Browsing the .NET Reference Source
You can browse through the .NET Framework source code online at https://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.
[View:https://blogs.msdn.com/cfs-file.ashx/__key/communityserver-blogs-components-weblogfiles/00-00-01-48-94/5658.HowToUseTheNetSourceBrowser_5F00_mid_5F00_xvid.mp4]
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