C# 0 The Complete Reference

TwoDShape

 used in the preceding example. 

The definition of 

Area( )

 is simply a placeholder. It will not compute and display the area of 

any type of object.

You will see as you create your own class libraries that it is not uncommon for a method 

to have no meaningful definition in the context of its base class. You can handle this situation 

two ways. One way, as shown in the previous example, is to simply have it report a warning 

message. Although this approach can be useful in certain situations—such as debugging—it 

is not usually appropriate. You may have methods that must be overridden by the derived 

class in order for the derived class to have any meaning. Consider the class 

Triangle

. It is 

incomplete if 

Area( )

 is not defined. In such a case, you want some way to ensure that a 

derived class does, indeed, override all necessary methods. C#’s solution to this problem is 

the

abstract method.

An abstract method is created by specifying the

abstract

 type modifier. An abstract 

method contains no body and is, therefore, not implemented by the base class. Thus, a 

derived class must override it—it cannot simply use the version defined in the base class. 

As you can probably guess, an abstract method is automatically virtual, and there is no 

need to use the 

virtual

 modifier. In fact, it is an error to use