webservice 返回datatable无法序列化 DataTable。未设置 DataTable 姓名

webservice 返回datatable无法序列化 DataTable。未设置 DataTable 名称。
在做返回datatable时出现无法序列化 DataTable。未设置 DataTable 名称。
 
代码如下：
            Hashtable ht = new Hashtable();
            ht.Add("departure", departure);
            ht.Add("arrive", arrive);
            ht.Add("departuretime", departuretime);
            ht.Add("flytype", flytype);
            Class.ClassFlyfind flightfind = new Service.Class.ClassFlyfind();
            DataTable tmpdta = new DataTable();
            tmpdta.TableName = "tmp";
            tmpdta = flightfind.flightfind(ht);
            return tmpdta;

Class
            Hashtable ht = new Hashtable();
            ht.Add("departure", departure);
            ht.Add("arrive", arrive);
            ht.Add("departuretime", departuretime);
            ht.Add("flytype", flytype);
            Class.ClassFlyfind flightfind = new Service.Class.ClassFlyfind();
            DataTable tmpdta = new DataTable();
            tmpdta.TableName = "tmp";
            tmpdta = flightfind.flightfind(ht);
            return tmpdta;



报错如下

System.InvalidOperationException: 生成 XML 文档时出错。 ---> System.InvalidOperationException: 无法序列化 DataTable。未设置 DataTable 名称。
   在 System.Data.DataTable.WriteXmlSchema(XmlWriter writer, Boolean writeHierarchy)
   在 System.Data.DataTable.System.Xml.Serialization.IXmlSerializable.WriteXml(XmlWriter writer)
   在 System.Xml.Serialization.XmlSerializationWriter.WriteSerializable(IXmlSerializable serializable, String name, String ns, Boolean isNullable, Boolean wrapped)
   在 Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write4_DataTable(Object o)
   在 Microsoft.Xml.Serialization.GeneratedAssembly.DataTableSerializer.Serialize(Object objectToSerialize, XmlSerializationWriter writer)
   在 System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
   --- 内部异常堆栈跟踪的结尾 ---
   在 System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
   在 System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o, XmlSerializerNamespaces namespaces)
   在 System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o)
   在 System.Web.Services.Protocols.XmlReturnWriter.Write(HttpResponse response, Stream outputStream, Object returnValue)
   在 System.Web.Services.Protocols.HttpServerProtocol.WriteReturns(Object[] returnValues, Stream outputStream)
   在 System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues)
   在 System.Web.Services.Protocols.WebServiceHandler.Invoke()


不胜感激datatable不支持序列化，直接返回dataset吧。或者自定义一个类。