(I asked this question in another way, and got some interesting responses but I'm not too convinced.)
Is Mono's GtkSharp truly cross-platform? It seems to be Gnome based... how can that work with PC and Mac?
Can someone give me examples of a working Mac/PC/Linux app that is written with a single codebase in Microsoft .Net?
Gtk# is cross platform. However the only platform where it looks nice is Linux/BSD running GNOME. If possible somehow, separate frontend and backend and develop separate user interfaces for Linux, Windows and OS X. Even wx, which does a really good job in looking okay on all three platforms, has its limits.
Working Mac/PC/Linux app in Gtk#? Tomboy runs on all three I think.
It would be more correct to say that GNOME is GTK-based than it is to say that GTK is GNOME based. GTK is a toolkit that GNOME sits on top of, and you can get GTK for several platforms, including Windows. That's how GIMP works on Windows: you install GTK first.
The best example of a Gtk# app that runs on both Windows and Linux may be Medsphere's OpenVista. Granted, its not an app that many people need to run, but it is a very professional, polished, open-source Gtk# application. It shows how a professional Gtk# app can be written.
There are no GTK# apps that actually do anything that run on Windows, Linux and OSX. Not one. If you decide that I'm not right, just provide one single example, please.
Plastic SCM is supported on Windows, Linux, Solaris, and Mac OS X. The link includes screenshots on Windows and Linux.