VB.NET WPF NullReference 异常

问题描述

我有一个带有父节点和两个子节点的 TreeView.每个节点都包含一个存储在 TreeViewItem 中的复选框.当用户检查父节点时，我希望将两个子节点复选框设置为 IsChecked=true，当用户取消选中父节点时，我希望两个子节点复选框设置为 IsChecked=false.

I have a TreeView with a parent node and two children node. Each of the nodes contain a checkbox stored in a TreeViewItem. I want the two children node checkboxes to be set to IsChecked=true when the user checks the parent node and I want the two children node checkboxes to be IsChecked=false when the user unchecks the parent node.

我有一个 for 循环，其中子节点复选框存储在列表中.父节点复选框选中/取消选中事件应该遍历子节点复选框列表，但我在访问列表时遇到问题.出于某种原因，该列表在父节点检查/取消检查事件中等于无".谁能解释一下我应该如何访问该列表?

I have a for loop in which the child node checkboxes are stored in a list. The parent node checkbox check/uncheck event should iterate through the child node checkbox list but I am having a problem aceess the list. For some reason the list equals "nothing" in the parent node check/uncheck event. Can anyone explain how I should be accessing that list?

这是我的代码:

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

感谢您的帮助！

推荐答案

你能不能试试换行:

Dim childCheckboxes() As CheckBox

致:

Dim childCheckboxes as new list(of CheckBox)

这篇关于VB.NET WPF NullReference 异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！