Unparsing skips arguments with values equal to the argument type's default value

使用提供的 Program 和 TestArgs 应用重现该问题，重点关注 settings.SkipDefault = true 时的 Parser.FormatCommandLine。将其对值 0、1 和 2 的输出与预期的命令行进行比较，然后验证每个结果经过往返解析后都能还原为原始值。

Using CommandLineParser 2.9.1, the unparsing behaviour with SkipDefault = true seems to be incorrect. Here is a test app to reproduce the behaviour.

using CommandLine;

namespace UnparsingTest
{
    public class Program
    {
        public class TestArgs
        {
            [Option('c', Required = true)]
            public int CompulsoryWithoutDefault { get; set; }

            [Option('d', Required = true, Default = 1)]
            public int CompulsoryWithDefault { get; set; } = 1;

            [Option('o', Required = false, Default = 1)]
            public int Optional { get; set; } = 1;
        }

        static void Main()
        {
            Parser parser = new Parser();

            for (int i = 0; i <= 2; ++i)
            {
                TestArgs args = new TestArgs()
                {
                    CompulsoryWithoutDefault = i,
                    CompulsoryWithDefault = i,
                    Optional = i,
                };

                string commandLine = parser.FormatCommandLine(args, settings => settings.SkipDefault = true);
                System.Console.WriteLine($"CompulsoryWithDefault: {args.CompulsoryWithDefault}, CompulsoryWithoutDefault: {args.CompulsoryWithoutDefault}, Optional: {args.Optional}, Command line: \"{commandLine}\"");
            }
        }
    }
}

There are 2 compulsory arguments, one with a Default value and one without a Default value. CompulsoryWithDefault and Optional have their Default values set to any value that is not default(int).

The test app's console output shows the unparsed command line for the 3 sets of arguments

CompulsoryWithDefault: 0, CompulsoryWithoutDefault: 0, Optional: 0, Command line: ""
CompulsoryWithDefault: 1, CompulsoryWithoutDefault: 1, Optional: 1, Command line: "-c 1"
CompulsoryWithDefault: 2, CompulsoryWithoutDefault: 2, Optional: 2, Command line: "-c 2 -d 2 -o 2"

The unparsed command line for the first case will fail round trip parsing because the compulsory -c and -d arguments are missing and -o is missing so Optional will have a value of 1 instead of 0. The unparsed command line for the second case will also fail round trip parsing because the compulsory -d argument is missing. Only the third case is giving correct output.

The bug appears to be that the unparser is skipping Required = true arguments, arguments with value equal to Default = {value} and arguments with value equal to default(typeof(argument)).

The correct behaviour would be to only skip arguments that do not have Required = true and whose value equals the value specified in Default = {value}.

The expected console output would be

CompulsoryWithDefault: 0, CompulsoryWithoutDefault: 0, Optional: 0, Command line: "-c 0 -d 0 -o 0"
CompulsoryWithDefault: 1, CompulsoryWithoutDefault: 1, Optional: 1, Command line: "-c 1 -d 1"
CompulsoryWithDefault: 2, CompulsoryWithoutDefault: 2, Optional: 2, Command line: "-c 2 -d 2 -o 2"

These command lines would then correctly round trip parse/unparse.