How can an interface implementation be protected?

Question

I just came across one thing in the .NET framework (v4.0) that I cannot understand:

There is the class SortDescriptionCollection

namespace System.ComponentModel
{
    public class SortDescriptionCollection : Collection<SortDescription>, INotifyCollectionChanged
    {
        ....
        protected event NotifyCollectionChangedEventHandler CollectionChanged;
        ....
    }
}

implementing the Interface INotifyCollectionChanged:

public interface INotifyCollectionChanged
{
    event NotifyCollectionChangedEventHandler CollectionChanged;
}

I wanted to use the event on the class but I cannot because it's protected.

That's not the big problem because I can cast the implementation to the Interface and then use it.

But how can that be built? If I try to do with

class MyDerivedType : INotifyCollectionChanged
{
    protected event NotifyCollectionChangedEventHandler CollectionChanged;
}

so the compiler says:

'MyDerivedType' does not implement interface member 'System.Collections.Specialized.INotifyCollectionChanged.CollectionChanged'. 'MyDerivedType' cannot implement an interface member because it is not public.

---

EDIT: I don't think it's a duplicate. I was not asking how to compile the code above, it was the question how the .NET framework seemed to could do that (and obviously it couldn't)

Answer 1

It's an explicitly implemented interface event:

event NotifyCollectionChangedEventHandler INotifyCollectionChanged.CollectionChanged
{
    add { CollectionChanged += value; }
    remove { CollectionChanged -= value; }
}

So it satisfies the requirements of INotifyCollectionChanged, but it isn't publicly visible on the class itself - perfectly valid.

This usually signalizes intent - this is not supposed to be a part of the public interface of the class. However, if you absolutely do want to access this from the outside, you can just do a cast to the interface:

((INotifyCollectionChanged)myCollection).CollectionChanged