InvalidOperationException while WCF Data Service on load test

Question

I have a method to insert a record in a Table, which works fine during my unit test. However, when I do a load test using Visual studio ultimate's load test, with just constant load of 10 users, I get below InvalidOperationException after around 1000 records are successfully inserted. I am not sure which collection is modified and how to get rid of this exception. I cannot catch this during debugging, as it throws exception only with moderate load.

Topic is a single table, having FKs of Topic Category and User tables.

I have to make this to be used by at-least 10,000 users. Is, WCF DataService a bad choice for this?

Exception:

Test method AgentCollaborationTest.TestDataLayer.TestCreateTopic threw exception: 
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.

Stack Trace:

System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
System.Collections.Generic.Dictionary`2.ValueCollection.Enumerator.MoveNext()
System.Linq.Enumerable.<UnionIterator>d__88`1.MoveNext()
System.Linq.Enumerable.<UnionIterator>d__88`1.MoveNext()
System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
System.Linq.Buffer`1..ctor(IEnumerable`1 source)
System.Linq.OrderedEnumerable`1.<GetEnumerator>d__0.MoveNext()
System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
System.Data.Services.Client.BaseSaveResult..ctor(DataServiceContext context, String method, DataServiceRequest[] queries, SaveChangesOptions options, AsyncCallback callback, Object state)
System.Data.Services.Client.BaseSaveResult.CreateSaveResult(DataServiceContext context, String method, DataServiceRequest[] queries, SaveChangesOptions options, AsyncCallback callback, Object state)
System.Data.Services.Client.DataServiceContext.SaveChanges(SaveChangesOptions options)
System.Data.Services.Client.DataServiceContext.SaveChanges()
DishTalk.Data.DataStream.CreateTopic(String Name, String UserName, String content, Guid catagoryId, Boolean isLocked) in c:\Users\<username>\Documents\Visual Studio 2012\Projects\DataLayer\DataAccessLayer\DataStream.cs: line 239
AgentCollaborationTest.TestDataLayer.TestCreateTopic() in c:\Users\<userName>Documents\Visual Studio 2012\Projects\DataLayer\AgentCollaborationTest\UnitTest1.cs: line 67

Method:

    public Guid? CreateTopic(string name, string userName, string content, Guid catagoryId)
    {      
         context.IgnoreResourceNotFoundException = true;

        //Get the category and user objects, which has a Foreign Key association with Topic           
        Category selectedCat = GetCatagory(catagoryId);
        User user = GetUser(userName);

        var topic = DataService.Topic.CreateTopic(
                Guid.NewGuid(),
                name,
                DateTime.Now,
                GetUser(userName).Id,
                false,
                catagoryId,
                false                      
                );            
            try
            {
                topic.Content = content;
                topic.Solved = false;

                //The context is created in the constructor
                context.AddToTopics(topic);

                context.AddLink(selectedCat, "Topics", topic);
                context.AddLink(user,"Topics",topic);

                // Send the insert to the data service.                
                DataServiceResponse response = context.SaveChanges();

            }           
            catch (DataServiceRequestException ex)
            {

                 logger.ErrorException(ex.GetType().ToString(), ex);

            }
            return topic.Id;            
    }

Answer 1

I fixed the problem by creating the context for each request, rather than sharing it among all the requests. My context was a singleton earlier.