On a language level, not that hard (though the result wouldn't be idiomatic in a lot of places). But on a library and API level ... Either you move everything t

On a language level, not that hard (though the result wouldn't be idiomatic in a lot of places). But on a library and API level ... Either you move everything that uses the stdlib to your own standard library excerpt that you maintain for both languages to be identical. Or you find compatible implementations in the other language (or translate Apache Harmony to C# as well). Or you'll deal with all the things that your program could call and emit code that does the same thing in the target language, with more code where standard library functionality differs slightly. And then there is the problem of external libraries and their APIs. Either translate them as well, or find API-compatible replacements, or find replacements and handle the API differences yourself. Actually, it comes down to the same thing either way, when considering the standard library just as a library, among the others.