void TreeView_DragOver (object sender, DragEventArgs e)
{
// Retrieve the client coordinates of the mouse position.
Point targetPoint = _treeView.PointToClient (new Point (e.X, e.Y));
// Retrieve the node at the mouse position
TreeNode targetNode = _treeView.GetNodeAt (targetPoint);
// Select the node at the mouse position
_treeView.SelectedNode = targetNode;
if (_treeView.SelectedNode.Text == "Node4")
e.Effect = DragDropEffects.Copy;
else
e.Effect = e.AllowedEffect;
}