[VB.NET]浅谈MDI窗体的多窗体编程2

---关于WITH结构内的MDI窗体实例--

在上一篇内,介绍了MDI窗体的实例

http://blog.csdn.net/allenle/archive/2005/02/18/293122.aspx

在第二段代码中写到

Private Shared fr As New frmName

Private Sub ToolBar1_ButtonClick()Sub ToolBar1_ButtonClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.ToolBarButtonClickEventArgs) Handles ToolBar1.ButtonClick

Select Case e.Button.Text

Case "OK" '"OK" is ToolBarButton.Text

If fr Is Nothing Or fr.IsDisposed Then

fr = New frmName

fr.MdiParent = Me

fr.Show()

Else

fr.MdiParent = Me

fr.Show()

fr.Focus()

End If

End Select

End Sub

今天又调试了一下,使用了WITH结构,代码如下

Private Shared fr As New frmName

Private Sub ToolBar1_ButtonClick()Sub ToolBar1_ButtonClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.ToolBarButtonClickEventArgs) Handles ToolBar1.ButtonClick

Select Case e.Button.Text

Case "OK" '"OK" is ToolBarButton.Text

With fr

If fr Is Nothing Or fr.IsDisposed Then

fr = New frmName

.MdiParent = Me

.Show()

Else

.MdiParent = Me

.Show()

.Focus()

End If

End With

End Select

End Sub

调试出现一错误:Cannot access a disposed object named "frmName".

想了一下,修改了代码,如下

就是把with的范围缩小到fr实例化之后,调试成功~

Private Shared fr As New frmName

Private Sub ToolBar1_ButtonClick()Sub ToolBar1_ButtonClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.ToolBarButtonClickEventArgs) Handles ToolBar1.ButtonClick

Select Case e.Button.Text

Case "OK" '"OK" is ToolBarButton.Text

If fr Is Nothing Or fr.IsDisposed Then

fr = New frmName

With fr

.MdiParent = Me

.Show()

End With

Else

With fr

.MdiParent = Me

.Show()

.Focus()

End With

End If

End Select

End Sub

所以大家使用的时候要当心点

分析,就是关闭了实例了一次的窗口后,fr is nothing ,所以fr是不被with所使用的,改了with的使用范围之后就没有错误出现.

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