System.InvalidOperationException : Collection was modified; enumeration operation may not execute.

[Sreejith]

01.
[Fact]
02.public void Reconcile_LocationGroupChangedEvent_ReconcileSuccessfully()
03.{
04.    using (var container = new MockingContainer<Assignment>(settings))
05.    {
06.        //Arrange
07.        container.Arrange<IFacade>(x =>x.Initialize()).OccursAtMost(1);
08.
09.        //Act
10.        container.Instance.Reconcile();
11.
12.        //Assert
13.        container.AssertAll();
14.    }
15.}
16.
17.// Method under test
18.
19.public void Reconcile()
20.{
21.    Task.Factory.StartNew(() => facade.Initialize());
22.}

 

I am getting intermittent errors when running the above Test. It fails so infrequently that I am unable to debug. When it fails, here is what I see in the logs.

Reconcile_Test [FAIL]
System.InvalidOperationException : Collection was modified; enumeration operation may not execute.
Stack Trace:
     at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
     at System.Collections.Generic.Dictionary`2.Enumerator.MoveNext()
     at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
     at System.Linq.Enumerable.<SelectManyIterator>d__23`3.MoveNext()
     at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
     at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
     at Telerik.JustMock.Core.MocksRepository.GetDebugView(Object mock)
     at Telerik.JustMock.DebugView.<>c.<get_CurrentState>b__1_0()
     at Telerik.JustMock.Core.ProfilerInterceptor.GuardInternal[T](Func`1 guardedAction)
     at Telerik.JustMock.DebugView.TraceEvent(IndentLevel traceLevel, Func`1 message)
     at Telerik.JustMock.Core.Context.MockingContext.ResolveRepository(UnresolvedContextBehavior unresolvedContextBehavior)
     at Telerik.JustMock.Helpers.FluentHelper.<>c__DisplayClass13_0`1.<AssertAll>b__0()
     at Telerik.JustMock.Core.ProfilerInterceptor.GuardInternal(Action guardedAction)
     at Telerik.JustMock.AutoMock.MockResolver.ForEachMock(Action`1 action)
     at Telerik.JustMock.Core.ProfilerInterceptor.GuardInternal(Action guardedAction)
  E:\agt01\COM-CN-JOB1\AssignmentTest.cs(14,0): at AssignmentTest.Reconcile_Test()

[Ivo]

Hello Sreejith,

Most probably the issue is caused by the asynchronous call inside the method under test. I was unable to reproduce such behavior using the code snippet you have sent combined with some assumptions around it. It is not clear enough what exactly is the use case, so I would like to ask you to share more details. Please clarify also, whether the test is failing when it runs in bundle with others or just standalone? As far as I know, xUnit executes tests in parallel, so it could be also a possible source of troubles. It would be nice to share what version of JustMock you are using free or commercial?

Regards,

Ivo
Progress Telerik

[Sreejith]

Hi Ivo,

This test is failing intermittently in our build server. There, tests are run sequentially if it is in a single file, but we run different test classes in parallel.

In my local machine, it is more or less impossible to reproduce.

I am using JustMock free version 2018.1.117.2

[Ivo]

Hello Sreejith, 

I am afraid that without stable reproduction it would be hard to identify the root cause of the issue. However, I encourage you to try the latest version, since R1 2018 we have addressed a few fixes regarding multithreading, they could resolve this particular problem too. 

I hope the provided information helps. Any other relevant information that might help for the reproduction and further investigation is also appreciated.  

Regards,

Ivo
Progress Telerik

[jh-bkraemer]

We have the same issue appearing with the latest version:

15-Feb-2022 10:28:48	  Error Message:
15-Feb-2022 10:28:48	   System.InvalidOperationException : Collection was modified; enumeration operation may not execute.
15-Feb-2022 10:28:48	  Stack Trace:
15-Feb-2022 10:28:48	     at System.Collections.Generic.Dictionary`2.Enumerator.MoveNext()
15-Feb-2022 10:28:48	   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
15-Feb-2022 10:28:48	   at System.Linq.Enumerable.SelectManyIterator[TSource,TCollection,TResult](IEnumerable`1 source, Func`2 collectionSelector, Func`3 resultSelector)+MoveNext()
15-Feb-2022 10:28:48	   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
15-Feb-2022 10:28:48	   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
15-Feb-2022 10:28:48	   at Telerik.JustMock.Core.MocksRepository.GetDebugView(Object mock)
15-Feb-2022 10:28:48	   at Telerik.JustMock.DebugView.<>c.<get_CurrentState>b__1_0()
15-Feb-2022 10:28:48	   at Telerik.JustMock.Core.ProfilerInterceptor.GuardInternal[T](Func`1 guardedAction)
15-Feb-2022 10:28:48	   at Telerik.JustMock.DebugView.TraceEvent(IndentLevel traceLevel, Func`1 message)
15-Feb-2022 10:28:48	   at Telerik.JustMock.Core.Context.MockingContext.ResolveRepository(UnresolvedContextBehavior unresolvedContextBehavior)
15-Feb-2022 10:28:48	   at Telerik.JustMock.Mock.<>c__DisplayClass1_0`1.<Arrange>b__0()
15-Feb-2022 10:28:48	   at Telerik.JustMock.Core.ProfilerInterceptor.GuardInternal[T](Func`1 guardedAction)

In the console runner, we are not expecting the DebugView to do anything. Especially not trying to run over IEnumerables that per definition might only be runnable once. Is it possible to disable the DebugView when not needed to avoid issues like this? Your internals seem to access DebugView.CurrentState which leads to this issues.

It is triggered by a simple call, that is not related to anything. So this seems to be some behind-the-scene issue.

Mock.Arrange(() => Task.Delay(Arg.AnyInt, Arg.IsAny<CancellationToken>())).Returns(Task.CompletedTask);

[Ivo]

Hello Benjamin,

Many thanks for your findings, they could be extremely helpful for us to find the root cause of the issue. I am going to experiment a little bit and come back with a conclusion in a day or two.

Regards,
Ivo
Progress Telerik

[Karina]

Hello, 

Just to give more input, the problem Benjamin describes is only happening when the DebugView is enabled.

DebugView.IsTraceEnabled = true;

Regards.

[Ivo]

Hello Karina, thanks a lot for the input, it puts another important detail to the whole picture.