string .format

想生成类似20090409001的序号。

 string sUsername = "";
                string date = DateTime.Now.ToString("yyyyMMdd");
                DataTable dtmax = Conn.query("select sUserName from xUser where sUserName like '" + date + "%' order by sUserName desc ");
                if (dtmax.Rows.Count == 0) sUsername = date + "001";
                else sUsername =date +String .Format ("{0:D3}", (Convert.ToInt32(dtmax.Rows[0]["sUserName"].ToString().Substring(7)) + 1));
                txtsUserName.Text = sUsername;