The Book of Visual Studio .NET - OOP in VB .NET Crash Course

Adding Polymorphism or Interface-based Inheritance

This example features an interface called IPerson and a class named Employee that uses the IPerson interface:

1. First create a new Windows Form with the same controls as used in the encapsulation example and name it "frmPolymorphism".

2. Add a new Module to the PersonProj project and name it "MyInterfaces.vb".

3. Apply the following code to the MyInterface.vb module. The code defines

the interface.
Public Interface IPerson
Property FName() As String
Property LName() As String
ReadOnly Property FullName() As String
Property BirthDate() As Date
ReadOnly Property Age() As Integer
'Method for adding hours to m_TotalHours worked.
Sub Work(ByVal intHours As Integer)
ReadOnly Property TotalHoursWorked() As Integer
End Interface

4. Right- click and select Properties, then change the StartUp object to "frmPolymorphism".

5. Create a new class to the Person.vb module. You will use it to inherit the new interface:

Public Class clsPerson3
Implements IPerson
Private m_FName As String
Private m_LName As String
Private m_BirthDate As Date
Private m_TotalHours As Integer
Private m_HrRate As Integer
Private m_TotalPay As Double
Public Property FName() As String _
Implements IPerson.FName
Get
Return m_FName
End Get
Set(ByVal Value As String)
m_FName = Value
End Set
End Property
Public Property LName() As String _
Implements IPerson.LName
Get
Return m_LName
End Get
Set(ByVal Value As String)
m_LName = Value
End Set
End Property
Public ReadOnly Property FullName() As String _
Implements IPerson.FullName
Get
Return m_FName & " " & m_LName
End Get
End Property
Public Property BirthDate() As Date _
Implements IPerson.BirthDate
Get
Return m_BirthDate
End Get
Set(ByVal Value As Date)
If IsDate(Value) Then
m_BirthDate = Value
End If
End Set
End Property
Public ReadOnly Property Age() As Integer _
Implements IPerson.Age
Get
If DatePart(DateInterval.Year, _
m_BirthDate) = 1 Then Exit Property
Return DateDiff(DateInterval.Year, _
m_BirthDate, Now)
End Get
End Property
'Method for adding hours to m_TotalHours worked.
Public Sub Work(ByVal intHours As Integer) _
Implements IPerson.Work
m_TotalHours += intHours
End Sub
Public ReadOnly Property TotalHoursWorked() As Integer _
Implements IPerson.TotalHoursWorked
Get
Return m_TotalHours
End Get
End Property
'Additional Properties:
'HrRate
Public WriteOnly Property HrRate()
Set(ByVal Value)
m_HrRate = Value
End Set
End Property
'TotalPay
Public ReadOnly Property TotalPay() As Double
Get
Return m_HrRate * m_TotalHours
End Get
End Property
End Class

6. Now run this example just as you ran the previous ones. You will not notice a difference in how the application runs, although the plumbing has changed quite a bit.

With this simple example, it is easy to question the usefulness of polymorphism. However, if you were to continue building an application that dealt with several aspects of a person, you might find polymorphism more helpful to use if you had to deal with Employees, Customers, Managers, and Contractors.

Working with Inheritance

This inheritance example inherits the interface and implementation code of the clsPerson3 class:

1. Add a new Windows Form item named "frmInheritance.vb". Use the same controls as we used in the encapsulation example.

2. Add the following code to the initialization section of the form:

' Here we are initializing the Person class. Normally this would be done
' when the class was needed for data access but in this case we are using
' the Person class to maintain our data.
Dim objPerson As New Person.clsEmployee()

3. Add the following class to the Person.vb module of the PersonProj project:

Public Class clsEmployee
Inherits clsPerson3
End Class

4. Right- click and select Properties, then change the StartUp object to "frmInheritance".

5. Now Run the inheritance example as you did the previous ones. Notice that the clsEmployee class inherits the functionality of the clsPerson3 class, which in turn implements the IPerson interface. This example demonstrates both polymorphism and inheritance that have been combined to form a single solution.

Summary

In this chapter, you learned that Visual Basic has come a long way from a reduced featured-set language that promoted RAPID application development to a full featured language. Now employing full inheritance, Visual Basic promises to aid in the delivery of enterprise level applications that may previously have been better delivered in another OOP language.

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This is the final part of a sample chapter from The Book of Visual Studio .NET, ISBN 1-886411-69-7 from No Starch Press

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This article was originally published on February 3, 2003