You can find .NET source code just like you Android source code with super easy to use website. It took me less than few seconds to find the source code for String.Join method. You could do this for last several years but it was never easy as this. Latest version was not updated, Stepping through them with Visual Studio was that easy.
If you ever get curious about how a particular method is implemented in .NET framework, now you can find out. Hopefully Microsoft keeps this site updated, and it seems it is indexed with Roslyn.
Here is link for beta site.
Via [ Scott Hanselman ]