Abstract classes and interfaces allow for abstraction in object oriented programming. Abstract classes can contain both concrete and abstract methods, with the abstract methods requiring implementation in derived classes. Interfaces are fully abstract and can only contain abstract methods and properties, requiring any class implementing an interface to provide an implementation of its methods. Both abstract classes and interfaces enable security through abstraction and multiple inheritance by allowing classes to share common methods while still providing individual implementations.

1. Object Oriented
Programming
Lecture#9: Abstract Classes and Interfaces

2. Abstraction
Data Abstraction is the process of hiding certain details and showing only
essential information to the user.
Means displaying what is necessary and encapsulating other details
Abstraction means hiding the implementation details and only showing the
functionality

3. Abstract Classes
C# allows designing a class that can only be implemented to be used. Such
classes are incomplete and thus cannot be instantiated.
Abstract class can have method that are common for all derived classes
(concrete methods)
And it can have methods that are specific for different classes
Such methods are referred to as abstract methods

4. Interfaces
An interfaces is completely an abstract class, which can only contain abstract
methods and properties
To access the interface methods, the interface must be "implemented" (kind of
like inherited) by another class.
Why And When To Use Interfaces?
1. To achieve security object (interface).
2. Multiple Inheritance