When I started trying to write C# code, I spent most of my time with the Reflector trying to figure out what the .Net Framework was doing under the hood. My start routine was: start the debugger, see unexpected results, read source code in Reflector, repeat. I spent a lot of time searching for functions in the Reflector.
I spent years doing MFC programming. MFC shipped with complete source code from the very early days. Without that source code, it would have been almost impossible to solve many of the problems I ran into. (See my earlier blog about fixing problems with MFC symbols in Visual Studio 2005.)
I was tickled to discover today that Microsoft has made the source code of the Framework available for debugging purposes - even specialized code like Asp.Net. You must be running Visual Studio 2008. Take a look at Shawn Burke's Blog showing how to set it up.
Make sure you install the Visual Studio hotfix (QFE)!