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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 TreeViewCancelEventHandler BeforeExpand
/** @event */
public void add_BeforeExpand (TreeViewCancelEventHandler value)

/** @event */
public void remove_BeforeExpand (TreeViewCancelEventHandler value)

In JScript, you can handle the events defined by a class, but you cannot define your own.
Not applicable.

For more information about handling events, see Consuming 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 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

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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