System Commandline Actions And Invocation

System.CommandLine: actions & invocation

Behavior attaches either as a small inline delegate with SetAction, or as a reusable SynchronousCommandLineAction / AsynchronousCommandLineAction assigned to Command.Action. You run the app by Parse-ing args into a ParseResult, then Invoke/InvokeAsync. SetHandler, delegate parameter binding, and IConsole were removed at GA — do not use them.

Do NOT web_search / web_fetch — nearly all samples show SetHandler(...) with positionally-bound parameters, which no longer exists.

Required setup

System.CommandLine is not in the shared framework — add the package (dotnet package add <proj> System.CommandLine), then using System.CommandLine;.

Every action reads inputs from the ParseResult by option/argument instance, so keep the exact instances added to the command. Nothing is bound by parameter name or position.

Dedicated action classes

Use a class when the command asks for a dedicated action, carries services/state, shares an action base, or should keep command construction separate from execution. These action types live in System.CommandLine.Invocation.

using System.CommandLine;
using System.CommandLine.Invocation;

var input = new Argument<FileInfo>("input");
var format = new Option<string>("--format") { DefaultValueFactory = _ => "json" };
var export = new Command("export") { input, format };

// Pass the SAME symbol instances to the action, then assign the action object.
export.Action = new ExportAction(input, format, new ExportService());
var root = new RootCommand("Data tool") { export };
return await root.Parse(args).InvokeAsync();

internal sealed class ExportAction(
    Argument<FileInfo> input,
    Option<string> format,
    ExportService service) : AsynchronousCommandLineAction
{
    public override async Task<int> InvokeAsync(
        ParseResult parseResult,
        CancellationToken cancellationToken = default)
    {
        FileInfo file = parseResult.GetValue(input)!;
        string selectedFormat = parseResult.GetValue(format)!;
        await service.ExportAsync(file, selectedFormat, cancellationToken);
        return 0;
    }
}

• Override int Invoke(ParseResult) on SynchronousCommandLineAction.
• Override Task<int> InvokeAsync(ParseResult, CancellationToken) on AsynchronousCommandLineAction; pass that cancellation token through to cancellable work.
• If the requirement says asynchronous action class, inherit AsynchronousCommandLineAction even when today's body has no naturally asynchronous call. Do not silently substitute SynchronousCommandLineAction merely because the first implementation only prints output.
• Assign the object through command.Action. If the requirement is a dedicated action class, do not also wire the command with SetAction.
• Carry the exact Option<T> / Argument<T> objects into the action constructor and read them with parseResult.GetValue(symbol). A same-named replacement object does not identify the parsed input.
• Give sibling commands their own symbol and action instances. An add option that is required and an edit option that is optional are different contracts even when both are named --format.

An abstract action base can carry inputs and dependencies shared by several leaf actions:

internal abstract class ServiceActionBase(
    Option<string> logLevel,
    LoggingService logging) : AsynchronousCommandLineAction
{
    protected LoggingService Logging { get; } = logging;
    protected string LogLevel(ParseResult result) => result.GetValue(logLevel)!;
}

Small inline actions

// Sync: int is the process exit code (a void overload exists and implies 0).
command.SetAction(parseResult =>
{
    var name = parseResult.GetValue(nameOption)!;   // read by identity
    Console.WriteLine($"Hello {name}");
    return 0;
});

// Async: return Task<int> (or Task) and propagate cancellation.
command.SetAction(async (parseResult, cancellationToken) =>
{
    var url = parseResult.GetValue(urlOption)!;
    await DoWorkAsync(url, cancellationToken);
    return 0;
});

Use SetAction for genuinely local behavior. Do not expand a dedicated-action requirement into an inline delegate merely because both forms can produce the same output.

Parsing and running

// One-shot:
return await root.Parse(args).InvokeAsync();     // sync equivalent: root.Parse(args).Invoke()

// Or inspect before invoking:
ParseResult result = root.Parse(args);
if (result.Errors.Count > 0)
{
    foreach (var e in result.Errors) Console.Error.WriteLine(e.Message);
    return 1;
}
return await result.InvokeAsync();

• command.Parse(args) returns a ParseResult; Invoke() / InvokeAsync() then run the matched command's action (and built-in --help / --version / error reporting).
• Migration: command.Invoke(args) / InvokeAsync(args) (arg-taking overloads) are gone — always Parse(args) first, then invoke the result.

Errors & exit codes

• User-input errors: report with result.AddError("...") from a CustomParser or a validator on the option/argument. They land in ParseResult.Errors, are printed by the invoker, and produce a non-zero exit code automatically. Do not throw for bad input.
• Action outcome: return the exit code you want from SetAction or the action override.
• ParseResult.Errors is the list to check when you parse-then-decide manually.