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Figure 6-8.

The Employee hierarchy

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Source Code

The Employees project is included under the Chapter 6 subdirectory.

Building a Polymorphic Interface

When a class has been defined as an abstract base class (via the abstract keyword), it may define

any number of abstract members. Abstract members can be used whenever you wish to define a

member that does 

not supply a default implementation. By doing so, you enforce a polymorphic

interface on each descendent, leaving them to contend with the task of providing the details behind

your abstract methods.

Simply put, an abstract base class’s polymorphic interface simply refers to its set of virtual and

abstract methods. This is much more interesting than first meets the eye, as this trait of OOP allows

us to build very extendable and flexible software applications. To illustrate, we will be implementing

(and slightly modifying) the hierarchy of shapes briefly examined in Chapter 5 during our overview

of the pillars of OOP. To begin, create a new C# Console Application project named Shapes.

In Figure 6-9, notice that the Hexagon and Circle types each extend the Shape base class. Like

any base class, Shape defines a number of members (a PetName property and Draw() method in this

case) that are common to all descendents.

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