using System;

using FakeItEasy;

public class Program

{

    public static void Main()

    {

        MyI inter = new MyObject(){ Integer = 2 };

​

        inter.Sum();

        MyObject obj = new MyObject(){ Integer = 2 };

        obj.Sum();

        (obj as MyI).Sum();

    }

}

​

public static class extensions{

    public static void Sum(this MyI self)

    {

        self.Long = self.Integer * 2;

        Console.WriteLine("summing interface");

        if (self is MyObject){

            (self as MyObject).Sum();

        }

    }

    public static void Sum(this MyObject self)

    {

        self.NotFromInterface = self.Integer + 1;

        Console.WriteLine("summing myobject");

    }

}

​

public interface MyI{

    public int Integer {get;set;}

    public long Long {get;set;}

}

​

public class MyObject: MyI{

    public int Integer {get;set;}

    public long Long {get;set;}

    public long NotFromInterface {get;set;}

}