C # Learning abstract classes

Features:

1. abstract classes cannot be instantiated.

2. abstract classes can contain abstract members, but not abstract classes.

3. the abstract class also has constructor, because the abstract class needs to be inherited by other classes, and its subclass needs to be instantiated. when instantiating the subclass, it needs to call the constructor of the subclass, by default, the base class constructor must be called before the constructor of the subclass is called.

4. abstract classes cannot be sealed.

5. abstract methods in abstract classes are not implemented.

Purpose:

1. Provide public definitions of shared base classes for multiple Derived classes