I would be certain that this question addresses something that would have been brought up in a previous question, but I was unable to find it.
There is a method in a
Here is an answer that will exclude any objects in your list of the wrong type. This is a much safer way in my opinion:
List ListOfA = ListOfB.OfType().ToList();
The OfType method will exclude items of the wrong derived class where as the Cast will throw an error.