Designing Fluent Interfaces in C#

The concept of Fluent Interface was coined by Martin Fowler to create various objects and wire them up together. This pattern is often used for object configuration and setup. For example in ASP.NET Core applications we have following code for configuring the server:

using System;
using Microsoft.AspNetCore.Hosting;

namespace aspnetcoreapp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var host = new WebHostBuilder()
                .UseKestrel()
                .UseStartup<Startup>()
                .Build();

            host.Run();
        }
    }
}

As you can see we can chain methods together easily and the intention is to produce an API that is readable and flows. This pattern is heavily used in programming, Here’s an example of using this approach in jQuery:

$("#result")
  .css("background", "yellow")
  .width(200)
  .height(200)
  .text("This is the result");

For C#, there is LINQ:

var person = PersonDataSource
	.Where(p => p.Age > 20)
	.OrderBy(p => p.Name)
	.Select(p => new {
		Name = p.Name,
		LastName = p.LastName
});

You might ask how to implement this pattern, well, it is very simple you just need to return this during method call. The Calculator defined below contains three methods. These methods are returing Calculator object using this keyword, which allow to chain the methods:

public class Calculator
{
	public int Initial { get; private set; }
	public Calculator(int intial)
	{
		Initial = intial;
	}

	public Calculator Add(int x)
	{
		Initial = Initial + x;
		return this;
	}

	public Calculator Divide(int x)
	{
		Initial = Initial / x;
		return this;
	}

	public Calculator Substract(int x)
	{
		Initial = Initial - x;
		return this;
	}
}

Now we can use the class this way:

var calc = new Calculator(5)
			.Add(5)
			.Divide(5)
			.Substract(2);

System.Console.WriteLine(calc.Initial);

As you can see this approach makes our code concise and easier to read.