用于PC或MAC的桌面应用程序

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?I've had a quick look on the interweb and can't