此程序输出：
               Room Size: 100
               Desk Size:  18

3. 特类别继承与母子对象

    您已很熟悉父子类别关系了﹐这继承关系的另一面是母子对象关系。例如﹐

''''ex06.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
''''----------------------------------------------------
Class Room
    Private mother As MyRoom
   
    Public Sub New(ByVal mo As MyRoom)
        mother = mo
    End Sub
    Public Function GetID() As String
        GetID = mother.yourID()
    End Function
End Class

Class MyRoom
    Inherits Room
   
    Public Sub New()
        MyBase.New(Me)
    End Sub
    Public Function yourID() As String
        yourID = "VIP888"
    End Function
End Class
''''----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form
   
    Public Sub New()
        MyBase.New()
        Form1 = Me
        ''''This call is required by the Win Form Designer.
        InitializeComponent()
        ''''TODO: Add any initialization after the InitializeComponent() call
    End Sub
    ''''Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    ......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        MessageBox.Show(r.GetID())
    End Sub
End Class

此程序输出﹕
             VIP888

MyRoom()建构者New()将Me值给 Room的mother参考。于是mother参考到母对象r。Room类别是MyRoom之父类别﹐但Room之对象却是MyRoom之子对象。
 

 如果将上述Room的指令：

Public Function GetID() As String
     GetID = mother.yourID()
End Function

更改为：

Public Function GetID() As String
     GetID = Me.yourID()    ''''Error!!
End Function

就错了。因为Room类别里没有定义yourID()程序。事实上，VB的继承机制里已经有母对象的参考值了，VB的Overridable机制就是基于它而呼叫到子类别（母对象）的程序的。例如上述程序相当于：

''''ex07.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
''''----------------------------------------------------
Class Room
    Public Function GetID() As String
        GetID = Me.yourID()
    End Function
    Public Overridable Function yourID() As String
    End Function
End Class

Class MyRoom
    Inherits Room
   
    Public Sub New()
        MyBase.New()
    End Sub
    Public Overrides Function yourID() As String
        yourID = "VIP888"
    End Function
End Class
''''----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form
   
    Public Sub New()
        MyBase.New()
        Form1 = Me
        ''''This call is required by the Win Form Designer.
        InitializeComponent()
        ''''TODO: Add any initialization after the InitializeComponent() call
    End Sub
    ''''Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    .......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim r As New MyRoom()
        MessageBox.Show(r.GetID())
    End Sub
End Class

    此程序输出﹕
                 VIP888

上述ex07.bas比ex06.bas好的地方是：
    ex06.bas程序的Room类别里面用到MyRoom的名称。而ex07.bas程序的Room类别里面并没用到MyRoom的名称，因此Room类别可以先设计，MyRoom类别能后来才设计，这是继承机制的目的之一。不过，如果您一定不想用继承与Overridable概念的话，可使用VB的Interface机制来改善ex06.bas程序，如下：

''''ex08.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
''''----------------------------------------------------
Interface IMyRoom
    Function yourID() As String
End Interface

Class Room
    Private mother As IMyRoom
   
    Public Sub New(ByVal mo As IMyRoom)
        mother = mo
    End Sub
    Public Function GetID() As String
        GetID = "RoomID: " + mother.yourID()
    End Function
End Class

Class MyRoom
    Implements IMyRoom
    Private base As Room
   
    Public Sub New()
        base = New Room(Me)
    End Sub
    Public Function yourID() As String Implements IMyRoom.yourID
        yourID = "VIP888"
    End Function
    Public Function GetID() As String
        GetID = base.GetID()
    End Function
End Class
''''----------------------------------------------------
Public Class Form1
    Inherits System.WinForms.Form
   
    Public Sub New()
        MyBase.New()
        Form1 = Me
        ''''This call is required by the Win Form Designer.
        InitializeComponent()
        ''''TODO: Add any initialization after the InitializeComponent() call
    End Sub
    ''''Form overrides dispose to clean up the component list.
    Public Overrides Sub Dispose()
        MyBase.Dispose()
        components.Dispose()
    End Sub
#Region " Windows Form Designer generated code "
    .......
#End Region
    Protected Sub Form1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim r As New MyRoom()
   &

上一页  [1] [2] [3] [4]  下一页