How is it possible to have an interface implemented as a protected method?

Question

I just stumbled across IController and noticed that it has a method Execute . My question is that given that Controller derives from ControllerBase which implements the interface IController , how is it that ControllerBase can implement Execute as protected virtual ?

My understanding is that an interface must be implemented as a public method. My understanding of this is further complicated as you can't call Execute on an instantiated Controller and you must instead cast it to an instance of IController .

How is it possible to have an interface implemented as a protected method?

To add a bit more, I know about explicit interface implementation, however if you view the source code for ControllerBase at you will see that the method is implemented as protected virtual void Execute(RequestContext requestContext)

Answer 1

It's called explicit Interface Implementation.

A class that implements an interface can explicitly implement a member of that interface. When a member is explicitly implemented, it cannot be accessed through a class instance, but only through an instance of the interface .

Read more on MSDN: Explicit Interface Implementation Tutorial .

Simple sample:

interface IControl
{
    void Paint();
}

public class SampleClass : IControl
{
    void IControl.Paint()
    {
        System.Console.WriteLine("IControl.Paint");
    }

    protected void Paint()
    {
        // you can declare that one, because IControl.Paint is already fulfilled.
    }
}

And usage:

var instance = new SampleClass();

// can't do that:
// instance.Paint();

// but can do that:
((IControl)instance).Paint();