Second async operation started...don't see how - entity-framework

Question

I'm getting the error about a second async operation starting on the context before the previous async operation ended, but I'm not seeing where it's happening. This is the code I'm using. Each of the async calls are using await, so what am I doing wrong?
This is a method on a Web API 2 call.
public async Task<IHttpActionResult> SendAssessmentArsAsync(int assessmentId) {
using (var context = new LAMPEntities()) {
var assessment = await context.EHS_Assessment_Audit.AsNoTracking().Where(x => x.id == assessmentId).FirstOrDefaultAsync();
var arsQuery = from r in context.EHS_Assessment_Audit_AR.AsNoTracking()
where r.EHS_Assessment_Audit_Id == assessmentId
join w in context.Worker on r.Assignee_WWID equals w.WWID
select new {
w.Email,
w.Full_Name,
r.AR,
r.Due_Date
};
var ars = await arsQuery.ToArrayAsync();
var lab = from s in context.Lab_Space.AsNoTracking()
where s.id == assessment.Lab_Space.id
join w in context.Worker.AsNoTracking() on s.Contact_WWID equals w.WWID
where w.Email != null
join d_join in context.Worker.AsNoTracking() on s.Delegate equals d_join.WWID into d_grp
from d in d_grp.DefaultIfEmpty()
select new {
Owner = w.Email,
Delegate = d.Email,
Barcode = s.Entry_Bar_Code,
Label = s.Floor_Space_Label,
Id = s.id
};
var mails = await lab.FirstAsync();
When it hits that last line is where it is throwing the exception.
Exception message:
A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe.
Here's the exception as shown via the webservice call:
b__a()
at System.Data.Entity.Core.Objects.ObjectContext.d__3d`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.d__9`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Data.Entity.Utilities.TaskExtensions.CultureAwaiter`1.GetResult()
at System.Data.Entity.Core.Objects.ObjectQuery`1.d__e.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Data.Entity.Utilities.TaskExtensions.CultureAwaiter`1.GetResult()
at System.Data.Entity.Internal.LazyAsyncEnumerator`1.d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Data.Entity.Infrastructure.IDbAsyncEnumerableExtensions.d__1d`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at LabSORService.Controllers.EHSController.d__21.MoveNext() in ...\\Controllers\\EHSController.cs:line 854
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Threading.Tasks.TaskHelpersExtensions.d__1`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ApiControllerActionInvoker.d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ActionFilterResult.d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Dispatcher.HttpControllerDispatcher.d__15.MoveNext()" }
{
"message": "An error has occurred.",
"exceptionMessage": "A second operation started on this context before a previous asynchronous operation completed. Use 'await' to ensure that any asynchronous operations have completed before calling another method on this context. Any instance members are not guaranteed to be thread safe.",
"exceptionType": "System.NotSupportedException",
"stackTrace": " at System.Data.Entity.Core.Objects.ELinq.QueryParameterExpression.TryGetFieldOrPropertyValue(MemberExpression me, Object instance, Object& memberValue)
at System.Data.Entity.Core.Objects.ELinq.QueryParameterExpression.TryEvaluatePath(Expression expression, ConstantExpression& constantExpression)
at System.Data.Entity.Core.Objects.ELinq.QueryParameterExpression.EvaluateParameter(Object[] arguments)
at System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClassc.

Answer 1

Most likely
where s.id == assessment.Lab_Space.id
is triggering lazy loading of Lab_Space navigation property in inappropriate time.
Assuming you have no explicit FK property like Lab_Space_Id (if you do have, just use it instead of Lab_Space.id), either eager load it (preferable):
var assessment = await context.EHS_Assessment_Audit.AsNoTracking()
.Include(x => x.Lab_Space) // <--
.Where(x => x.id == assessmentId).FirstOrDefaultAsync();
or evaluate it outside the query:
var labSpaceId = assessment.Lab_Space.id; // <--
var lab = from s in context.Lab_Space.AsNoTracking()
where s.id == labSpaceId // <--
...
Note that in the original query, assessment.Lab_Space access is just recorded in the query expression tree. The actual evaluation (hence triggering the lazy load) happens as part of the execution of the query.