Had a discussion yesterday about Java & .Net integration. Of course, the usual suspects came up; JNI, J-Integra , JNBridge, JuggerNET, OOJNI etc. However if you do not only need to convert the objects, but also need to have the objects cached for fast transparent access from both Java and .Net, using Coherence is definitely an option.
Coherence provides transparent conversion to and from Java and .Net data types, including custom application user types. This enables .Net applications to access cached Java objects as native .Net objects and Java applications, including data grid members and Java clients, to access cached .Net objects as native Java objects.
Here you can find more details about Coherence for .Net. It is available for download on OTN. The download includes a .Net demo. To run this demo you either need Visual Studio, or you can use SharpDevelop (an open source IDE for the .Net platform).