Visual Basic Books

Inheritance

Inheritance is the ability, offered by many OOP languages, to derive a new class (the derived or inherited class) from another class (the base class). The derived class automatically inherits the properties and methods of the base class. For example, you could define a generic Shape class with properties such as Color and Position and then use it as a base for more specific classes (for example, Rectangle, Circle, and so on) that inherit all those generic properties. You could then add specific members, such as Width and Height for the Rectangle class and Radius for the Circle class. It’s interesting to note that, while polymorphism tends to reduce the amount of code necessary to use the class, inheritance reduces the code inside the class itself and therefore simplifies the job of the class creator. Unfortunately, Visual Basic doesn’t support inheritance, at least not in its more mature form of implementation inheritance. In the next chapter, I show how you can simulate inheritance by manually writing code and explain when and why this can be useful.

Creating a class in Visual Basic is straightforward: just issue an Add Class Module command from the Project menu. A new code editor window appears on an empty listing. By default, the first class module is named Class1, so the very first thing you should do is change this into a more appropriate name. In this first example, I show how to encapsulate personal data related to a person, so I’m naming this first class CPerson.

The first version of our class includes only a few properties. These properties are exposed as Public members of the class module itself, as you can see in this code and also in Figure 6-1 on the following page: