Another short one this time, as I still haven’t been up to anything particularly interesting recently. This should change in the coming weeks as we start work on a brand new service, which will be nice. We learnt a lot from the first one, but we still have some unanswered questions like database schema versioning and deploying/automating functional tests as part of a continuous integration environment, which should make for some good topics.

Anyway this weeks post is a simple one about a component I ran across the other day while refactoring an application.

Its called Fluent Command Line Parser and it fills one of those niche roles that you come across all the time, but never really spend any time thinking about, which is supplying command line arguments to an application.

Yes Commander?

Most of the time when you see an application that takes input from the command line, you’ll probably see assumptions about the order of elements supplied (like args[0] is the database, args[1] is the username, etc). Typically there is little, if any, validation around the arguments, and you would be lucky if there was help or any other documentation to assist the people who want to use the application from the command line. There are many reasons for this, but the most common is usually effort. It takes effort to put that sort of thing in, and unless the command line tool is going to be publically consumed, that effort probably isn’t necessary.

Fluent Command Line Parser (to be referred to as FCLP from here on) makes this sort of thing much much easier, reducing the effort to the level of “Why not?”.

Essentially you configure a parser object to recognise short and long tokens to indicate a particular command line argument, and then use the presence of those tokens to fill properties in a strongly typed object. The parser can be configured to provide descriptions of the tokens, whether or not they are required, callbacks to be run when the token is encountered and a few other things that I haven’t really looked into yet.

FCLP also offers the ability to set a help callback, executed whenever /? or /help is present, which is nice. The help callback automatically ties in with the definitions and descriptions for the tokens that have been configured, so you don’t need to document anything twice.

Enough rambling about the code, lets see an example:

public static void Main(string[] args)
    var parser = new FluentCommandLineParser<Arguments>();
        .Setup(a => a.Address)
        .As('a', "address")
        .WithDescription("The URL to connect to the service.");

        .Setup(a => a.Username)
        .As('u', "username")
        .WithDescription("The username to use for authentication.");
        .Setup(a => a.Password)
        .As('p', "password")
        .WithDescription("The password to use for authentication.");

    var parsedArgs = parser.Parse(args);

    // Use parsedArgs in some way.

private class Arguments
    public string Address { get; set; }
    public string Username { get; set; }
    public string Password { get; set; }

As you can see its pretty clear what’s going on here. Its simply to add new arguments as well, which is nice.

The Commander is Down!

Its not all puppies and roses though. The FCLP library is very simple and is missing the ability to do at least 1 important thing that I needed, which was a shame.

It only supports simple/primitive objects (like strings, integers, booleans, enums and collections of those primitives) for the properties being set in your parsed arguments object. This means that if you want to supply a Uri on the command line you have to add an extra step or an extract function on your parsed arguments class that gets the string value passed in as a Uri. Its not a total deal breaker, its just unfortunate, because it seems like something you should be able to do with a callback or something similar, but the context (the object containing the parsed arguments) is just not available at that point in the workflow.

I might take a stab at adding that ability to the library (its open source) if I get a chance. I feel like you can probably accomplish it by adding another method to override the default parsing of the object from the entered string (supplying a Func<string, T> to transform the string with custom logic).


If you’re looking for a nice simple library to make your command line parsing code a bit clearer, this is a good one to grab. It really does help to improve the quality of a typically messy area of an application, which makes it a lot easier for someone coming in later to understand. Just add a reference to the FluentCommandLineParser package via Nuget (you are using Nuget aren’t you? No reason not to) and off you go.

Remember, every little bit like this helps, because we don’t write code for ourselves. We write it for the people who come after us. Even if that person is you 6 months from now, when you’ve forgotten everything.