We use public static methods decorated with [WebMethod] to support our Ajax Postbacks.

Recently, I received an error from a UI developing stating he was receiving the following error when attempting his post back:

{
  "Message": "Operation is not valid due to the current state of the object.",
  "StackTrace": "   at System.Web.Script.Serialization.ObjectConverter.ConvertDictionaryToObject(IDictionary`2 dictionary, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject)\r\n   at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeInternal(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject)\r\n   at System.Web.Script.Serialization.ObjectConverter.ConvertObjectToTypeMain(Object o, Type type, JavaScriptSerializer serializer, Boolean throwOnError, Object& convertedObject)\r\n   at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeInternal(Int32 depth)\r\n   at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeDictionary(Int32 depth)\r\n   at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeInternal(Int32 depth)\r\n   at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeDictionary(Int32 depth)\r\n   at System.Web.Script.Serialization.JavaScriptObjectDeserializer.DeserializeInternal(Int32 depth)\r\n   at System.Web.Script.Serialization.JavaScriptObjectDeserializer.BasicDeserialize(String input, Int32 depthLimit, JavaScriptSerializer serializer)\r\n   at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize(JavaScriptSerializer serializer, String input, Type type, Int32 depthLimit)\r\n   at System.Web.Script.Serialization.JavaScriptSerializer.Deserialize[T](String input)\r\n   at System.Web.Script.Services.RestHandler.GetRawParamsFromPostRequest(HttpContext context, JavaScriptSerializer serializer)\r\n   at System.Web.Script.Services.RestHandler.GetRawParams(WebServiceMethodData methodData, HttpContext context)\r\n   at System.Web.Script.Services.RestHandler.ExecuteWebServiceCall(HttpContext context, WebServiceMethodData methodData)",
  "ExceptionType": "System.InvalidOperationException"
}

Goggling this error brought me little support.  All the results talked about increasing the aspnet:MaxJsonDeserializerMembers value to handle larger payloads.  Since 1) I’m not using the asp.net ajax model and 2) the payload is very small, this clearly was not the cause of my issue.

Here’s the payload the UI developer was sending to the endpoint:

{
  "FundingSource": {
    "__type": "XX.YY.Engine.Contract.Funding.EvidenceBasedFundingSource,  XX.YY.Engine.Contract",
    "MeansType": 13,
    "FundingMethodName": "LegalTender",
  },
  "AddToProfile": false,
  "ProfileNickName": "",
  "FundingAmount": 0
}

By tweaking the JSON I’ve found the culprit.

Apparently the default JSS Serializer used doesn’t like the assembly name in the __type value.  Removing the assembly portion of the type name resolved my issue.

{
"FundingSource": {
"__type": "XX.YY.Engine.Contract.Funding.EvidenceBasedFundingSource",
"MeansType": 13,
"FundingMethodName": "LegalTender",
},
"AddToProfile": false,
"ProfileNickName": "",
"FundingAmount": 0
}