C# 0 The Complete Reference

PART I

C h a p t e r   1 1 :  

I n h e r i t a n c e  

301

PART IPART I

Let’s examine this program closely. First, as explained, 

Area( )

 is declared as 

virtual

 in 

the

TwoDShape

 class and is overridden by 

Triangle

 and 

Rectangle

. Inside 

TwoDShape

,

Area( )

 is given a placeholder implementation that simply informs the user that this method 

must be overridden by a derived class. Each override of 

Area( )

 supplies an implementation 

that is suitable for the type of object encapsulated by the derived class. Thus, if you were to 

implement an ellipse class, for example, then 

Area( )

 would need to compute the area of an 

ellipse.

There is one other important feature in the preceding program. Notice in