Question

What are extension methods in .NET?

EDIT: I have posted a follow up question at Usage of Extension Methods

Answer 1

Extension methods allow developers to add new methods to the public contract of an existing CLR type, without having to sub-class it or recompile the original type.

Extension Methods help blend the flexibility of ‘duck typing’ support popular within dynamic languages today with the performance and compile-time validation of strongly-typed languages.

Reference: http://weblogs.asp.net/scottgu/archive/2007/03/13/new-orcas-language-feature-extension-methods.aspx

Here is a sample of an Extension Method (notice the this keyword infront of the first parameter):

public static bool IsValidEmailAddress(this string s) {     Regex regex = new Regex(@'^[\w-\.]+@([\w-]+\.)+[\w-]{2,4}$');     return regex.IsMatch(s); } 

Now, the above method can be called directly from any string, like such:

bool isValid = '[e-mail]'.IsValidEmailAddress(); 

The added methods will then also appear in IntelliSense:

_{(source: scottgu.com)}

As regards a practical use for Extension Methods, you might add new methods to a class without deriving a new class.

Take a look at the following example:

public class Extended {     public int Sum() {         return 7+3+2;     } }  public static class Extending {     public static float Average(this Extended extnd) {         return extnd.Sum() / 3;     } } 

As you see, the class Extending is adding a method named average to class Extended. To get the average, you call average method, as it belongs to extended class:

Extended ex = new Extended();  Console.WriteLine(ex.average()); 

Reference: http://aspguy.wordpress.com/2008/07/03/a-practical-use-of-serialization-and-extension-methods-in-c-30/