CommandLineParser - Value not changing

Question

I created a small project, which uses the CommandLineParser library.

I took a small code snippet from the guide C# Quick Start:

using System;
using CommandLine;

namespace QuickStart
{
    class Program
    {
        public class Options
        {
            [Option('v', "verbose", Required = false, HelpText = "Set output to verbose messages.")]
            public bool Verbose { get; set; }
        }

        static void Main(string[] args)
        {
            Parser.Default.ParseArguments<Options>(args)
                   .WithParsed<Options>(o =>
                   {
                       if (o.Verbose)
                       {
                           Console.WriteLine($"Verbose output enabled. Current Arguments: -v {o.Verbose}");
                           Console.WriteLine("Quick Start Example! App is in Verbose mode!");
                       }
                       else
                       {
                           Console.WriteLine($"Current Arguments: -v {o.Verbose}");
                           Console.WriteLine("Quick Start Example!");
                       }
               });
        }
    }
}

I can start the program via command promt with

dotnet CommandLineParserTestProject.dll

The output is as expected:

Current Arguments: -v False
Quick Start Example!

When I then put:

dotnet CommandLineParserTestProject.dll -v true

The output is again as expected:

Verbose output enabled. Current Arguments: -v True
Quick Start Example! App is in Verbose mode!

---

Now the problem:

After these two commands from above I enter:

dotnet CommandLineParserTestProject.dll -v false

but the output then is still:

Verbose output enabled. Current Arguments: -v True
Quick Start Example! App is in Verbose mode!

Only when I enter dotnet CommandLineParserTestProject.dll false it changes, dotnet CommandLineParserTestProject.dll -v false doesn't seem to work.

Does anybody know why this happens?

Answer 1

Boolean command line options do not take parameters (i.e. true/false); you either include them or you don't. Therefore the true/false parameter at the end of your command line is being ignored. Verbose output is turned on when the parser sees the -v option, and it's turned off when you omit -v from the command line.

Referring back to the documentation, you should consider handling unparsed options in the recommended way:

.WithNotParsed<Options>((errs) => HandleParseError(errs));

... which would have informed you that your true/false parameters were being ignored.