There is a very new feature in Visual Studio 2017 version 15.6
You can see decompiled source code without any plugin! Btw, Ref12 doesn’t work for VS2017.
Tools > Options , expand Text Editor > C# > Advanced, and enable “Enable navigation to decompiled sources (experimental)“.
Ref: https://learn.microsoft.com/en-us/visualstudio/ide/go-to-and-peek-definition?view=vs-2017#view-decompiled-source-definitions-instead-of-metadata-c