Fluent interfaces seem to be everywhere these days, probably because they can be used to produce highly readable code. They use method cascading/chaining to achieve this, whereby method calls are made one after another on the same object reference.
Good examples are the LINQ component in .NET and Fluent NHibernate, obviously!
Here is a really simple example to demonstrate how you would go about writing a fluent API, in which values can be added to or subtracted from a numeric value. This is encapsulated in a Number class:
public class Number { private int currentValue; public Number(int value = 0) { currentValue = value; } public int Value { get { return currentValue; } } public Number Add(int value) { currentValue += value; return this; } public Number Subtract(int value) { currentValue -= value; return this; } }
Note that each of the chained methods returns the current instance, via the this keyword.
The Number class can be used as follows:
public static class Program { public static void Main() { var number = new Number(); number.Add(1).Add(2).Add(3); Console.WriteLine(number.Value); number.Subtract(4).Subtract(1).Add(2); Console.WriteLine(number.Value); Console.ReadLine(); } }
As an aside, adding the ability to multiply and divide would significantly increase the complexity of this class, since the order of operations would need to be taken into account.
However, hopefully my example shows how fluent APIs are put together.