C# xml序列化 datatime字段

　　　　 [XmlIgnore]
        public DateTime ApplicationDatetime { get; set; }

        [XmlElement("ApplicationDatetime")]
        public string ApplicationDatetimeString
        {
            get { return ApplicationDatetime.ToString("yyyy-MM-dd HH:mm:ss"); }
            set { ApplicationDatetime = DateTime.Parse(value); }
        }

        static public string SerializeToXml<T>(T obj)
        {
            string xmlString = string.Empty;
            //XmlSerializer xmlSerializer = new XmlSerializer(typeof(T));
            //using (MemoryStream ms = new MemoryStream())
            //{
            //    xmlSerializer.Serialize(ms, obj);
            //    xmlString = Encoding.UTF8.GetString(ms.ToArray());
            //}
            Encoding encoding = Encoding.UTF8;
            using (MemoryStream memoryStream = new MemoryStream())
            {
                XmlSerializer xmlSerializer = new XmlSerializer(obj.GetType());
                XmlSerializerNamespaces namespaces = new XmlSerializerNamespaces();
                namespaces.Add("", "");

                XmlTextWriter xmlTextWriter = new XmlTextWriter(memoryStream, encoding);

                xmlTextWriter.Formatting = Formatting.None;
                xmlSerializer.Serialize(xmlTextWriter, obj, namespaces);
                xmlTextWriter.Flush();
                xmlTextWriter.Close();

                xmlString = encoding.GetString(memoryStream.ToArray());
            }
            return xmlString;
        }

            string queryParameters = WebUtils.SerializeToXml(posQueryParameters);

<Entity><EntityName>我和coffee</EntityName><PId></PId><LoanId>c70215bbc6dd4722b51d2bb93c1a81bd</LoanId><BankCardNo /><MobilePhone /><CheckEmployee>dkx4003</CheckEmployee><ApplicationDatetime>-- ::</ApplicationDatetime><Role>PR</Role></Entity>