c#读取xml 老是显示为空，什么原因？

悬赏园豆：10 [已解决问题] 解决于 2013-12-20 18:10

                WebClient wc = new WebClient();
                Stream sm = wc.OpenRead(AutoUpdateFileName);
                XmlTextReader xml = new XmlTextReader(sm);
                while (xml.Read())
                {
                    if (xml.Name == "UpdateTime")//获取升级日期
                    {
                        LastUpdateTime = xml.GetAttribute("value");
                        break;
                    }
                }
                sm.Close();
                xml.Close();