Inheritance in C# - The Unified Modeling Language
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The Unified Modeling Language (UML) is a standardized language for describing an object-oriented system. The UML has many different visual representations, but in this case, all you need to know is that classes are represented as boxes. The name of the class appears at the top of the box, and (optionally) methods and members can be listed in the sections within the box.
In the UML, you model specialization relationships, as shown in Figure 11-1. Note that the arrow points from the more specialized class up to the more general class. In the figure, the more specializedButton andListBoxclasses point up to the more general Window class.
Figure 11-1. An is-a relationship
It is not uncommon for two classes to share functionality. When this occurs, you can factor out these commonalities into a shared base class, which is more general than the specialized classes. This provides you with greater reuse of common code and gives you code that is easier to maintain, because the changes are located in a single class rather than scattered among numerous classes.
For example, suppose you started out creating a series of objects, as illustrated in Figure 11-2. After working with RadioButtons, CheckBoxes, and Command buttons for a while, you realize that they share certain characteristics and behaviors that are more specialized than Window, but more general than any of the three. You might factor these common traits and behaviors into a common base class,Button, and rearrange your inheritance hierarchy, as shown in Figure 11-3. This is an example of how generalization is used in object-oriented development.
Figure 11-2. Objects deriving from Window
Figure 11-3. Factoring a Button class
The UML diagram in Figure 11-3 depicts the relationship among the factored classes and shows that bothListBoxandButton derive from Window, and thatButtonis specialized into CheckBox and Command. Finally, RadioButton derives from Check-Box. You can thus say that RadioButton is a CheckBox, which in turn is aButton, and thatButtons are Windows.
This is not the only, or even necessarily the best, organization for these objects, but it is a reasonable starting point for understanding how these types (classes) relate to one another.
Actually, although this discussion might reflect how some widget hierarchies are organized, I am very skeptical of any system in which the model does not reflect how I perceive reality, and when I find myself saying that a RadioButton is a CheckBox, I have to think long and hard about whether that makes sense. I suppose a RadioButton is a kind of CheckBox. It is a checkbox that supports the idiom of mutually exclusive choices. That said, it is a bit of a stretch and might be a sign of a shaky design.
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 This article is excerpted from chapter 11 of Learning C# 2005, Second Edition, written by Jesse Liberty and Brian MacDonald (O'Reilly, 2006; ISBN: 0596102097). Check it out today at your favorite bookstore. Buy this book now.
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