Imitation of OperationCanceledException scenario with unit-test fails

Question

I use Moq 4.18.2 framework for my tests.

The RtspClient might throw an OperationCanceledException from ConnectAsync. So, I try to test this scenario. My test below throws an exception System.OperationCanceledException: The operation was canceled. and the catch (OperationCanceledException) never gets executed. What am I doing wrong here?

RTSP

public interface IRtspClient : IDisposable
{
    event EventHandler<RawFrame> FrameReceived;

    Task ConnectAsync(CancellationToken token);
    Task ReceiveAsync(CancellationToken token);
}

Method that uses IRtspClient

public async Task ConnectAsync(CancellationToken token = default)
{
    try
    {
        await _rtspClient.ConnectAsync(token).ConfigureAwait(false);
        OnConnected();
    }
    catch (OperationCanceledException ex)
    {
        OnConnectAttemptCanceled(ex);
        throw;
    }
    catch(Exception ex)
    {
        OnFailedToConnect(ex);
        throw;
    }
}

Test

[TestMethod]
public async Task ConnectAsync_Canceled()
{
    var expectedCanceledTaskStatus = true;

    var tcs = new CancellationTokenSource();
    tcs.Cancel();
    var rtspClient = new Mock<IRtspClient>();
    rtspClient
        .Setup(_ => _.ConnectAsync(It.IsAny<CancellationToken>()))
        .Returns(Task.FromException<OperationCanceledException>(new OperationCanceledException()))

    var actualCanceledTaskStatus = false;
    var camera = new MyCamera(rtspClient.Object);
    camera.FailedToConnect += () => actualCanceledTaskStatus = true;
    await camera.ConnectAsync(tcs.Token);

    Assert.AreEqual(expectedCanceledTaskStatus, actualCanceledTaskStatus);
}

UPDATE Added missing await as suggested by @Dai, but my test still fails. Can anyone take a look at the test code?

Dai · Answer 1 · 2022-08-15T05:25:56.057

You need to await the returned Task inside the try{} block - otherwise synchronous control will immediately leave the try{} block.
Exceptions thrown inside an anonymous function (or local function, or lambda method, or ye olde school delegate() local) will not be caught by the catch.
Also, CancellationTokenSource is IDisposable, so you should change your ConnectAsync_Canceled test to wrap it in a using() block.
Also, don't swallow exceptions - so my code below captures both exceptions for possible investigation and re-throws them with throw; (don't use throw ex;: it resets the recorded stack-trace; instead just do throw; by itself).
- With OperationCanceledException and/or TaskCanceledException specifically, the exception needs to be re-thrown right back to the original caller (i.e. the owner of the CancellationTokenSource); whereas if a TaskCanceledException is caught and handled without the corresponding Task "seeing" then the caller will assume the operation succeeded despite the cancelation request.

Change your code to this:

public async Task ConnectAsync( CancellationToken cancellationToken = default )
{
    try
    {
         await this.rtspClient.ConnectAsync(cancellationToken ).ConfigureAwait(false);

         this.OnConnected();
    }
    catch( OperationCanceledException cEx )
    {
        this.OnConnectAttemptCanceled( cEx );

        throw; // Re-throw so the `Task` representing *this method* (`ConnectAsync`) will report as Cancelled rather than Succeeded.
    }
    catch( Exception ex ) 
    {
        this.OnFailedToConnect( ex );

        throw; // Re-throw so the `Task` representing *this method* (`ConnectAsync`) will report as Failed rather than Succeeded.
    }
}

you're right I forgot the await. Can you take a look at the test too? — theateist, Aug 14 '22 at 22:39
@theateist I did look at your test code when I wrote my answer but I don't know enough about your test code to say anything useful at this point. — Dai, Aug 14 '22 at 22:43
I've updated my post. What additional information do you need to find the issue with my test? — theateist, Aug 14 '22 at 22:47
I found my mistake in the test. I added the in my answer that I posted. — theateist, Aug 14 '22 at 23:28

score 0 · Accepted Answer · answered Aug 14 '22 at 23:27

0

I found my mistake (in addition to what @Dai noticed). I should have either put await camera.ConnectAsync from my test in try-catch or used Assert.ThrowsExceptionAsync. I chose the latter. Here is the working test:

[TestMethod]
public async Task ConnectAsync_Canceled()
{
    var expectedTaskCanceledStatus = true;

    var rtspClient = new Mock<IRtspClient>();
    rtspClient
        .Setup(_ => _.ConnectAsync(default(CancellationToken)))
        .Returns(Task.FromException(new OperationCanceledException()));

    var actualTaskCanceledStatus = false;
    var camera = new MyCamera(rtspClient.Object);
    camera.ConnectAttemptCanceled += () => actualTaskCanceledStatus = true;

    await Assert.ThrowsExceptionAsync<OperationCanceledException>(async () => await camera.ConnectAsync());
    Assert.AreEqual(expectedTaskCanceledStatus, actualTaskCanceledStatus);
}

answered Aug 14 '22 at 23:27

theateist

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Don't you have Code Analysis on? There's a warning on by default that's raised whenever a `Task` is left abandoned without being `await`'d. – Dai Aug 14 '22 at 23:30
@Dai, I probably didn't notice. I want to accept your answer since it helped me to find my bug. What's the best way to do this? – theateist Aug 15 '22 at 05:20
May I enquire as to how you have 13.000+ karma on StackOverflow... yet you're asking how to accept an answer? – Dai Aug 15 '22 at 05:22
No. To be fair - your answer didn't solve my issue. It helped me to find the real bug in my test, which I described in my posted answer. So, since yours and my answers make 100% together, I wonder what should I do? I've never been in this situation. Should I select your answer or mine? – theateist Aug 15 '22 at 05:37
Ah, a simple misundrestanding on my part :) - I thought you _literally_ meant that you didn't know how to click the green checkmark, lol – Dai Aug 15 '22 at 05:37

Imitation of OperationCanceledException scenario with unit-test fails

2 Answers2