openEHR.NET
The first release of openEHR.NET is available at http://openehr.codeplex.com/. openEHR.NET is a C# implementation of the openEHR Reference Model (RM) and Archetype Model (AM) specifications (Release 1.0.1), allowing developers to build openEHR applications by composing RM objects, validate against AM objects and serialise to/from XML. The aim of this release is to help all .Net development share the same core classes, and of course collaborate on a common high-quality library rather than having separate developmets.
openEHR.Net is already in use on a demonstration project called Gastros (http://gastros.codeplex.com/) led by Koray Atalag at Auckland University.
If you are interested in open source openEHR development on .Net platform, please visit the openEHR codeplex site and join the development group.