Take a look at Harmony 2007 (www.domialifestyle.com) - it's a home automation platform, with support for several hardware options (X10, BBS, CBus, Dupline, Rako etc).

Harmony 2007 is a complete rewrite of the former Harmony 5.x range - written in VB.Net and C#

It comes with a DotNet API that enables full control over the "core" messaging system.

There's an active community on the forum (see support section of domialifestyle).

*disclosure* I wrote most of the components, and I'm now the manager of the Harmony / Domialifestyle Business).

Hope this is of interest,

Rob Iles

P.S. feel free to contact me directly if you feel it's appropriate.