Re: Um, what?
.NET Core is .NET from now on. You run .NET core and install any other bits you need via nuget.
The 4.x framework isn't fully open source yet - and it may never be fully - but that's not really the point. Everything you need to develop new code is open and you shouldn't be using 4.x (after VS2015 is fully released) for anything except supporting old code.
Biting the hand that feeds IT
