How to catch exception thrown by a task that's not awaited in a third party library?

Question

I have a Windows Service (using Topshelf) that relies on the third party library EasyRedisMQ . Unfortunately, I just discovered the following code in one of the library methods:

    public async Task InitializeAsync()
    {
        if (SubscriberInfo == null) throw new NullReferenceException("SubscriberInfo is required.");
        if (string.IsNullOrWhiteSpace(SubscriberInfo.SubscriberId)) throw new NullReferenceException("SubscriberId is required");
        if (string.IsNullOrWhiteSpace(SubscriberInfo.ExchangeName)) throw new NullReferenceException("ExchangeName is required");
        if (string.IsNullOrWhiteSpace(SubscriberInfo.QueueName)) throw new NullReferenceException("QueueName is required");
        if (OnMessageAsync == null) throw new NullReferenceException("OnMessageAsync is required");

        await _cacheClient.SubscribeAsync<string>(SubscriberInfo.ExchangeName, DoWorkAsync);

        DoWorkAsync("").FireAndForget();
    }

Here DoWorkAsync returns a task, but as FireAndForget indicates, this is unfortunately not awaited. Actually, FireAndForget is a method with an empty body (the only purpose is to make it explicitly that the task is not awaited). See the full source code here .

The problem is that an exception occasionally is thrown in DoWorkAsync, which causes the service to crash:

{
  "Depth": 0,
  "ClassName": "StackExchange.Redis.RedisConnectionException",
  "Message": "SocketFailure on RPOP",
  "Source": "mscorlib",
  "StackTraceString": "   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at StackExchange.Redis.Extensions.Core.StackExchangeRedisCacheClient.<ListGetFromRightAsync>d__67`1.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at EasyRedisMQ.Models.Subscriber`1.<GetNextMessageAsync>d__12.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at EasyRedisMQ.Models.Subscriber`1.<DoWorkAsync>d__13.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at StackExchange.Redis.Extensions.Core.StackExchangeRedisCacheClient.<>c__DisplayClass59_0`1.<<SubscribeAsync>b__0>d.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<>c.<ThrowAsync>b__6_1(Object state)\r\n   at System.Threading.QueueUserWorkItemCallback.WaitCallback_Context(Object state)\r\n   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)\r\n   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)\r\n   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()\r\n   at System.Threading.ThreadPoolWorkQueue.Dispatch()\r\n   at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback()",
...
}

I would very much like to catch the exception (as it's actually harmless and simply retry the operation). However, after reading this StackOverflow question and experimenting with AppDomain.CurrentDomain.FirstChanceException, I do not have high hopes. The problem with FirstChanceException is that it occurs outside Tophelf's

HostFactory.Run(hostConfigurator => { ... }

method, in which I have the reference to the service (where the business logic resides):

hostConfigurator.Service<IConsumer>(serviceConfigurator =>
{
    serviceConfigurator.ConstructUsing(() => IocContainer.IocContainer.Instance.Resolve<IConsumer>());
    serviceConfigurator.WhenStarted(consumer => { /* Here I have control */ });
    ...
}

Has anyone any ideas on how to deal with this situation?

Answer 1

Subscribe to event of TaskScheduler:

TaskScheduler.UnobservedTaskException += TaskScheduler_UnobservedTaskException;
//...

private void TaskScheduler_UnobservedTaskException(object sender, UnobservedTaskExceptionEventArgs e)
{
    e.SetObserved();
}