Complete and working sample of CLI

Program.cs

using McMaster.Extensions.CommandLineUtils;
using System;
using System.IO;

namespace CSVUtil
{
    class Program
    {
        public static void Main(string[] args)
        {
            var app = new CommandLineApplication<Program>(throwOnUnexpectedArg: false);

            #region global-argumetns-options
            var inputFileOption = app.Option<string>("-i|--input", "Input file to convert"
                , CommandOptionType.SingleValue)
                .IsRequired();

            var verboseOption = app.Option("-v|--verbose", "Display operation details"
                , CommandOptionType.NoValue);
            #endregion

            #region convert-command
            app.Command("convert",
                (convert) =>
                {
                    var outputOption = convert.Option("-o|--output", "Output File"
                        , CommandOptionType.SingleValue);

                    var firstRowOption = convert.Option("-f|--first-row-header", "First row as header"
                        , CommandOptionType.NoValue);

                    convert.HelpOption("-? | -h | --help");

                    convert.OnExecute(() =>
                    {
                        if (verboseOption.HasValue())
                            Console.WriteLine($"Init process file {inputFileOption.ParsedValue}.");

                        var inputFile = GetInputFile(inputFileOption);

                        var outputFile = outputOption.Value()
                            ?? Path.GetFileNameWithoutExtension(inputFile);

                        if (verboseOption.HasValue() && firstRowOption.HasValue())
                            Console.WriteLine("Find first row as header.");

                        if (verboseOption.HasValue())
                            Console.WriteLine($"Convert to {outputFile}.xlsx");

                        // do the conversation!
                    });

                }
            );
            #endregion convert-command

            #region print-command
            app.Command("print", (print) =>
            {
                var tailOptions = print.Option<bool>("-t|--tail", "Show n rows from the tail."
                    , CommandOptionType.NoValue);

                var rowOption = print.Option<int>("-r|--rows",
                    "Number of rows to show", CommandOptionType.SingleValue);

                print.HelpOption("-? | -h | --help");

                print.OnExecute(() =>
                {
                    var inputFile = GetInputFile(inputFileOption);

                    // Zero = all rows
                    var rows = rowOption.HasValue() ? Math.Abs(rowOption.ParsedValue) : 0;

                    if (verboseOption.HasValue())
                        Console.WriteLine(tailOptions.HasValue() && rows > 0
                            ? $"Print {rows} records from the tail."
                            : $"Print {rows} records from the top.");

                    // print file
                    Console.WriteLine("print.");
                });
            });
            #endregion

            app.HelpOption("-h|--help|-?");
            app.Execute(args);
        }

        private static string GetInputFile(CommandOption<string> inputOption)
        {
            var inputFile = inputOption.ParsedValue;
            if (!Path.IsPathFullyQualified(inputOption.ParsedValue))
                inputFile = Path.GetFullPath(inputOption.ParsedValue);

            //if (!File.Exists(inputFile))
            //    Console.WriteLine("File not found.");

            return inputFile;
        }
    }
}