System.InvalidOperationException: There was an error generating the XML document. ---> System.InvalidOperationException: The type WebService.Property.Property_Users was not expected. Use the XmlInclude or SoapInclude attribute to specify types that are not known statically. at System.Xml.Serialization.XmlSerializationWriter.WriteTypedPrimitive(String name, String ns, Object o, Boolean xsiType) at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write1_Object(String n, String ns, Object o, Boolean isNullable, Boolean needType) at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write8_ArrayOfAnyType(Object o) at Microsoft.Xml.Serialization.GeneratedAssembly.ListOfObjectSerializer.Serialize(Object objectToSerialize, XmlSerializationWriter writer) at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id) --- End of inner exception stack trace --- at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id) at System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o, XmlSerializerNamespaces namespaces) at System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o) at System.Web.Services.Protocols.XmlReturnWriter.Write(HttpResponse response, Stream outputStream, Object returnValue) at System.Web.Services.Protocols.HttpServerProtocol.WriteReturns(Object[] returnValues, Stream outputStream) at System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues) at System.Web.Services.Protocols.WebServiceHandler.Invoke()

public List<object> GetDataByModuleName(string ModuleName)
    {
       List<Property_Users> obj_UserList = new List<Property_Users>(); 
      // performing some operation that add data to obj_UserList

     List < Object > myList = new List<object>();
            return ConvertToObjectList<Property_Users>(obj_UserList); 
    }
     public List<Object> ConvertToObjectList<N>(List<N> sourceList)
        {
            List<Object> result = new List<Object>();

            foreach (N item in sourceList)
            {
                result.Add(item as Object);
            }
            return result;
        } 

     [WebMethod]
        public List<object> GetDataByModuleName(string ModuleName)
        {
            List<object> obj_list = new List<object>();
            obj_list = BAL_GeneralService.GetDataByModuleName(ModuleName);
            return obj_list;
        }