I just changed computer and installed VS2015.
My projects are running and no problem on VS2013 on my old computer, but opening the same project on VS2015, I have red underlining with message "The type Object is defined in an unreferenced assembly, you should add the reference to assembly "System.Runtime, Version=126.96.36.199, ..." "
The code is still compiling without any errors, but the message and the red underlining is still here.
My references are here, that's why it's compiling I guess, but my editor is filled with many "fake errors".
Did anyone face this issue ?