This podcast interview explains the compiler platform RemObjects built and customized for various popular languages: C# and Swift. Not only it naively supports iOS, Mac, Windows, Android, but it also supports multiple language platforms: super-set of C#/.NET and super-set of Swift.
One can program in C#, or Swift, or Pascal-like Oxygene and and call natively Java libraries on Android and objective-C libraries on iOS or Windows libraries (trough .NET).
It even integrates with Visual Studio.
RemObjects Silver | Elements
RemObjects Elements: Oxygene and C# | Elements
FAQ | RemObjects C# | Elements
"Q: What is the difference between RemObjects C# and Xamarin?
A: Xamarin is a commercial toolchain that brings the .NET Framework to Android and iOS. RemObjects C# looks at this idea of reusing C# on a different level: It brings the C# Language to the Cocoa and Java/Dalvic platforms, instead.
What does that mean, exactly?
Xamarin always compiles your C# code to .NET (or Mono – same difference) IL code to run in the Mono runtime. It uses the .NET class libraries.
At first glance that might seem (and sometimes truly is) a benefit: it lets you work with the .NET classes you are already familiar with. But this approach also has a lot of downsides: Xamarin apps always require the large(ish) Mono runtime to be bundled and run with your applications. This makes your applications bigger and, oftentimes, slower. What's more, because you are building against the .NET class libraries, the real platform - be it Java/Android or Cocoa - is always kept at arms length, and your access to platform features always goes through an intermediate layer.
The RemObjects C# compiler builds directly against the native platforms - such as the Java and Android class libraries provided by Google, or the Cocoa frameworks provided by Apple. This brings you "closer to the metal" and lets you create apps using the same APIs that all other Android or iOS developers use. "
Silver Swift compilers are free (optional donation),
and C# and Oxygene are $500 each per developer, 1/2 price of Xamarin.
Swift - Overview - Apple Developer