C# Unleashed

Abstraction

This is a good time to make a point about abstraction. Some objects are pure abstractions and others are real things. For example, something called an animal does not exist. It's merely a description of a class of objects. Furthermore, there is no physical entity that is only a bird. These things are only classifications and there are no tangible instances of them.

On the other hand, there definitely is a thing called a duck. It has physical characteristics such as webbed feet and a bill. It looks like a duck, walks like a duck, and quacks like a duck. Therefore, it must be an instance of a duck.

These things cannot be said about “animal” and “bird,” which merely exist to add a useful classification mechanism to a hierarchy. Although ...