[G_Morgan]

It's similar to WX yes but suffers from the same problem as both WX and AWT of subtle differences between different back ends. Definitely write once, debug everywhere. I think it has the highest quality of the multiple back end tool kits though. Mainly because it doesn't sacrifice functionality and falls back to Swing if it cannot implement a particular function in a tool kit.

I'm not sure how many back ends it has altogether. I've seen Win32, GTK+, Cocoa and Swing backends (yeap, SWT can run purely via Swing). There are probably a good deal more. I personally still use Swing even though it sucks. I just separate the logic into a controller class, create an interface for the GUI class (so the controller class has a common API to work to) and then you can move from tool kit to tool kit with impunity. I tend to write a GTK interface then and suffer Swing on Windows.

Back on C# v Java. If you really want some of the functional aspects of C# try out Scala. Scala has all the functional aspects of C#* and has a type inference system that means you need only provide types for a few variables (you can of course type every variable if you are one of those people) It can use Java objects with impunity and with some care you can go the other way and use Scala objects in Java.

*and more. The only C#ism it's missing is LINQ.