Visual Basic.NET How to Program (PDF)

Chapter 10 Object-Oriented Programming: Polymorphism 423
21 ' property Age
22 ReadOnly Property Age() As Integer _
23 Implements IAge.Age
24
25 Get
26 Return mRings
27 End Get
28
29 End Property ' Age
30
31 ' property Name implementation of interface IAge
32 ReadOnly Property Name() As String _
33 Implements IAge.Name
34
35 Get
36 Return "Tree"
37 End Get
38
39 End Property ' Name
40
41 End Class ' CTree
Fig. 10.17 CTeee class implements IAge interface (part 2 of 2).
Class CTest (Fig. 10.18) demonstrates polymorphism on the objects of disparate
classes CPerson and CTree. Line 11 instantiates object tree of class CTree, and line 12
instantiates object person of class CPerson. Line 15 declares iAgeArray—an array
of two references to IAge objects. Line 18 and 21 assign tree and person to the first
and second reference in iAgeArray, respectively. Lines 24–26 invoke method
ToString on tree, then invoke its properties Age and Name to return age and name
information for object tree. Lines 29–31 invoke method ToString on person, then
invoke its properties Age and Name to return age and name information for object
person. Next, we manipulate these objects polymorphically through the iAgeArray of
references to IAge objects. Lines 36–39 define a For-Each structure that uses properties
Age and Name to obtain age and name information for each IAge object in iAgeArray.
Note that we use Name so that each object in iAgeArray can “identify” itself in our program’s
output. Objects tree and person can use method ToString to do this, because
classes CTree and CPerson both inherit from class Object. However, when CTest
interacts with these objects polymorphically, CTest can use only properties Age and
Name for each interface object. Because interface IAge does not provide method
ToString, clients cannot invoke method ToString through interface IAge references.
1 ' Fig. 10.18: Test.vb
2 ' Demonstrate polymorphism.
3
4 Imports System.Windows.Forms
5
6 Class CTest
7
Fig. 10.18 Demonstrate polymorphism on objects of disparate classes (part 1 of 2).