You can browse through the .NET Framework source code online at http://referencesource.microsoft.com/.
You can also download the source code as a solution and browse through it in Visual Studio. This gives you the ability to debug into .NET Framework code from your project code. This 2 minutes video present the high level idea of how to achieve that.
You may optionally choose to view a rather detailed video on the subject by Alok Shriram of the .NET Framework team and Kirill Osenkov of the Roslyn team from Channel 9 here, https://channel9.msdn.com/Shows/Visual-Studio-Toolbox/Browse-the-NET-Reference-Source