Section 3.2. Classes and Objects

3.2 Classes and Objects

The most important metaphors in object-oriented programming are the class and the object .

A class defines a new type of thing . The class defines the common characteristics of every object of that new type. For example, you might define a class Car. Every car will share certain characteristics (wheels, brake, accelerator, and so forth). Your car and my car both belong to the class of Cars; they are of type Car.

An object is an individual instance of a class. Each individual car (your particular car, my particular car) is an instance of the class Car, and thus is an object. An object is just a thing.

We perceive the world to be composed of things . Look at your computer. You do not see various bits of plastic and glass amorphously merging with the surrounding environment. You naturally and inevitably see distinct things: a computer, a keyboard, a monitor, speakers , pens, paper. Things.

More importantly, even before you decide to do it, you've categorized these things. You immediately classify the computer on your desk as a specific instance of a type of thing: this computer is one of the type computer. This pen is an instance of a more general type of thing, pens. It is so natural you can't avoid it, and yet the process is so subtle it's difficult to articulate . When I see my dog Milo, I can't help also seeing him as a dog , not just as an individual entity. Milo is an instance, Dog is a class.

The theory behind object-oriented programming is that for computer programs to accurately model the world, the programs should reflect this human tendency to think about individual things and types of things. In C# you do that by creating a class to define a type and creating an object to model a thing.