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
I think the above references may be useful.