I am using Visual Studio 2012. I want to Debug .NET Framework source code. I have tried nearly all the options but I am still getting There is source code available for Cu
I've had much luck in the past following the procedure as adviced by SymbolSource.
If you combine this with Resharper, you'll be able to CTRL-click your way through the framework.
There's a long history however of MS not publishing the code of all versions of their binaries. That's why often debugging won't work.
Still, Resharper seems a bit more flexible in downloading source code, and if it can't, it'll give you the disassembled source code, yet undebuggable. Note that JetBrains considers the debugging of decompiled sources a critical feature for dotPeek (the decompiler that comes with Re#), so it might be possible within a not too distant future.