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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)

public class TreeViewCancelEventArgs : CancelEventArgs
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 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.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.BeforeExpand -= checkForCheckedChildren;

    // Enable redrawing of treeView1.
    treeView1.EndUpdate();
}

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

// Returns a value indicating whether the specified 
// TreeNode has checked child nodes.
private bool HasCheckedChildNodes(TreeNode node)
{
    if (node.Nodes.Count == 0) return false;
    foreach (TreeNode childNode in node.Nodes)
    {
        if (childNode.Checked) return true;
        // Recursively check the children of the current child node.
        if (HasCheckedChildNodes(childNode)) return true;
    }
    return false;
}

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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