I need to re-write an old application, that is currently Windows only, to OS X plus do a few updates on the Windows version.
I was thinking of just doing a full rewrite and using a framework to keep most of the code base the same for the OS X and Windows version. I'm not sure how easy this is. I've checked QT, Xamarin and Monodevelop. I'd prefer to do it in C#, so my preference would be either Xamarin or Monodevelop but I'm not sure how much of the code-base the app can share between the 2 OSes, especially the GUI part.
Which of the two (three) platforms can you recommend if I want to rewrite the Windows code as little as possible?