You can build C# OSX apps with native UI if you use Mono with Monobjc: http://www.monobjc.net/ http://www.monobjc.net/ MonoTouch for iOS works similarly, and

You can build C# OSX apps with native UI if you use Mono with Monobjc: http://www.monobjc.net/ http://www.monobjc.net/ MonoTouch for iOS works similarly, and bridges into native Objective C UI libraries (and you continue to use Interface Builder). Last time I tried it, it had the downsides of long startup time (2-3 seconds) and big runtime. However, if you're writing a big app, let's say 5MB, then going from 5MB to 7MB is probably ok. I think MonoTouch for Android also uses native UI, but I have not tried it yet.