I don't mean your own code. I mean the .Net Framework's code.

You have been able to do that for quite some time but now it is even easier.

Just go to http://referencesource-beta.microsoft.com/ and have fun! Also check http://blogs.msdn.com/b/dotnet/archive/2014/02/24/a-new-look-for-net-reference-source.aspx for some information.