[Closed] Place to store user data

I have been working on a game using C#, and need a place to store player files. I want my game to be cross platform for at LEAST windows/mac/linux although I may want to port it to other places later. However, I am not sure where to store my save data. I wish to store it by user. If you know how to obtain a directory for any of the 3 operating systems (and how to determine if it’s that operating system) and/or have one to suggest please reply.

Thanks.

There are more ways to do this but from what i hear XML is one very good choice.