, -o)
// name is the first parameter; additional params are aliases
var outputOption = new Option<FileInfo>("--output", "-o")
{
    Description = "Output file path",
    Required = true  // was IsRequired in beta4
};

// Option with default value via DefaultValueFactory
var verbosityOption = new Option<int>("--verbosity")
{
    Description = "Verbosity level (0-3)",
    DefaultValueFactory = _ => 1
};

Argument<T> -- Positional Parameters

// Argument<T> -- positional parameter
// name is mandatory in 2.0 (used for help text)
var fileArgument = new Argument<FileInfo>("file")
{
    Description = "Input file to process"
};

rootCommand.Arguments.Add(fileArgument);

Constrained Values

var formatOption = new Option<string>("--format")
{
    Description = "Output format"
};
formatOption.AcceptOnlyFromAmong("json", "csv", "table");

rootCommand.Options.Add(formatOption);

Aliases

// Aliases are separate from the name in 2.0
// First constructor param is the name; rest are aliases
var verboseOption = new Option<bool>("--verbose", "-v")
{
    Description = "Enable verbose output"
};

// Or add aliases after construction
verboseOption.Aliases.Add("-V");

Global Options

// Global options are inherited by all subcommands
var debugOption = new Option<bool>("--debug")
{
    Description = "Enable debug mode",
    Recursive = true  // makes it global (inherited by subcommands)
};
rootCommand.Options.Add(debugOption);

Setting Actions (Command Handlers)

In 2.0.0 GA, SetHandler is replaced by SetAction. Actions receive a ParseResult directly (no InvocationContext).

Synchronous Action

var outputOption = new Option<FileInfo>("--output", "-o")
{
    Description = "Output file path",
    Required = true
};
var verbosityOption = new Option<int>("--verbosity")
{
    DefaultValueFactory = _ => 1
};

rootCommand.Options.Add(outputOption);
rootCommand.Options.Add(verbosityOption);

rootCommand.SetAction(parseResult =>
{
    var output = parseResult.GetValue(outputOption)!;
    var verbosity = parseResult.GetValue(verbosityOption);
    Console.WriteLine($"Output: {output.FullName}, Verbosity: {verbosity}");
    return 0; // exit code
});

Asynchronous Action with CancellationToken

// Async actions receive ParseResult AND CancellationToken
rootCommand.SetAction(async (ParseResult parseResult, CancellationToken ct) =>
{
    var output = parseResult.GetValue(outputOption)!;
    var verbosity = parseResult.GetValue(verbosityOption);
    await ProcessAsync(output, verbosity, ct);
    return 0;
});

Getting Values by Name

// Values can also be retrieved by symbol name (requires type parameter)
rootCommand.SetAction(parseResult =>
{
    int delay = parseResult.GetValue<int>("--delay");
    string? message = parseResult.GetValue<string>("--message");
    Console.WriteLine($"Delay: {delay}, Message: {message}");
});

Parsing and Invoking

// Program.cs entry point -- parse then invoke
static int Main(string[] args)
{
    var rootCommand = BuildCommand();
    ParseResult parseResult = rootCommand.Parse(args);
    return parseResult.Invoke();
}

// Async entry point
static async Task<int> Main(string[] args)
{
    var rootCommand = BuildCommand();
    ParseResult parseResult = rootCommand.Parse(args);
    return await parseResult.InvokeAsync();
}

Parse Without Invoking

// Parse-only mode: inspect results without running actions
ParseResult parseResult = rootCommand.Parse(args);
if (parseResult.Errors.Count > 0)
{
    foreach (var error in parseResult.Errors)
    {
        Console.Error.WriteLine(error.Message);
    }
    return 1;
}

FileInfo? file = parseResult.GetValue(fileOption);
// Process directly without SetAction

For detailed examples (custom parsing, validation, configuration, tab completion, DI, testing, migration), see examples.md in this skill directory.

Agent Gotchas

1. Do not use beta4 API patterns. The 2.0.0 GA API is fundamentally different. There is no SetHandler -- use SetAction. There is no InvocationContext -- actions receive ParseResult directly. There is no CommandLineBuilder -- configuration uses ParserConfiguration/InvocationConfiguration.
2. Do not reference discontinued packages. System.CommandLine.Hosting, System.CommandLine.NamingConventionBinder, and System.CommandLine.Rendering are discontinued. Use the single System.CommandLine package.
3. Do not confuse Option<T> with Argument<T>. Options are named (--output file.txt), arguments are positional (mycli file.txt). Using the wrong type produces confusing parse errors.
4. Do not use AddOption/AddCommand/AddAlias methods. These were replaced by mutable collection properties: Options.Add, Subcommands.Add, Aliases.Add. The old methods do not exist in 2.0.0.
5. Do not use IConsole or TestConsole for testing. These interfaces were removed. Use InvocationConfiguration with StringWriter for Output/Error to capture test output.
6. Do not ignore the CancellationToken in async actions. In 2.0.0 GA, CancellationToken is a mandatory second parameter for async SetAction delegates. The compiler warns (CA2016) when it is not propagated.
7. Do not write Console.Out directly in command actions. Write to InvocationConfiguration.Output for testability. If no configuration is provided, output goes to Console.Out by default, but direct writes bypass test capture.
8. Do not set default values via constructors. Use the DefaultValueFactory property instead. The old getDefaultValue constructor parameter does not exist in 2.0.0.

References

• System.CommandLine overview
• System.CommandLine migration guide (beta5+)
• How to parse and invoke
• How to customize parsing and validation
• System.CommandLine GitHub

Attribution

Adapted from Aaronontheweb/dotnet-skills (MIT license).