We often get asked about running Drools and jBPM on .net.
Today FrankVhh pointed out, in this post, that there has been some success using just pure IKVM. Frank pointed to the previous post by Corneil du Plessis, which can be found here:
http://drools-java-rules-engine.46999.n3.nabble.com/Drools-Java-and-Net-td2290536.html#a2414244
He also pointed out success with the jni4net project, which now comes with a Drools sample, to help you get started.
http://jni4net.sourceforge.net/