There: C# SocketAsyncEventArgs High Performance Socket Code; based on things I have learnt from this (and some other resources) I have written a high performance TCP server which is handling more than 7000 clients.
Edit: Other good .NET code bases I studied to some extend are fracture (F#), SocketAwaitable and SuperSocket. I especially like fracture because of it's simple (not naive) and smart buffer pool handling but (as the version I've worked with) it does not provide a separate pool for acceptors; which I've done myself easily based on the already provided pool.