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BeforeExpand Event

TreeView.BeforeExpand Event

Occurs before the tree node is expanded.

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

Public Event BeforeExpand As TreeViewCancelEventHandler

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

The following code 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 for the BeforeExpand event. Next, all the nodes are expanded. The 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 standard node expansion when the plus sign next to a node is clicked, the BeforeExpand event handler is then removed.

This behavior can also be implemented by handling the BeforeCollapse event, as illustrated in the example for that topic.

For the complete example, see the CheckBoxes reference topic.

Private Sub showCheckedNodesButton_Click(ByVal sender As Object, ByVal e As EventArgs)
    ' Disable redrawing of treeView1 to prevent flickering  
    ' while changes are made.

    ' Collapse all nodes of treeView1.

    ' 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.

    ' 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.
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

.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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