private void tvBooks_NodeMouseClick(object sender, TreeNodeMouseClickEventArgs e) { TreeNode tn = e.Node; if (tn.Level == 1 && tn.Nodes.Count == 0) { foreach (Book book in BookManage.getBooksByGenId(int.Parse(tn.Tag.ToString()))) { TreeNode btn = new TreeNode(book.Name); btn.Tag = book.Id; tn.Nodes.Add(btn); } } }