From a quick Google (which you should have tried first),
these guys[
^] offer a paid-for solution, though I don't know how much it is.
There is no simple way, as the two VMs are completely incompatible. You need to write communication/marshalling code on both sides, or use a tool which writes it for you (for example web services).
If you are in control of the source code on both sides it may be best to rewrite the Java part in a .Net language, if .Net is your target final platform. That depends on how deeply integrated the code is with Java libraries etc that there is no direct equivalent for in the Framework.