Well, you cannot have 2 List<T>s, right? Anyway, that's what I read once one some blog, a long time ago.

If you ask me, this "exposes implementation detail" is a bit pompous. They also changed Hashtable to Dictionary but it's not like you can hide the fact that the Dictionary uses a hashtable internally. You have to document the fact that it requires GetHashCode to work properly, so much for hiding implementation details...