As far as I know, RM should mock anything that can be accessed by a derived class. Since a protected virtual member is accessible from a subclass, shouldn't it be mockable?
My understanding is that RM does not support mocking protected virtual members.
As far as I know, RM should mock anything that can be accessed by a derived class.
No. The idea is that you should be able to mock the behavior of dependencies of the class under test. The word dependencies is used here in the same sense as in dependency injection.
In test driven development, one typically favors composition over inheritance. Dependencies are typically hidden behind interfaces.
My understanding is that RM does not support mocking protected virtual members.
That's correct. There are two concise and type-safe ways to specify which class member you're talking about when setting up behavior: by calling the member in a record mode, or with a lambda expression. Rhino Mocks supports both, but neither can work for protected members.
If I want to mock a protected member I usually skip over Rhino Mocks and just subclass it with my own fake implementation.