VB.NET WPF空引用異常

Question

我有一個父節點和兩個子節點的TreeView。每個節點都包含一個存儲在TreeViewItem中的複選框。我希望在用戶檢查父節點時將兩個子節點複選框設置爲IsChecked = true，並且當用戶取消選中父節點時，我希望兩個子節點複選框爲IsChecked = false。

我有一個for循環，其中子節點複選框存儲在列表中。父節點複選框選中/取消選中事件應該遍歷子節點複選框列表，但我在列表中遇到問題。出於某種原因，該列表在父節點檢查/取消選中事件中等於「無」。任何人都可以解釋我應該如何訪問該列表？

這裏是我的代碼：

Public Class Question 

Dim childCheckbox As CheckBox 
Dim childCheckboxes() As CheckBox 

Public Sub Window_Loaded(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs) Handles MyBase.Loaded 

    Dim parentCheckbox = New CheckBox 
    Dim parentNode = New TreeViewItem 

    parentCheckbox.Uid = "All Sites" 

    AddHandler parentCheckbox.Checked, AddressOf chkbox_AllChecked 
    AddHandler parentCheckbox.Unchecked, AddressOf chkbox_AllUnchecked 

    parentCheckbox.Content = "All Sites" 
    parentNode.Header = parentCheckbox 

    For Each osite In sites 

       Dim childNode = New TreeViewItem 
       Dim childCheckbox = New CheckBox 

       AddHandler childCheckbox.Checked, AddressOf chkbox_Checked 
       AddHandler childCheckbox.Unchecked, AddressOf chkbox_Unchecked 

       childCheckbox.Uid = osite.SiteName.ToString 

       childCheckbox.Content = osite.SiteName.ToString 
       childNode.Header = childCheckbox 
       parentNode.Items.Add(childNode) 

       'Add all childCheckbox to an array for use by parentChildbox methods to check/uncheck all 
       childCheckboxes(i) = childCheckbox 

       i += 1 

      Next 
      TreeView1.Items.Add(parentNode) 


    End Sub 

Private Sub chkbox_AllChecked(ByVal sender As Object, ByVal e As RoutedEventArgs) 
    Dim chk = DirectCast(sender, CheckBox) 

      'MessageBox.Show(chk.Uid.ToString) 


      'This part doesn't work. 
      For Each child In childCheckboxes 
       child.IsChecked = True 
      Next 

    End Sub 

Private Sub chkbox_Checked(ByVal sender As Object, ByVal e As RoutedEventArgs) 
    Dim chk = DirectCast(sender, CheckBox) 

      'MessageBox.Show("Check!") 
      MessageBox.Show(chk.Uid.ToString) 

End Sub 

Private Sub chkbox_Unchecked(ByVal sender As Object, ByVal e As RoutedEventArgs) 
    Dim chk = DirectCast(sender, CheckBox) 

      'MessageBox.Show("Uncheck!") 
      MessageBox.Show(chk.Uid.ToString) 

End Sub 

End Class 

感謝您的幫助！

Answer 1

你能嘗試更改行：

Dim childCheckboxes() As CheckBox 

要：

Dim childCheckboxes as new list(of CheckBox)