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TreeView.BeforeCheck Event

Occurs before the tree node check box is checked.

Namespace:  System.Windows.Forms
Assembly:  System.Windows.Forms (in System.Windows.Forms.dll)

'Declaration
Public Event BeforeCheck As TreeViewCancelEventHandler

NoteNote

Setting the TreeNode.Checked property from within the BeforeCheck or AfterCheck event causes the event to be raised multiple times and can result in unexpected behavior. For example, you might set the Checked property in the event handler when you are recursively updating the child nodes, so that the user does not have to expand and check each one individually. To prevent the event from being raised multiple times, add logic to your event handler that only executes your recursive code if the Action property of the TreeViewEventArgs is not set to TreeViewAction.Unknown.

For more information about how to handle events, see Handling and Raising Events.

The following code example updates all the child tree nodes of a TreeNode when the user changes its checked state. This code requires that you have a Form with a TreeView that has TreeNode objects in its TreeNodeCollection. The TreeNodeCollection should have tree nodes with child nodes.

' Updates all child tree nodes recursively. 
Private Sub CheckAllChildNodes(treeNode As TreeNode, nodeChecked As Boolean)
   Dim node As TreeNode
   For Each node In  treeNode.Nodes 
      node.Checked = nodeChecked
      If node.Nodes.Count > 0 Then 
         ' If the current node has child nodes, call the CheckAllChildsNodes method recursively. 
         Me.CheckAllChildNodes(node, nodeChecked)
      End If 
   Next node
End Sub 

' NOTE   This code can be added to the BeforeCheck event handler instead of the AfterCheck event. 
' After a tree node's Checked property is changed, all its child nodes are updated to the same value. 
Private Sub node_AfterCheck(sender As Object, e As TreeViewEventArgs) Handles treeView1.AfterCheck
   ' The code only executes if the user caused the checked state to change. 
   If e.Action <> TreeViewAction.Unknown Then  
      If e.Node.Nodes.Count > 0 Then 
         ' Calls the CheckAllChildNodes method, passing in the current  
         ' Checked value of the TreeNode whose checked state changed.  
         Me.CheckAllChildNodes(e.Node, e.Node.Checked)
      End If 
   End If 
End Sub

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
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