Interview Question in C#

Question

A Technical Lead asked me the following:
He created a class, declared an object and initialized it. But in some circumstance we may get "null reference" exception.
He commented that there are 1000 possible reasons for such exception and asked me to guess a single reason.
I am unable to figure it out. What is (are) the reason(s) ,we may get such an exception?

Answer 1

1. You have used an object reference you have explicitly set to null, or
2. You have used an object reference you have implicitly set to null, or
3. Somewhere in your code, or in code called by you, there is the statement throw new NullReferenceException() (which you shouldn't do, by the way). I don't know if this counts, since it's not a real null reference.

I can't think of any of the other 997 reasons.

Edit: Thanks, Mark Byers, for point 3.

Answer 2

If it's a multi-threaded app, then some other thread could come along and set the object to a null reference.