Example for an erroneous run for the command line parser

calc.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="commandlineparser" Version="2.8.0" />
  </ItemGroup>

</Project>

Program.cs

using System;
using System.Collections.Generic;
using CommandLine;

namespace calc
{
    [Verb("fib")]
    internal sealed class FibonacciOption
    {
        [Value(0, Required = true, MetaName = "position of the fibonacci number to calculate" )]
        public uint Position { get; set; }
    } 

    [Verb("square")]
    internal sealed class SquareOption
    {
        [Value(0, Required = true, MetaName = "value to create a square of")]
        public uint Value {get; set;}
    }

    class Program
    {
        static int Main(string[] args)
        {
            using (Parser parser = new Parser(ConfigureParser))
            {
                return parser.ParseArguments<FibonacciOption, SquareOption>(args).MapResult(
                    (FibonacciOption fo) => HandleFibonacciOption(fo), 
                    (SquareOption so) => HandleSquareOption(so), 
                    err => HandleErrors(err));
            }
            
        }

        private static void ConfigureParser(ParserSettings settings)
        {
            settings.AutoHelp = true;
            settings.AutoVersion = true;
            settings.HelpWriter = System.Console.Out;
            settings.IgnoreUnknownArguments = false;
        }

        private static int HandleErrors(IEnumerable<Error> _)
        {
            return 1;
        }

        private static int HandleFibonacciOption(FibonacciOption option)
        {
            Console.Out.WriteLine("Calculate {0}-th fibonacci number", option.Position);
            uint fibonacci = Fibonacci(option.Position);
            Console.Out.WriteLine(fibonacci);
            return 0;
        }

        private static int HandleSquareOption(SquareOption option)
        {
            Console.Out.WriteLine("Calculate square of {0}", option.Value);
            Console.Out.WriteLine(option.Value * option.Value);
            return 0;
        }

        private static uint Fibonacci(uint position)
        {
            switch (position)
            {
                case 0u:
                case 1u: 
                {
                    return 1u;
                }
                default:
                {
                    return Fibonacci(position - 1) + Fibonacci(position - 2);
                }
            }
        }
    }
}