USB + Power Jack at the same time...Is it possible?

Is it okay to connect an arduino uno r3 to a pc via the usb jack while it is connected to a power source (i.e. 9 volt battery) via the 2.1mm power jack?

What happens when i do so?(circuit wise)
Will the arduino get damaged/fried?
Will the pc get damaged/fried?

Thanks in advance
Merry Christmas and happy holidays everyone :slight_smile:

This thread has a discussion on that question: External Power and USB connected//Conflicts