Where could an UnobservedTaskException be occurring?

Question

I have a WPF client that accesses a WCF service. The service runs along very nicely using a timer to execute some processing every n minutes. The client UI has a ProcessNow command, which overrides the service's timer, and performs the processing immediately. I am trying to sort out a usability problem where clicking the Process Now button more than once, while processing is still busy, results in a "connection closed" exception, from the WCF comms channel I assume, followed by a UnobservedTaskException .

The exception message explains that its cause is either not waiting on the task, or not accessing its Exception property. I only have two apparent uses of a Task anywhere in my solution, and one is in my view model:

async void ExecuteProcessNowCommand()
{
        await _proxy.ProcessAsync(true);
}

The other is in a small utility method in the same viewmodel:

private void ExecuteWithTimeout(Action task, int? timeout = null)
{
    timeout = timeout ?? HostServiceConstants.ServiceControlTimeout;
    var source = new CancellationTokenSource();
    var timedTask = new Task(task, source.Token);
    timedTask.Start();
    bool done = timedTask.Wait(timeout.Value);
    if (!done)
    {
        source.Cancel();
        var frame = new StackFrame(1);
        var methodName = frame.GetMethod().Name;
        _logger.Warn("'{0}' timed out after {1} milliseconds.", methodName, timeout);
    }
}

In the first code excerpt, I do seem to be waiting on the task, but I am more suspicious of the second, based on an example I found somewhere online. I use that to try and perform non-blocking checks like:

bool CanExecuteProcessNowCommand()
{
    bool result = false;
    ExecuteWithTimeout(() =>
        {

            try
            {
                result = _proxy.CanUserForceProcessing();
            }
            catch (EndpointNotFoundException)
            {
                result = false;
            }
        });
    return result;
}

The last piece of code is called in my WCF client when I try and keep my command button updated with:

ProcessNow.RaiseCanExecuteChanged();

I am very new to both async and WCF, so please excuse any obvious blunders here.

Answer 1

ExecuteProcessNowCommand will not cause this. If an unhandled exception is thrown in async void method, process is taken down. So that's not the case.

Only thing left is ExecuteWithTimeout . Yes it can cause result in UnObservedTaskException if unhandled exception is thrown in the timedTask .

You're waiting on it, waiting should observe the exception, but you're waiting with a Timeout. If the exception is thrown after the Timeout has been passed, you'll see this behavior.

You need to attach continuation to the timedTask and observe the exception.