GTK # may be an option. I do not agree that this is convenient, but it is a matter of taste.
In my humble opinion, you should use different GUI tools for each platform (this is usually a topic of holy wars, but I think if your budget is enough for this, you really should think about it). I can name only a few completely cross-platform applications that we wrote using only one GUI toolkit, which is really completely suitable for all systems.
n535
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