Designate Required vs Optional Parameters/Options

Currently, it is up to the application developer using CommandLineUtils to validate command line input and display an appropriate error if an Option that is required is left out. I propose that CommandLineUtils take on this responsibility, allowing the library user to designate parameters as required and provide appropriate feedback to the user when the parameter is not provided.

This may be a nice feature to do after #8 so the user of the library need only add an attribute of [Required] to make this designation.

[natemcmaster]

Thought about it more, here is what I’m thinking. Would like your feedback.

Attributes

Design:

Support any implementation of System.ComponentModel.DataAnnotations.ValidationAttribute. This includes RequiredAttribute and others.

Result: When validation fails, the application exits with code 1 and prints an error message.

Usage:

using System.ComponentModel.DataAnnotations;
using McMaster.Extensions.CommandLineUtils;

class Program
{
    static int Main(string[] args) => CommandLineApplication.Execute<Program>(args);

    [Required]
    [Option(Description = "Required. The message")]
    private string Message { get; }

    [Required(ErrorMessage = "my custom error message")]
    [Option("--to <EMAIL>", Description = "Required. The recipient.")]
    public string To { get; }

    private void OnExecute(IConsole console)
    {
        console.WriteLine("To = " + To);
        console.WriteLine("Message = " + Message);
    }
}

Builder api

Design: Add extension methods that build of CommandOption, such as .IsRequired(). These should roughly map to the same values provided by RequiredAttribute and others.

Usage:

class Program
{
    static int Main(string[] args)
    {
        var app = new CommandLineApplication();

        var optionMessage = app.Option("-m|--message <MSG>", "Required. The message.", CommandOptionType.SingleValue)
            .IsRequired();

        var optionReceiver = app.Option("--to <EMAIL>", "Required. The recipient.", CommandOptionType.SingleValue)
            .IsRequired();
        app.OnExecute(() =>
        {
            Console.WriteLine("To = " + optionReceiver.Value());
            Console.WriteLine("Message = " + optionMessage.Value());
        });

        return app.Execute(args);
    }
}

[natemcmaster]

This is now available in 2.1.0-rc.

https://www.nuget.org/packages/McMaster.Extensions.CommandLineUtils/2.1.0-rc