Moq to return one set of values on first invocation and then to return second set of values for all susequent invocations

Question

I have tried

Option 1

         var invocations = 0;
        _mockSqsMessageManager.Setup(x => x.GetMessages())
            .ReturnsAsync(() => invocations > 0 ? new List<SqsMessage>() : messages)
            .Callback(() =>
            {
                invocations++;
            });

Option 2

            _mockSqsMessageManager.Setup(x => x.GetMessages())
            .ReturnsAsync(messages)
            .Callback(() =>
            {
                _mockSqsMessageManager.Setup(x => x.GetMessages())
                    .ReturnsAsync(new List<SqsMessage>());
            });

Option 3

            var invocations = 0;
        _mockSqsMessageManager.Setup(x => x.GetMessages())
            .ReturnsAsync(messages)
            .Callback(() =>
            {
                invocations++;
                if (invocations > 0)
                {
                    messages = new List<SqsMessage>();
                }
            });

Option 4

        _mockSqsMessageManager.Setup(x => x.GetMessages())
            .ReturnsAsync(() =>
            {
                var invocations = _mockSqsMessageManager.Invocations.Count(x => x.MatchingSetup.Expression.Body.ToString() == "x.GetMessages()");
                return invocations > 0 ? new List<SqsMessage>() : messages;
            });

Option 5

            _mockSqsMessageManager.SetupSequence(x => x.GetMessages())
            .ReturnsAsync(messages)
            .ReturnsAsync(new List<SqsMessage>());

None have worked reliably. Option 5 works but if I only have the exact amount of invocations. I may have more than 100+ invocations so I can't use SetupSequence.

Here's a similar question https://stackoverflow.com/questions/11308328/setupsequence-in-moq with some other options. — juharr, Sep 23 '22 at 13:34
I have answered my own question, please check. Your recommendation for the post prompted me to think about thread safety. I have a working solution now with locks. — Vergil C., Oct 04 '22 at 09:44

Vergil C. · Accepted Answer · 2022-10-04T09:48:40.870

My issue was thread safety since the mock was being called from a TransformBlock of TPL

Creating a lock and ensuring unique access to the resource worked.

object getMessagesLocker = new();
var fetchedMessages = false;
var noMessages = Enumerable.Empty<SqsMessage>();

_mockSqsMessageManager.Setup(x => x.GetMessages())
.ReturnsAsync(() =>
{
    lock (getMessagesLocker)
    {
        if (fetchedMessages == false)
        {
            fetchedMessages = true;
            return messages;
        }

        return noMessages;
    }
});

BTW, I tested option 4 on single-threaded execution where mock is called only on from the main thread and that works fine.

Moq to return one set of values on first invocation and then to return second set of values for all susequent invocations

1 Answers1