TreeView.BeforeCollapse Event

Occurs before the tree node is collapsed.

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

public event TreeViewCancelEventHandler BeforeCollapse

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 expanded, and a handler is added for the BeforeCollapse event. Next, all the nodes are collapsed. The BeforeCollapse event handler determines whether a given node has child nodes that are checked. If a node has checked children, the collapse is canceled for that node. In order to allow standard node collapse when the minus sign next to a node is clicked, the BeforeCollapse event handler is then removed.

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

For the complete example, see the TreeView constructor 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.ExpandAll();

    // Add the checkForCheckedChildren event handler to the BeforeExpand event.
    treeView1.BeforeCollapse += checkForCheckedChildren;

    // Expand all nodes of treeView1. Nodes without checked children are  
    // prevented from expanding by the checkForCheckedChildren event handler.
    treeView1.CollapseAll();

    // Remove the checkForCheckedChildren event handler from the BeforeExpand  
    // event so manual node expansion will work correctly.
    treeView1.BeforeCollapse -= checkForCheckedChildren;

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

// Prevent collapse of a node that has 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;
}

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Was this page helpful?
(1500 characters remaining)
Thank you for your feedback
Show:
© 2014 Microsoft