ASP.NET Core Debugging MVC Source
ASP.NET Core makes it very easy to debug the MVC source code in Visual Studio. In the current version you just need to clone the github repository or download the mvc source. Then create a solution folder called global and add a global.json file. Inside this file add, { "sources": ["C:\\YourMVCSourcePath"] }. That's it. You can also watch this small clip.
Update: You can also check the official blog at here.
For further detail you can watch these videos,