Well,I think it doesn't matter program the game use c# or c++. In my work experience, you should learn something about network,GDI drawer, threads and so on .Actually , the responsibility of client is draw something and send your information to server, when server get the message from client ,it will decide what to do and send a reply message to client.The client do something against the message from server.
And the simplest method is search on GitHub,there are many open source games.