Question

I am newbie to C#, and have a problem with understanding how should I implement an extension method of interface. I didn’t found any material about this issue. From the material I found about general extension methods in C# I expected the following simple demonstrating example work:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace testExtension
{
public  interface  IcheckThingsOut
{
   bool x();
}
public static class extensionInterface
{
   public static bool y(this IcheckThingsOut bla) { return true;}
}
public class implementInteface : IcheckThingsOut
{
   bool IcheckThingsOut.x()
   {
       return true;
   }
   bool IcheckThingsOut.y()
   {
       return false;

   }
}
}

But the compiler is still not satisfied. What did I miss, and what is the right syntax here (with the same semantics as the code)?

Answer 1

You have messed up the concepts of extension methods and interfaces. Extension methods are just a compiler convenient way of calling static methods on static classes. They do not extend the interface. What the compiler does when using an extensions method is:

  IcheckThingsOut instance = new IcheckThingsOutImpl();
  instance.y();

… will be converted to:

  IcheckThingsOut instance = new IcheckThingsOutImpl();
  extensionInterface(instance, y);

So as you can see, y is not a method on your interface. This is why you can’t implement it explicitly in the implementation.