Desktop application for PC or MAC


I've had a quick look on the interweb and can't seem to easily find the answer to my question which is, what do I need to do differently to make a desktop application, WPF, function on both PC and MAC environments? I suppose I could buy a mac and try to run an app I've written but seems a bit expensive?

There are comments that WPF on mac is a no-go. I could convert to Silverlight with some effort but my question now is what would you develop a desktop app in for both PC and MAC, my guess is poor old WinForms? If so would the pitfalls simply be directory structure / accessing logged in user from WIN32 API commands, et cetera?

You can't run WPF on a mac.

The best you can get is Silverlight and its suitability depends on your needs.

Do you need OS level access?