Protected Classes in .NET

Yes, you just cannot make them top level classes, they must be inner classes

public class Outer
{
    protected class Foo
    {
    }
}

This is fine, it means that the only classes allowed to see Foo are sub classes of Outer

class X 
{
    // 'Outer.Foo' is inaccessible due to its protection level
    private void Flibble(Outer.Foo foo)
    {
    }
}

class X : Outer
{
    // fine
    private void Flibble(Outer.Foo foo)
    {
    }
}

Note that you cannot declare any outer class as private, protected (or protected internal) in c# since the access modifier for outer level classes defines their visibility in relation to other assemblies. Specifically visible within the assembly only (or to friends via InternalsVisibleTo) or outside the assembly.

Thus whilst the public/internal identifiers are used here for consistency really the state in IL is simply ‘Public’ or ‘NonPublic’ (as the Reflection.Emit flags show)