Debug .NET Framework Source Code in Visual Studio 2012?

This feature was broken in VS2010 SP1, it might be broken in VS2012 as well. Or Microsoft might release incorrect PDB files for the latest .NET Framework version (this had happened before). Try alternative solutions. I'm using .NET Reflector VSPro - it allows to decompile assemblies, load decompiled symbols during debugging and use full debugging features for these decompiled assemblies.

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.

In VS 2008 I was able to debug .Net source, but in VS2012 I've got .NET Reflector (VS extension) installed, so it automatically decompiles and allows to debug any .Net code.

Another open source tool is ilspy.net , it saves the decompiled assembly as C# project and you can use the debugging tools.

There are also debugging tools for Windows, that come as part of SDK, but i haven't tried those: http://msdn.microsoft.com/en-us/windows/hardware/gg463009.aspx