private void treeView1_NodeMouseDoubleClick(object sender, TreeNodeMouseClickEventArgs e)
        {
            if (e.Node.Nodes.Count > 0)
                return;
        }

------解决方案--------------------

    private void treeView1_NodeMouseDoubleClick(object sender, TreeNodeMouseClickEventArgs e)
       {
           if (e.Node.Nodes.Count==0&&e.Node.NextNode==null)
               {}
       }