R# is written in C# while their IDEs is in Java. Most likely they "convert" some of the C# code to Java. It'll be ugly...
"instead of reimplementing ReSharper’s features on the IntellIJ Platform, which runs on the JVM, we’re using ReSharper in a headless mode, out of process, and communicating with it via a very fast custom binary protocol. "
R# is written in C# while their IDEs is in Java. Most likely they "convert" some of the C# code to Java. It'll be ugly...