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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 {
	void add (TreeViewCancelEventHandler^ value);
	void remove (TreeViewCancelEventHandler^ value);
}

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:
   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. 
   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. 
   bool HasCheckedChildNodes( TreeNode^ node )
   {
      if ( node->Nodes->Count == 0 )
            return false;

      System::Collections::IEnumerator^ myEnum = node->Nodes->GetEnumerator();
      while ( myEnum->MoveNext() )
      {
         TreeNode^ childNode = safe_cast<TreeNode^>(myEnum->Current);
         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.

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