Extension methods in C#

What is extension method?

Extension methods in C# are methods applied to some existing class and they look like regular instance methods. This way we can "extend" existing classes we cannot change. Perhaps the best example of extension methods are HtmlHelper extensions used in ASP.NET MVC.

Extension methods are static methods of static class and they use "this" keyword in argument list to specify the type they extend. The following demo shows how to build extension method that returns word count in string.