In very short:
Read Solution #1 from CPallini. He is explaining why the situation you described is not multiple inheritance.
To give you a basic idea what multiple inheritance is:
You have e.g. two classes
class A
class B
and from this you do (not possible in c#)
class AB: A, B
So class AB inherits from two independent classes.
Very short, but you will find a lot about this with Google.