Extension methods

Extension methods enable you to "add" methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods on the extended type. For client code written in C# and Visual Basic, there is no apparent difference between calling an extension method and the methods that are actually defined in a type.

Extension methods are defined as static methods but are called by using instance method syntax. Their first parameter specifies which type the method operates on, and the parameter is preceded by the “this” modifier. Extension methods are only in scope when you explicitly import the namespace into your source code with a using directive.

The following example shows an extension method defined for the System.String class. Note that it is defined inside a non-nested, non-generic static class:

namespace ExtensionMethods

{

    public static class MyExtensions

    {

        public static int WordCount(this String str)

        {

            return str.Split(new char[] { ' ', '.', '?' }, StringSplitOptions.RemoveEmptyEntries).Length;

        }

    }  

}

The WordCount extension method can be brought into scope with this using directive:

using ExtensionMethods;

And it can be called from an application by using this syntax:

string s = "Hello Extension Methods";

int i = s.WordCount();

Points to remember

When using an extension method to extend a type whose source code you cannot change, you run the risk that a change in the implementation of the type will cause your extension method to break.

If you do implement extension methods for a given type, remember the following two points:

·      An extension method will never be called if it has the same signature as a method defined in the type.

·      Extension methods are brought into scope at the namespace level. For example, if you have multiple static classes that contain extension methods in a single namespace named Extensions, they will all be brought into scope by the using Extensions; directive.