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Editorial Team
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Asked: 2026-06-12T02:46:18+00:00

I have a simple data model with 3 tables (Account, Contact, and User) with

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I have a simple data model with 3 tables (Account, Contact, and User) with the following relationships:

User -> Account (1 – Many) Account -> Contact (Many – 1)

I am exposing my data via an OData (v3) WCF Data Service, which is consumed by a .NET client that uses the WCF Data Service Client Library. I used the Add Service utility to generate the client proxy code to call the data service.

All methods in the client class uses the class’s single DataServiceContext object for calling the web service. i.e.:

DC.WhEntities svcClient = new DC.WhEntities(new Uri(BaseUrl));

What I am having a hard time trying to figure out is why the same query request to the service starts failing after the 6th time. I have literally tried all possible ways to construct a call to the data service:

First approach:

DataServiceQuery<DC.User> users = svcClient.Users.Expand("Accounts");
QueryOperationResponse<DC.User> response = users.Execute() as QueryOperationResponse<DC.User>;
var user = response.FirstOrDefault(u => u.Id == long.Parse(key.ToString()));

Second approach:

string queryString = string.Format("Users({0}L)?$expand=Accounts", key.ToString());
foreach (var user in response) {...}

The last statement in both of the above solution starts failing with a message below after it has executed successfully 6 times in a row:

The response payload is a not a valid response payload. Please make sure that the top level element is a valid Atom element or belongs to 'http://schemas.microsoft.com/ado/2007/08/dataservices' namespace.

**StackTrace:**
   at System.Data.Services.Client.Materialization.ODataMaterializer.CreateODataMessageReader(IODataResponseMessage responseMessage, ResponseInfo responseInfo, Boolean projectionQuery, ODataPayloadKind& payloadKind)
   at System.Data.Services.Client.Materialization.ODataMaterializer.CreateMaterializerForMessage(IODataResponseMessage responseMessage, ResponseInfo responseInfo, Type materializerType, QueryComponents queryComponents, ProjectionPlan plan, ODataPayloadKind payloadKind)
   at System.Data.Services.Client.DataServiceRequest.Materialize(ResponseInfo responseInfo, QueryComponents queryComponents, ProjectionPlan plan, String contentType, IODataResponseMessage message, ODataPayloadKind expectedPayloadKind)
   at System.Data.Services.Client.QueryResult.ProcessResult[TElement](ProjectionPlan plan)
   at System.Data.Services.Client.DataServiceRequest.Execute[TElement](DataServiceContext context, QueryComponents queryComponents)

When this happens, my WCF Data Service just stopped working and returns a response with

error on line 1 at column 83: Unescaped ‘<‘ not allowed in attributes values.

I am not sure if I am missing anything fundamental or if I’m constructing the WCF Data Service Client request incorrectly or if there is something on the WCF Data Service side that doesn’t like the same client requesting the same thing more than 6 times.

I’ve already spent a few days and I meant 3+ days trying to figure this out. I am new to WCF Data Service and I thought I could learn from this tutorial, but so far I got more pain than gain.

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1 Answer

  1. Editorial Team

    I tried setting logMessagesAtTransportLevel to false and still got the error.

    Then i remembered seeing this issue before when I had an assembly conflict. I went and created a brand new service and this solved my problem even when I had logMessagesAtTransportLevel set to true on my client. This ensured me that the problem was the service.

    Although my solution solved my problem, I still don’t know the exact issue and I already ran out of time to find it out. However, It is good to see that people are willing to help out and I truly appreciate the effort.

    Thanks everyone again for your help.

    Qster123.

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