Extension method in C#

Extension methods are used in C# to add extra methods to classes which you haven't coded yourself. There would be litle point in adding them to classes which you have coded yourself as you could just include those methods in the class directly.

They are also frequently used to add methods to all classes which implement a particular interface.

A good example of the latter is the extension methods in the System.Linq.Enumerable class which extend all classes which implement IEnumerable and form the basis of 'LINQ to object' queries:

http://msdn.microsoft.com/en-us/library/system.linq.enumerable.aspx

You can also use extension methods to add methods to enums, which is something which you cannot do directly. Check out my article on the subject:

http://www.c-sharpcorner.com/uploadfile/b942f9/how-to-add-methods-to-an-enum-in-C-Sharp/