Protocol pattern in C#

optional_interface.cs

// http://www.knowing.net/index.php/2014/07/10/

interface IFoo
{
    //Methods defined here, as always, must be implemented
    void Necessary ();
}
 
static class IFoo_Extensions
{
    //"Optional" methods defined here with default implementations
    public static void Optional (this IFoo self)
    {
    }
}
 
class ImplementingClass : IFoo
{
    public void Necessary ()
    {
        Console.WriteLine ("Necessary");
    }
 
//    public void Optional()
//    {
//        Console.WriteLine("Overridden");
//    }
 
}