Introduction - If you have any usage issues, please Google them yourself
(1) create a set of related classes: the second class of response to first class event, the third class and second class response to the event. When the first class of the object specified event occurs, the object will trigger a series of chain reactions. (2) to create a student class and its derived classes Student Undergraduate and Graduate, and in the Windows form to display different types of student information. (3) analog bank card payment system, define the payment interface IPayable, implemented in the base class BankCard basic payment functions in the derived class override base class methods to achieve polymorphism payment functions.