Debugging .NET Framework sources was broken for a long time. See the relevant suggestion on VisualStudio UserVoice Allow developers to step through .NET Framework sources. Good news: it was finally been fixed! See the following blog post from Microsoft:
A new look for .NET Reference Source
In short:
- New web interface for browsing sources: http://referencesource-beta.microsoft.com/
- All sources, including project files, can be downloaded directly: http://referencesource-beta.microsoft.com/download.html (not compilable though)
- There’s a new extension for F12 navigation: http://visualstudiogallery.msdn.microsoft.com/f89b27c5-7d7b-4059-adde-7ccc709fa86e
- Sources are always up to date and debugging isn’t broken by patches and updates.
It’s currently “beta”. Eventually it’s going to replace http://referencesource.microsoft.com/. Microsoft also promises to add sources of more assemblies.