C # Extension methods

The Using system;using system.collections.generic;using system.linq;using system.text;/* * extension method allows you to write and declare methods that are associated with classes outside of its class  * 1, the extension method must be declared as Static * 2, the class where the extension method declaration resides must also be declared as static * 3, the extension method must contain the keyword this as his first argument type, and followed by the name of the class it extends to */ Namespace extendmethod{    Sealed class mydata    {        Private double D1, D2, d3;        public MyData (double d1, double D2, double D3)         {    &N Bsp       D1 = D1; D2 = D2; D3 = d3;       }        public double Sum ()         {  &N Bsp         return D1 + D2 + d3;       }   }    Static class Ext endmydata    {        public static double Average (this MyData MD)//Extension class MyData method   &NBSP ;     {             return MD. Sum ()/3;       }&NBSp  }    Static class extendmydata1    {        public static double Average (M Ydata MD)//non-extended class MyData method         {            return MD. Sum ()/3;       }   }    class program    {      &NBSP ; static void Main (string[] args)         {            MyData MD = new Myda Ta (3, 4, 5);            Console.WriteLine ("Sum: {0}", Md. Sum ());            Console.WriteLine ("extension method Md. Avrage: {0} ", Md. Average ());            Console.WriteLine ("Non-extended method extendmydata1.average:{0}", Extendmydata1.average (MD));            Console.readkey ();       }& nbsp  }}

C # Extension methods