On some it is an emergency - you dont want the door to unlatch on an Aztec for example.
Having owned an Aztec I am curious as to how this is an emergency? Certainly is not considered one in the POH.
I have had doors open on a PA28 (my fault) a B58 (copilot fault) & on one very memorable occasion a pax opened the back door on a C404 at 10,000'
In all cases, continue to fly the aeroplane and make a determination of whether & where you want to land. I know of two fatal prangs caused by distraction due to open doors, neither of which the door had any physical effect on the flight.