It should be possible to access all the command configuration

AcceptOnlyFromAmong と、Program.cs/Main に示されている rootCommand.Parse の呼び出しに関係する Argument および Option API から始めます。completion 構成がどのように保存され、公開されているかを追跡し、続いて、インタラクティブな呼び出し元が reflection を使わずにコマンド、引数、オプション、許可された値のメタデータを読み取れる公開 API を特定します。Issue のユースケースがサポート対象の API を通じてその構成にアクセスできれば完了です。

I originally raised this as #2526, but that was on beta4 and the internals have changed since then.

I have a large command line application, with lots of sub commands and options, it's used both in scheduled tasks (where any parse fail should error) and by users (where it should be interactive).

I want to use --interactive as a global option - if parse succeeds this is ignored (it just runs) but if parse fails (and this is true) the CLI app should prompt the user for the missing command/argument/options.

The information is all there for building the help, but when --interactive is set I want to prompt for just the next bit, not stop and present the help.

Unfortunately, that information is all hidden in the private implementation. For instance, I can set AcceptOnlyFromAmong in my argument or option, but then when I prompt the user for that argument I can't access that list.

Here is an example use-case (using Spectre.Console for the prompt formatting, but you can use any console input):

static class InteractiveParsePrompt
{
    /// <summary>Limit of error depth - users can CTRL+C cancel out but this is a saftey check.</summary>
    const int MaxDepth = 20;

    /// <summary>Interative option, set globally and available on all commands.</summary>
    public static readonly Option<bool> OptInteractive = new("--interactive", "--i", "-i") {
        Description = "Run in interactive mode, any missing arguments or options will be requested",
        Recursive = true,
    };

    /// <summary>Escape/quote text so that it can be passed on the command line</summary>
    static string EscapeCommandLine(string value)
    {
        bool allSafe = true;
        foreach (char c in value)
            if(!char.IsLetter(c) &&
                !char.IsNumber(c) &&
                c != '-')
            {
                allSafe = false;
                break;
            }

        if (allSafe) return value;

        return $"\"{value.Replace("\"", "\\\"")}\"";
    }

    /// <summary>Strip leading hyphens from the name, as it might include them but doesn't have to</summary>
    static string NormaliseOptionName(string name)
    {
        if (name.StartsWith("--"))
            return name[2..];
        else if (name.StartsWith("-"))
            return name[1..];
        else
            return name;
    }

    /// <summary>Get the allowed values for an argument.</summary>
    static string[] GetAllowedValues(Argument argument) {

        // System.CommandLine makes all this info about the allowed values private, so we need to use reflection to get it
        var target = argument.CompletionSources?.FirstOrDefault()?.Target;
        if (target is null) return [];

        // We expect this to be Func<CompletionContext, IEnumerable<CompletionItem>>
        // Why we need to decompile and reflect to get values set by AcceptOnlyFromAmong is baffling
        // Obviously this is fragile AF so expect bugs
        var method = target.GetType().GetMethods(BindingFlags.NonPublic | BindingFlags.Instance)?.FirstOrDefault();
        if (method is null) return [];

        // Invoke the method to get the list of CompletionItem set by AcceptOnlyFromAmong
        var allowedValues = method.Invoke(target, BindingFlags.NonPublic | BindingFlags.Instance, null, new CompletionContext[1], null) as IEnumerable<CompletionItem>;
        if (allowedValues is null) return [];

        return [..from v in allowedValues let t = v.InsertText where !string.IsNullOrEmpty(t) select t];
    }

    /// <summary>Get the markup text for the option or argument description.</summary>
    static string PromptText(Symbol symbol) {
        if (symbol.Description is not null)
            return $"[bold yellow]{symbol.Name}[/] [italic]{symbol.Description.EscapeMarkup().TrimEnd(' ', '.')}[/]" ;

        return $"[bold yellow]{symbol.Name}[/]";
    }

    /// <summary>Prompt the user to provide the value for an argument</summary>
    static async Task<string> PromptArgument(
        Argument argument, 
        IAnsiConsole console, 
        CancellationToken cancellationToken)
    {
        string[] allowedValues = GetAllowedValues(argument);
        IPrompt<string> prompt;
        if (allowedValues.Length > 0)
            prompt = new SelectionPrompt<string>().
                Title(PromptText(argument)).
                PageSize(20).
                AddChoices(allowedValues.Order());
        else
            prompt = new TextPrompt<string>(PromptText(argument));

        string argResponse = await prompt.ShowAsync(console, cancellationToken);
        console.MarkupLine($"Argument [bold yellow]{argument.Name}[/] = [green]{argResponse}[/]");
        return argResponse;
    }

    /// <summary>Prompt the user to provide the value for an option</summary>
    static async Task<string[]> PromptOption(
        Option option, 
        IAnsiConsole console, 
        CancellationToken cancellationToken)
    {
        if (option.ValueType == typeof(bool)) {
            // Boolean, give them a y/n confirmation prompt
            var promptConfirm = new TextPrompt<bool>(PromptText(option)).
                AddChoice(true).
                AddChoice(false).
                DefaultValue(false).
                WithConverter(choice => choice ? "y" : "n");

            bool optConfirm = await promptConfirm.ShowAsync(console, cancellationToken);
            if (!optConfirm) return [];

            console.MarkupLine($"Option set [bold green]{option.Name}[/]");
            return [$"--{NormaliseOptionName(option.Name)}"];
        }

        TextPrompt<string> prompt = new(PromptText(option));

        // Get the default value, if the option has one
        // option.GetDefaultValue() throws an (undocumented) exception if there is no default value factory set
        string? defaultValue = option.HasDefaultValue ? option.GetDefaultValue()?.ToString() : null;
        if (defaultValue is not null)
            prompt.DefaultValue(defaultValue);

        string optResponse = await prompt.ShowAsync(console, cancellationToken);
        console.MarkupLine($"Option [bold yellow]{option.Name}[/] = [green]{optResponse}[/]");
        return [$"--{NormaliseOptionName(option.Name)}", optResponse];
    }

    /// <summary>Prompt the user to choose a subcommand, if that has arguments or options prompt for them too, return a new set of arguments to parse from the prompts</summary>
    static async IAsyncEnumerable<string> PromptCommand(
        Command command, 
        IAnsiConsole console, 
        [EnumeratorCancellation] CancellationToken cancellationToken)
    {
        int maxL = command.Subcommands.Select(c => c.Name.Length).Max() + 1;

        string subCommand = console.Prompt(
            new SelectionPrompt<string>().
                Title("Choose command?").
                PageSize(20).
                AddChoices(command.Subcommands.Select(c => $"{c.Name.PadRight(maxL)}: {c.Description}").Order()));

        string commandName = subCommand.Split(":")[0].Trim();
        console.MarkupLine($"Command [green]{commandName}[/] selected");
        yield return commandName;

        var subCommandFound = command.Subcommands.FirstOrDefault(c => c.Name == commandName);
        if(subCommandFound is null) yield break;

        if(subCommandFound.Arguments.Count > 0)
            foreach (var argument in subCommandFound.Arguments)
                yield return await PromptArgument(argument, console, cancellationToken);

        if (subCommandFound.Options.Count > 0)
            foreach (var option in subCommandFound.Options)
                foreach(string optionValue in await PromptOption(option, console, cancellationToken))
                    yield return optionValue;

        if (subCommandFound.Subcommands.Count > 0)
            await foreach (string sub in PromptCommand(subCommandFound, console, cancellationToken))
                yield return sub;
    }

    /// <summary>Intercept the command line parse on failure if --interactive option is set to prompt the user for the missing commands, arguments and options.</summary>
    public static async Task<ParseResult> AskUserForArgs(IServiceProvider services, RootCommand rootCommand, ParseResult original, CancellationToken cancellationToken)
    {
        // If no errors or not in interactive mode, continue
        if (!original.GetValue(OptInteractive) ||
            original.Errors.Count == 0)
            return original;

        // Use Spectre.Console for interactive prompts, set up in the DI
        var console = services.GetRequiredService<IAnsiConsole>();
        console.WriteLine("Interactive mode");

        var parseResult = original;
        try { 
            int retry = 0;
            while(retry++ < MaxDepth &&
                parseResult.Errors.Count != 0 && 
                !cancellationToken.IsCancellationRequested)
            {
                var command = parseResult.CommandResult.Command;

                List<string> interactiveArgs = [.. parseResult.Tokens.Select(t => t.Value)];
                foreach (var error in parseResult.Errors)
                {
                    if (cancellationToken.IsCancellationRequested) break;

                    if (error.Message == "Required command was not provided.")
                    {
                        await foreach (string arg in PromptCommand(command, console, cancellationToken))
                        {
                            if (cancellationToken.IsCancellationRequested) break;

                            interactiveArgs.Add(arg);
                        }
                    }
                    else if (error.Message.StartsWith("Required argument missing for command:"))
                    {
                        string argumentName = error.Message.Split(":")[1].Trim(' ', '\'', '.');
                        var argument = command.Arguments.FirstOrDefault(a => a.Name == argumentName);
                        if (argument is null)
                        {
                            console.MarkupLine($"[red]{error.Message.EscapeMarkup()}[/]");
                            break;
                        }

                        interactiveArgs.Add(await PromptArgument(argument, console, cancellationToken));
                    }
                    else
                    {
                        console.MarkupLine($"[red]{error.Message.EscapeMarkup()}[/]");
                        break;
                    }
                }

                parseResult = rootCommand.Parse(interactiveArgs);
            }
        }
        catch (TaskCanceledException) { } // Ignore cancellation errors

        if (cancellationToken.IsCancellationRequested)
            console.MarkupLine("[red]Cancelled[/]");
        else if (parseResult.Errors.Count == 0)
        {
            string newArgs = string.Join(' ', parseResult.Tokens.Select(t => EscapeCommandLine(t.Value)));
            console.MarkupLine($"New arguments set: [green]{newArgs.EscapeMarkup()}[/]");
        }
        else
            console.MarkupLine("[red]Failed[/]");

        return parseResult;
    }
}

And then I cam hook this up in Program.cs/Main:

var parsed = rootCommand.Parse(args);

// If --interactive and there were parsing errors, prompt the user to fix them
if (parsed.GetValue(InteractiveParsePrompt.OptInteractive) &&
    parsed.Errors.Count > 0)
    parsed = await InteractiveParsePrompt.AskUserForArgs(services, rootCommand, parsed, cancellationToken);

Why do I need reflection to get the values from AcceptOnlyFromAmong that I set from my code?