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Note

Under the .NET platform,

System.Object

is always the topmost parent in any class hierarchy, which

defines some bare-bones functionality fully described in Chapter 6.

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D E F I N I N G   E N C A P S U L AT E D   C L A S S  T Y P E S

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Figure 5-8.

The “is-a” relationship

There is another form of code reuse in the world of OOP: the containment/delegation model

(also known as the 

“has-a” relationship or aggregation). This form of reuse is not used to establish

parent/child relationships. Rather, the “has-a” relationship allows one class to define a member

variable of another class and expose its functionality (if required) to the object user indirectly.

For example, assume you are again modeling an automobile. You might want to express the

idea that a car “has-a” radio. It would be illogical to attempt to derive the Car class from a Radio, or

vice versa (a Car “is-a” Radio? I think not!). Rather, you have two independent classes working

together, where the Car class creates and exposes the Radio’s functionality:

class Radio

{

public void Power(bool turnOn)

{

Console.WriteLine("Radio on: {0}", turnOn);

}

}

class Car

{