Browse dot net framework source code

Microsoft has launched a browsable version of the .Net reference source code at https://referencesource.microsoft.com/.

Also, there is an extension available at Visual Studio marketplace which lets you press F12 to open the source code for anything in the .Net framework in a browser
https://marketplace.visualstudio.com/items?itemName=SchabseSLaks.Ref12#overview
https://github.com/SLaks/Ref12