Reflection explained here
http://msdn.microsoft.com/en-us/library/cxz4wk15(v=vs.95).aspx[
^] can be used for this purpose.
This Code Project article
100% Reflective Class Diagram Creation Tool[
^] by
Sacha Barber is a very good article to obtain class diagram by reflection.
The Reflector by Redgate,
http://reflector.red-gate.com/Download.aspx[
^], is a also very good tool, which was free earlier, has been made
commercial now.
I think the above references may be useful.