To maintain only one login per user at a time in asp c#

The interviewer did not accept that answer? Sounds like you don't want that job. You are correct, mostly.

It's hard to know when a user "logs" out of the application unless they specifically click a Logout link because they could close their browser and you wouldn't know it. Their computer could crash and you wouldn't know it. They could lose power, etc, etc. So, the only thing I would clarify to your answer is to just check each time they login if they are already logged in under a different session. You'd want to store a timestamp as well.

However, it's still tricky. I don't know if IE still has this issue but when IE introduced tabs sometimes opening a link from my app in a different tab would kick off a new session and sometimes it wouldn't. So, it's still the same login per user but could potentially kick off another session.

With my bank, it used to be that if you did not click logout but just closed your browser and then came back to try and login before your session expired they displayed a message that you had to wait another 15 minutes at least since they only allowed 1 login at a time and this looked like more than one.

Bottom line, this is the web, unless you have a very good reason to do so, don't limit users to one session at a time. It's bad form.