I am having to port code from Java to C# (and soon the other way round) by copying and pasting and then editing compiler errors. (Please accept that this is necessary; I can
IKVM is a great option for running java code in C#. You don't need to modify your java code. It simply takes a jar file and generates a .NET dll.
http://www.ikvm.net/
Going the other direction, from C# to java, will be difficult, as C# is basically a superset of java. You would need to limit the use of constructs which are not available in java.