C# Language Extension methods in combination with interfaces


It is very convenient to use extension methods with interfaces as implementation can be stored outside of class and all it takes to add some functionality to class is to decorate class with interface.

public interface IInterface
   string Do()

public static class ExtensionMethods{
    public static string DoWith(this IInterface obj){
      //does something with IInterface instance

public class Classy : IInterface
   // this is a wrapper method; you could also call DoWith() on a Classy instance directly,
   // provided you import the namespace containing the extension method
   public Do(){
       return this.DoWith();

use like:

 var classy = new Classy();
 classy.Do(); // will call the extension
 classy.DoWith(); // Classy implements IInterface so it can also be called this way