As someone working on a compiler that turns a C# library into Java and JavaScript ... for us a significant benefit is that we can share code between our differe

As someone working on a compiler that turns a C# library into Java and JavaScript ... for us a significant benefit is that we can share code between our different products. We write the code in C# and about 95 % of the library code is the same for every platform, so implementing a feature for one platform automatically benefits all of them (which have been at one point: Java Swing, JavaFX, HTML5/JavaScript, .NET/WPF, .NET/WinForms, .NET/Silverlight, and Flash). In very many cases all that's needed is a simple merge and converting the code again. Yes, languages differ and language features rarely map 1:1 onto each other; that's about the other 5 % (as well as platform differences). But a lot of things can be done automatically if you control the conversion process, e.g. converting C#'s extension methods into default methods in Java. Of course, if you don't require the resulting converted code to be worked with, and just need it to run, then you can do a lot of shortcuts. We translate at the source code level because at least the Java code is still maintained and extended on the Java side. Doing it at the IL level would be simpler in some cases, but is impractical for a lot of others.