`Command.Validators` should be invoked only when the command options are arguments are valid

[9rnsr]

For Option<T> and Argument<T>, custom validators are invoked before the conversion from the token (string) to T. It's well behavior for the use case such the production of better error messages, even so the behavior is not well documented (related #2645).

For Command, as same as above, validators are invoked before its options and arguments today, but it's useless.

I expect to be able to use Command validators for the complex validation of our command line. In other words, the options and arguments should be validated with their definitions (Arity, AllowMultipleArgumentsPerToken, CustomParser, etc, and Validators) first, then only when no one have any errors, the command validators should be invoked.

Or, reading the values of options and arguments from Command.Validators is incorrect? If so, better documentation is needed.

[jonsequitur]

Can you provide some code showing what you're trying to do and where it doesn't work for you? If there's a bug here it would be helpful to isolate it.

[9rnsr]

I've written such the command validator, and hit the thrown exception from the GetValue method call from the incorrect command line myapp -i (missing the option value of -i).

static readonly Option<FileInfo[]> InputFileOption = new("--input", "-i") { ... };
static readonly Option<string[]> OutputFileOption = new("--output", "-o") { ... };

private static void ValidateRunCommand(CommandResult result)
{
    var inputs = result.GetValue(InputFileOption);
    var outputs = result.GetValue(OutputFileOption);
    if (outputs.Length != inputs.Length)
    {
        result.AddError($"{InputFileOption.Name} and {OutputFileOption.Name} should be equal in number");
        return;
    }
    ...
}

Just now, I found following validator sample from the test cases. If the validators are designed only for thee token based validation, this issue should be classified as a documentation issue rather than an implementation bug.

command-line-api/src/System.CommandLine.Tests/ParsingValidationTests.cs

Lines 329 to 345 in a3bae7d

	[Fact]
	public void A_custom_validator_can_be_added_to_a_command()
	{
	var command = new Command("the-command")
	{
	new Option<bool>("--one"),
	new Option<bool>("--two")
	};
	command.Validators.Add(commandResult =>
	{
	if (commandResult.Children.Any(sr => ((OptionResult)sr).Option.Name == "--one" &&
	commandResult.Children.Any(sr => ((OptionResult)sr).Option.Name == "--two")))
	{
	commandResult.AddError("Options '--one' and '--two' cannot be used together.");
	}
	});