c# - Using an extension method declared and implemented in interface -

i try understand extension method concept in c#, added interface :

 public interface iextension {    public static int multiple(this int a)    {        return * 2;    } } 

in main method

 public static void main()    {        console.writeline("entrer un numero");        int =int.parse( console.readline());        a.multiple();        console.readkey();    } 

i error in a.multiple(); indicates method not recognized.

even added static class implementing interface, same error.

how can fix problem?

extension methods must in static class, not in interface

public static class myextension {     public static int multiple(this int a)     {         return * 2;     } } 

and should print result:

console.writeline(a.multiple()); 

note extension method can't change value of a (because int value type), can return new value!.

check adding

console.writeline(a.multiple()); console.writeline(a); 

just curiousity, in java 8 oracle added default methods interfaces: https://docs.oracle.com/javase/tutorial/java/iandi/defaultmethods.html little c# extension methods, defined inside interface. types implement interface can override them, otherwise "inherit" these default methods.