A LINQy TicTacToe implementation in C# for this question on StackOverflow.
Its model is separated from presentation so the OP can write real GUI on top of it.
Because most of TicTacToe code is functional, it would be a fun excercize to make TicTacToe class immutable, so each MakeMove “forks” it. I'm sure there are plenty tutorials that do this already.
