The bits of C# code that could be plausibly ported to Java (read: doesn't rely on a Windows-only library) are pretty darn complete outside of Windows. Mono is

The bits of C# code that could be plausibly ported to Java (read: doesn't rely on a Windows-only library) are pretty darn complete outside of Windows. Mono is really a very solid project. Winforms (through version 2.0) is supported by Mono. So in the case of that example, before taking on the immense hassle of trying to port my C#/Winforms app to Java/Swing, I'd take a good long hard look at just downgrading my C#/Winforms app from Winforms >2.0 back a version or two. For any non-trivial case you'd be looking at a fraction of the cost.