Reference for extension method in C#

https://msdn.microsoft.com/en-IN/library/bb383977.aspx

Extension methods are defined as static methods but are called by using instance method syntax. Their first parameter specifies which type the method operates on, and the parameter is preceded by the this modifier. Extension methods are only in scope when you explicitly import the namespace into your source code with a using directive.

The following example shows an extension method defined for the System.String class. Note that it is defined inside a non-nested, non-generic static class:

C#

namespace ExtensionMethods
{
    public static class MyExtensions
    {
        public static int WordCount(this String str)
        {
            return str.Split(new char[] { ' ', '.', '?' }, 
                             StringSplitOptions.RemoveEmptyEntries).Length;
        }
    }   
}

The WordCount extension method can be brought into scope with this using directive:

using ExtensionMethods;

And it can be called from an application by using this syntax:

string s = "Hello Extension Methods";
int i = s.WordCount();