what is extender class?

Hi,

First of all you have to know about Extension methods.

Extension methods enables you to add methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.

An extension method is a special kind of static method, but they are called as if they were instance methods on the extended type.

Now take a look at below articles to get an idea about how Extension classes are created-

https://www.dotnetperls.com/extension
http://csharp.net-tutorials.com/csharp-3.0/extension-methods/
http://www.c-sharpcorner.com/uploadfile/puranindia/extension-methods-in-c-sharp-3-0/