TreeViewCancelEventHandler Delegate
Represents the method that will handle the BeforeCheck, BeforeCollapse, BeforeExpand, or BeforeSelect event of a TreeView.
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)
'Declaration Public Delegate Sub TreeViewCancelEventHandler ( _ sender As Object, _ e As TreeViewCancelEventArgs _ ) 'Usage Dim instance As New TreeViewCancelEventHandler(AddressOf HandlerMethod)
Parameters
- sender
- Type: System.Object
The source of the event.
- e
- Type: System.Windows.Forms.TreeViewCancelEventArgs
A TreeViewCancelEventArgs that contains the event data.
When you create a TreeViewCancelEventArgs delegate, you identify the method that will handle the event. To associate the event with your event handler, add an instance of the delegate to the event. The event handler is called whenever the event occurs, unless you remove the delegate. For more information about event handler delegates, see Events and Delegates.
The following example demonstrates how to change the collapse state of a TreeView so that all the checked nodes are visible. First, all the nodes are collapsed, and a handler is added to the TreeView.BeforeExpand event. Next, all the nodes are expanded. The TreeView.BeforeExpand event handler determines whether a given node has child nodes that are checked. If a node does not have checked children, the expansion is canceled for that node. In order to allow normal node expansion when the plus sign next to a node is clicked, the TreeView.BeforeExpand event handler is then removed.
This behavior can also be implemented by handling the TreeView.BeforeCollapse event, as illustrated in the example for that topic.
Imports System Imports System.Drawing Imports System.Windows.Forms Public Class Form1 Inherits Form Private treeView1 As TreeView Private showCheckedNodesButton As Button Public Sub New() treeView1 = New TreeView showCheckedNodesButton = New Button Me.SuspendLayout() ' Initialize treeView1. treeView1.Location = New Point(0, 25) treeView1.Size = New Size(292, 248) treeView1.Anchor = AnchorStyles.Top Or AnchorStyles.Left Or AnchorStyles.Bottom Or AnchorStyles.Right treeView1.CheckBoxes = True ' Add nodes to treeView1. Dim node As TreeNode Dim x As Integer For x = 0 To 3 ' Add a root node. node = treeView1.Nodes.Add(String.Format("Node{0}", x * 4)) Dim y As Integer For y = 1 To 4 ' Add a node as a child of the previously added node. node = node.Nodes.Add(String.Format("Node{0}", x * 4 + y)) Next y Next x ' Set the checked state of one of the nodes to ' demonstrate the showCheckedNodesButton button behavior. treeView1.Nodes(1).Nodes(0).Nodes(0).Checked = True ' Initialize showCheckedNodesButton. showCheckedNodesButton.Size = New Size(144, 24) showCheckedNodesButton.Text = "Show Checked Nodes" AddHandler showCheckedNodesButton.Click, AddressOf showCheckedNodesButton_Click ' Initialize the form. Me.ClientSize = New Size(292, 273) Me.Controls.AddRange(New Control() {showCheckedNodesButton, treeView1}) Me.ResumeLayout(False) End Sub 'New <STAThreadAttribute()> _ Shared Sub Main() Application.Run(New Form1) End Sub 'Main Private Sub showCheckedNodesButton_Click(ByVal sender As Object, ByVal e As EventArgs) ' Disable redrawing of treeView1 to prevent flickering ' while changes are made. treeView1.BeginUpdate() ' Collapse all nodes of treeView1. treeView1.CollapseAll() ' Add the CheckForCheckedChildren event handler to the BeforeExpand event. AddHandler treeView1.BeforeExpand, AddressOf CheckForCheckedChildren ' Expand all nodes of treeView1. Nodes without checked children are ' prevented from expanding by the checkForCheckedChildren event handler. treeView1.ExpandAll() ' Remove the checkForCheckedChildren event handler from the BeforeExpand ' event so manual node expansion will work correctly. RemoveHandler treeView1.BeforeExpand, AddressOf CheckForCheckedChildren ' Enable redrawing of treeView1. treeView1.EndUpdate() End Sub 'showCheckedNodesButton_Click ' Prevent expansion of a node that does not have any checked child nodes. Private Sub CheckForCheckedChildren(ByVal sender As Object, ByVal e As TreeViewCancelEventArgs) If Not HasCheckedChildNodes(e.Node) Then e.Cancel = True End If End Sub 'CheckForCheckedChildren ' Returns a value indicating whether the specified ' TreeNode has checked child nodes. Private Function HasCheckedChildNodes(ByVal node As TreeNode) As Boolean If node.Nodes.Count = 0 Then Return False End If Dim childNode As TreeNode For Each childNode In node.Nodes If childNode.Checked Then Return True End If ' Recursively check the children of the current child node. If HasCheckedChildNodes(childNode) Then Return True End If Next childNode Return False End Function 'HasCheckedChildNodes End Class 'Form1
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The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.