1:The Difference between Delegate  and function pointers
   delegate are object_oriented and type safe
   delegate can invoke static mothod and instance mothod,but function pointers only can invoke static mothod.

2: The difference between Delegate and interface
    Interface define a set method,but Delegate is smaller than Interface,bacause delegate are of finer
    granularity.
    that is say, if inherited from interface ,you have to implement all mothods of interface,but in face,
    you hope implement a mothod of interface.

3: the difference between event and delegate
    event is the sended message of object