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Posted 3 Oct 2009

Creating A Fluent Interface in C# - For training a bunch of Dogs

, 3 Oct 2009 CPOL
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A simple post on a simple subject

Today Vin just tweeted "FI(Fluent Interface) - when you use method chaining, after finishing the functionality, "return this", and that's how you make it fluent".

I thought about writing a detailed post on this, just to demonstrate various possibilities of Fluent programming.

Chaining Methods - A Simple Scenario

Assuming that you are interested in training animals, let us start with a simple ITrainable interface. :)

/// <span class="code-SummaryComment"><summary>
</span>/// Anything that is trainable
/// <span class="code-SummaryComment"></summary>
</span>public interface ITrainable
    ITrainable Train(string skill);
    ITrainable Do(string skill);

Well, nothing fancy there. Let us go ahead and create a Dog class, which implements this interface. The only interesting piece you may find in the Dog class is, all methods are returning a type of ITrainable. As long as our Dog class implements ITrainable, we can return 'this', i.e. the current Dog Instance.

/// <span class="code-SummaryComment"><summary>
</span>/// Our simple dog class
/// <span class="code-SummaryComment"></summary>
</span>public class Dog : ITrainable
    public string Name { get; set; }
    public List<string> Skills { get; set; }

    public Dog(string name)
        Console.WriteLine("Dog " + name +
               " created");

        Name = name;
        Skills = new List<string>();

    //Let us train this skill to our dog
    //Note that we are returning 'this', so that
    //we can continue training or ask the dog to perform
    public ITrainable Train(string skill)
        Console.WriteLine("Dog " + Name +
                " learned " + skill);
        return this;

    //Let us ask the dog to perform this skill
    public ITrainable Do(string skill)
        if (Skills.Contains(skill))
            Console.WriteLine("Dog " + Name + 
                " is doing " + skill);
            Console.WriteLine("Dog " + Name + 
                ": Don't know how to " + skill);

        return this;

Now, we are ready to train our Dog fluently. Like,

//Train one dog, Hope your name is not Bobby ;)
var dog = new Dog("Bobby");

As you can see, we are chaining the method calls, because in each call, we are returning an object of type ITrainable. You'll see what Bobby is doing in the console:

Dog Bobby created
Dog Bobby learned Running
Dog Bobby learned Eating
Dog Bobby is doing Running
Dog Bobby is doing Eating

Chaining Methods - For Collections

Now, let us do something more interesting. Let us create a couple of extension methods for all collections of ITrainable. We do this by writing an extension method for IEnumerable<ITrainable>. If you see, our extension methods are accepting a bunch of trainable organisms(?) (read, IEnumerable<ITrainable>) and return the same.

Leave out the Console.WriteLine(), it is there just for some pretty printing.

/// <span class="code-SummaryComment"><summary>
</span>/// Let us fluently train a bunch of Trainable
/// animals
/// <span class="code-SummaryComment"></summary>
</span>public static class TrainableExtensions
    //Note that we are returning the IEnumerable<ITrainable>
    public static IEnumerable<ITrainable>
        Train(this IEnumerable<ITrainable> flock, string skill)
        foreach (var member in flock)
        return flock;

    //Ask all members to perform a skill
    public static IEnumerable<ITrainable>
        Do(this IEnumerable<ITrainable> flock, string skill)
        foreach (var member in flock)
        return flock;

Now, let us create few dogs, and train them together :)

//Let us create the dogs and train them
var dogs = new List<ITrainable>{new Dog("Jimmy"),
                                     new Dog("Sando"),
                                     new Dog("Rob")};

//Now train them

And you'll see what they are doing, in this order.

Dog Jimmy created
Dog Sando created
Dog Rob created

Dog Jimmy learned EatingFood
Dog Sando learned EatingFood
Dog Rob learned EatingFood

Dog Jimmy learned Running
Dog Sando learned Running
Dog Rob learned Running

Dog Jimmy is doing EatingFood
Dog Sando is doing EatingFood
Dog Rob is doing EatingFood

Dog Jimmy learned JumpingToRing
Dog Sando learned JumpingToRing
Dog Rob learned JumpingToRing

Dog Jimmy is doing Running
Dog Sando is doing Running
Dog Rob is doing Running

Now, as LINQ methods are essentially extension methods on top of IEnumerable<T>, you can mix and match the extension methods we just wrote, with LINQ methods. For example, if we decide only Rob should learn a special skill (JumpingRing), you can do this.


And you'll see how this works:

Dog Jimmy learned EatingFood
Dog Sando learned EatingFood
Dog Rob learned EatingFood

Dog Rob learned JumpingRing

Dog Rob learned TakingPaper
Dog Jimmy learned TakingPaper
Dog Sando learned TakingPaper

And finally, Fluent APIs can be used for much more useful tasks (If you haven't yet realized, LOL). Few fluent API's I've come across are:

Happy coding!!


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


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