Apple Platforms
The Mono runtime has been ported to iOS, tvOS and watchOS.
The Xamarin Platform is the edition of Mono for iOS, tvOS and watchOS that provides access to all of the native iOS APIs. It is available both as part of Microsoft Visual Studio, or as an open source project - coming soon to Evolve 2016.
Please visit the Xamarin site for more information on Mono/.NET on iOS, tvOS and watchOS.