App.cs

using System.CommandLine;
using System.CommandLine.Builder;
using System.CommandLine.Parsing;

namespace scl
{
  public static class Usage
  {
    public struct CmdArgs
    {
      public int dpi;
      public bool? grayscale;
      public string? outputFile;
      public string[]? inputFileList;

      public CmdArgs() {
        dpi = 0;
        grayscale = false;
        outputFile = "";
        inputFileList = new string[10];
      }
    }

    public static CmdArgs GetOpts(string[] args)
    {
      CmdArgs cmdArgs = new CmdArgs();

      var dpiOption        = new Option<int>     (new[] {"--dpi"}, "Set image dots per inch.");
      var grayscaleOption  = new Option<bool?>   (new[] { "--grayscale", "-g" }, "Convert Image to grayscale.");
      var outputFileOption = new Option<string?> (new[] { "--output", "-o" }, description: "Output filename or basename when passing multiple output files, i.e. outputfile-1 outputfile-2 ...", getDefaultValue: () => { return ""; });

      // The line below produces the following build error: error CS1503: Argument 1: cannot convert from 'string[]' to 'string?'
      var inputFileListArg = new Argument<string[]> ("input_file", description: "One or more image filename(s) to work on: image_file1 image_file2 ...");

      var rootCommand = new RootCommand("SDSI Image Transform and Enhancement");
      rootCommand.Add(dpiOption);
      rootCommand.Add(outputFileOption);
      rootCommand.Add(inputFileListArg);

      rootCommand.SetHandler(
        (dpiOption, grayscaleOption, inputFileListArg, outputFileOption) =>
        {
          cmdArgs.dpi        = dpiOption;
          cmdArgs.grayscale  = grayscaleOption;
          cmdArgs.inputFileList  = inputFileListArg;

          if (outputFileOption != null) {
            cmdArgs.outputFile = outputFileOption.ToString();
          }

          Console.WriteLine("DEBUG:/Usage.cs:GetOpts(): DPI         = {0}", cmdArgs.dpi);
          Console.WriteLine("DEBUG:/Usage.cs:GetOpts(): OUTPUT_FILE = {0}", cmdArgs.outputFile);
          Console.WriteLine("DEBUG:/Usage.cs:GetOpts(): INPUT_FILE  = {0}", cmdArgs.inputFileList);
        },
        dpiOption, grayscaleOption, inputFileListArg, outputFileOption
      );

      rootCommand.Invoke(args);

      return cmdArgs;
    }
  }
}