System Commandline Adopter Probe

System.CommandLine — parse & dispatch a .NET CLI (current API)

System.CommandLine (namespace System.CommandLine) parses arguments into a command tree and runs an action. The 2.0 GA redesign removed the old invocation/binding stack, so most remembered snippets do not compile. Pin the current shapes below.

Use the System.CommandLine skills, not the web. Do NOT web_search / web_fetch for System.CommandLine usage — the web is dominated by the pre-GA beta API (SetHandler, AddOption, AddCommand, BinderBase<T>, IConsole, getDefaultValue: ctor args) that no longer exists. These skills are the current, authoritative API. This skill covers the core pattern; beta→GA migration, options/arguments in depth, actions/invocation, subcommands/help, and 3.x additions are covered separately.

The core pattern (current API)

using System.CommandLine;

// 1. Declare options/arguments. KEEP the instances — you read values back by identity.
var nameOption = new Option<string>("--name")            // "--name" is the name; extra strings are ALIASES
{
    Description = "Who to greet",                        // description is a PROPERTY, not a ctor arg
    Required = true,
};
nameOption.Aliases.Add("-n");
var countOption = new Option<int>("--count") { DefaultValueFactory = _ => 1 };

// 2. Build the command tree.
var root = new RootCommand("Greeter sample");
root.Options.Add(nameOption);
root.Options.Add(countOption);

// 3. Wire behavior with SetAction; read parsed values from the ParseResult by instance.
root.SetAction(parseResult =>
{
    string name = parseResult.GetValue(nameOption)!;
    int count = parseResult.GetValue(countOption);
    for (int i = 0; i < count; i++) Console.WriteLine($"Hello, {name}!");
    return 0;                                            // exit code
});

// 4. Parse then invoke.
return await root.Parse(args).InvokeAsync();

Gotchas (compile-clean but wrong, or removed-API)

• new Option<T>("--name", "description") is WRONG. The 2nd positional arg is an alias, so the description becomes a bogus alias and the help text is lost. Use new Option<T>("--name") { Description = "..." }; pass real aliases as extra strings (new Option<T>("--name", "-n")) or via .Aliases.Add(...). Same for Argument<T>.
• Read values by identity. parseResult.GetValue(theOptionInstance) — keep the exact instance you added. There is no delegate-parameter binding anymore.
• SetHandler is gone. Use SetAction(parseResult => ...) (sync) or SetAction(async (parseResult, ct) => ...) (async).
• AddOption / AddArgument / AddCommand are gone. Use the .Options / .Arguments / .Subcommands collections: root.Options.Add(o), cmd.Subcommands.Add(sub).
• Required, not IsRequired. Default values are DefaultValueFactory = _ => v, not getDefaultValue: / SetDefaultValue(...).
• IConsole / BinderBase<T> / HelpBuilder are gone. Use Console directly; customize help via a HelpAction (help customization is covered separately).