二三级下拉菜单

if (!Page.IsPostBack)
        {
            string sql = "select * from InfoType";
            DataTable dt = SqlHelp.ExecuteDataTable(sql);
            string js = "",ss = "";
            for (int i = 0; i < dt.Rows.Count; i++)
            {
                if (dt.Rows[i]["parenttypeid"].ToString()=="")
                {
                    js=dt.Rows[i]["typeid"].ToString();
                    DropDownList1.Items.Insert(i, new ListItem(dt.Rows[i]["tyepname"].ToString(), ""));
                }
                else
                {
                    if (dt.Rows[i]["parenttypeid"].ToString()==js)
                    {
                         ss = "+";
                    }
                    else
                    {
                        ss = "++";
                    }
                    DropDownList1.Items.Insert(i, new ListItem(ss + dt.Rows[i]["tyepname"].ToString(), ""));
                }
            }
        }