Inconsistent visibility of protected interface members

[zvrba]

Class implementing interface can access "inherited" protected static methods, but not protected instance methods.

Given the following:

public interface IFactAdapter {
    IReadOnlyList<Term> SchemaFields { get; }
    protected object[] CreateValues2() { ... }
    protected static object[] CreateValues(IReadOnlyList<Term> schemaFields) { ... }
}

class FactAdapter : IFactAdapter
{
    public FactAdapter() {
        var values1 = IFactAdapter.CreateValues(...);  // OK
        var values2 = ((IFactAdapter)this).CreateValues2(); // CS1540 ERROR:
    }
}

The full error is CS1540: Cannot access protected member IFactAdapter.CreateValues2() via a qualifier of type IFactAdapter; the qualifier must be of type DefaultFactAdapter<T> (or derived from it).

This is blatantly inconsistent. I know that a class technically does not "inherit" an interface, but then why is it allowed to access protected static interface members? Is it possible to fix this so that also inherited protected members are accessible?

[HaloFour]

You've never been able to cast this to another type in order to invoke protected members. I think this is one of those scenarios that fell under the base(T) feature, no?

#2337

var values2 = base(IFactAdapter).CreateValues2();

[zvrba]

You're absolutely correct in all of your points.

However, base(T) feature has been "in the works" for quite a while now, we don't know when it's going to be ready, and at the same time there's this blatant inconsistency wrt protected members on interfaces. The compiler could relax its rules about casts and protected access for just the scenario in question. (While we're waiting for base(T)... which is soon 5 and a half years old, so it's like waiting for Godot.)

[HaloFour]

I think the problem is that the runtime requires changes in order to support such an invocation, regardless of what syntax would be used. As soon as this is cast to anything you lose the ability to call members that require privileged accessibility. This is true with inheritance, too.