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TreeViewCancelEventArgs Class

Provides data for the BeforeCheck, BeforeCollapse, BeforeExpand, and BeforeSelect events of a TreeView control.

Namespace: System.Windows.Forms
Assembly: System.Windows.Forms (in system.windows.forms.dll)

'Declaration
Public Class TreeViewCancelEventArgs
	Inherits CancelEventArgs
'Usage
Dim instance As TreeViewCancelEventArgs

public class TreeViewCancelEventArgs extends CancelEventArgs
public class TreeViewCancelEventArgs extends CancelEventArgs
Not applicable.

For more information about handling events, see Consuming Events.

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.

For the complete example, see the TreeView.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.
    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

private void showCheckedNodesButton_Click(Object sender, EventArgs e)
{
    // 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.
    treeView1.add_BeforeExpand(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.
    treeView1.remove_BeforeExpand(checkForCheckedChildren);
    // Enable redrawing of treeView1.
    treeView1.EndUpdate();
} //showCheckedNodesButton_Click

// Prevent expansion of a node that does not have any checked child nodes.
private void CheckForCheckedChildrenHandler(Object sender,
    TreeViewCancelEventArgs e)
{
    if (!(HasCheckedChildNodes(e.get_Node()))) {
        e.set_Cancel(true);
    }
} //CheckForCheckedChildrenHandler

// Returns a value indicating whether the specified 
// TreeNode has checked child nodes.
private boolean HasCheckedChildNodes(TreeNode node)
{
    if (node.get_Nodes().get_Count() == 0) {
        return false;
    }
    for (int iCtr = 0; iCtr < node.get_Nodes().get_Count(); iCtr++) {
        TreeNode childNode = node.get_Nodes().get_Item(iCtr);
        if (childNode.get_Checked()) {
            return true;
        }
        // Recursively check the children of the current child node.
        if (HasCheckedChildNodes(childNode)) {
            return true;
        }
    }
    return false;
} //HasCheckedChildNodes

System.Object
   System.EventArgs
     System.ComponentModel.CancelEventArgs
      System.Windows.Forms.TreeViewCancelEventArgs
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0
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