Exception Thrown Rather than Error Message And Help When Command Has Both Custom Parser and Validator

[richardcox13]

In the code below, running (with SDK 10.0.100-rc.1.25451.107)

dotnet run ./TestOne.cs 12 23 34 --dats ab bc cd 

Leads to an exception being thrown

Unhandled exception. System.InvalidOperationException: Argument "--dats" is not two characters long     
   at System.CommandLine.Binding.ArgumentConverter.GetValueOrDefault[T](ArgumentConversionResult result)
   at System.CommandLine.Parsing.ArgumentResult.GetValueOrDefault[T]()
   at Program.<>c.<<Main>$>b__0_3(ArgumentResult argRes) in [HIDDEN]\TestOne.cs:line 52
   at System.CommandLine.Parsing.ArgumentResult.ValidateAndConvert(Boolean useValidators)
   at System.CommandLine.Parsing.ArgumentResult.GetArgumentConversionResult()
   at System.CommandLine.Parsing.CommandResult.ValidateArgumentsAndAddDefaultResults(Boolean completeValidation)
   at System.CommandLine.Parsing.CommandResult.Validate(Boolean isInnermostCommand)
   at System.CommandLine.Parsing.ParseOperation.ValidateAndAddDefaultResults()
   at System.CommandLine.Parsing.ParseOperation.Parse()
   at System.CommandLine.Parsing.CommandLineParser.Parse(Command command, IReadOnlyList`1 arguments, String rawInput, ParserConfiguration configuration)
   at System.CommandLine.Parsing.CommandLineParser.Parse(Command command, IReadOnlyList`1 args, ParserConfiguration configuration)
   at System.CommandLine.Command.Parse(IReadOnlyList`1 args, ParserConfiguration configuration)
   at Program.<Main>$(String[] args) in [HIDDEN]\TestOne.cs:line 71
   at Program.<Main>(String[] args)

Rather than the expected, and more helpful:

Argument "--dats" is not two characters long

Description:
  Test app for System.CommandLine

Usage:
  TestOne <TestOne>... [options]

Arguments:
  <TestOne>  Root command arguments. One or more pairs of characters separated by space

Options:
  --data <data>   Option data items
  -?, -h, --help  Show help and usage information
  --version       Show version information

I noticed this when typo'ing the command line in a rather more complex project.

The problem seems to be that the validator triggers the parser, so there is nothing to check for errors reported by the parser. Without the custom parser the expected error & help is reported.

---

#:package System.CommandLine@2.0.0-rc.1.25451.107
#nullable enable
using System.CommandLine;
using System.CommandLine.Parsing;
using System.Diagnostics;

string[] ParseArguments(ArgumentResult argRes) {
    List<string> res = new();
    foreach (var a in argRes.Tokens) {
        var s = a.Value;
        if (s.Length != 2) {
            argRes.AddError($"Argument \"{s}\" is not two characters long");
        } else {
            res.Add(s);
        }
    }
    return res.ToArray();
}

void ValidateArguments(string[] rawValues, Action<string> reportError) {
    if (rawValues.Length == 0) {
        reportError("No arguments provided");
        return;
    }

    foreach (var rawValue in rawValues) {
        if (rawValue.Length != 2) {
            reportError($"Argument \"{rawValue}\" is not two characters long");
        } else if (rawValue[0] >= rawValue[1]) {
            reportError($"In argument \"{rawValue}\" the first letter does not precede, in Unicode code point order, the second");
        }
    }
}

var dataOption = new Option<string[]>("--data") {
    Description = "Option data items",
    Arity = ArgumentArity.OneOrMore,
    Required = false,
    AllowMultipleArgumentsPerToken = true
};
dataOption.Validators.Add(optRes => {
    var args = optRes.GetValueOrDefault<string[]>();
    ValidateArguments(args, msg => optRes.AddError(msg));
});

var rootCmdArgs = new Argument<string[]>("TestOne") {
    Description = "Root command arguments. One or more pairs of characters separated by space",
    Arity = ArgumentArity.OneOrMore
};
rootCmdArgs.CustomParser = ParseArguments;
rootCmdArgs.Validators.Add(argRes => {
    var args = argRes.GetValueOrDefault<string[]>();
    ValidateArguments(args, msg => argRes.AddError(msg));
});

var rootCmd = new RootCommand("Test app for System.CommandLine") {
    dataOption,
    rootCmdArgs
};
rootCmd.SetAction(parseResult => {
    var args = parseResult.GetValue(rootCmdArgs);
    var dataArgs = parseResult.GetValue(dataOption);
    Debug.Assert(args is not null);
    Debug.Assert(dataArgs is not null);
    Console.WriteLine("Root command");
    Console.WriteLine($"   args: {String.Join(", ", args.Select(a => $"\"{a}\""))}");
    Console.WriteLine($"   data: {String.Join(", ", dataArgs.Select(a => $"\"{a}\""))}");
});

var parse = rootCmd.Parse(args);
await parse.InvokeAsync();

[jonsequitur]

This looks related to #2607.