What is Extension methods and how it works ?

Extension method is new in c# 3.0 which allows creating new methods for the existing classes without breaking the current class.

Exp:
Below method will be listing in String class , to get count of words in that particular string.

public static int NoOfWrods(this string instance)
{
    return instance.Split(' ',',','.',';').Length;
}

Note : this method will be listing in String class
**** extension methods should be static methods. The first parameter of the method should be the reference of the class we wish to extend refered by this operator.