A Task's exception(s) were not observed

Question

I'm watching an error in my logs of Application Insight web application, but I don't find where it's coming and I don't find any strange behaviour in my app or web api (.NET 4.5 with Web API 2.0, using OutputCache and Web Api 2 Cache, MongoDB in Azure like Web App with 2 instances):

A Task's exception(s) were not observed either by Waiting on the Task or accessing its Exception property. As a result, the unobserved exception was rethrown by the finalizer thread.Object reference not set to an instance of an object.

I can't find where the error comes from, I will show you some example code, I'm using a lot of my Web Api and Javascript to get the data:

Example Controller:

public Task<IActionResult> GetDataFromMyMongoDB(string id){

         var data = await _service.GetData(id);

         if(data == null)
               return BadRequest("Error");

         return Ok(data);
}

Service:

public Task<ICollection<MyDto>> GetData(id)
{
    //Check security operations for example, or business login like this:

    var data = await _repositoryData.Where(d=>d.Id == id);

    //For example, sometimes I do some business logic operations in which
    // I use try ... catch (this code is just for show example code)

    try
    {
       var dataToJson = JSON.parse(data);

       //Dummy code dataJson to obtain a value of this and changed and update
       //to repository

       //** Code omited **//

       var bool = await _repository.UpdateManyAsync(data);

       if(bool == false)
         return null;

       else 
         {
            //Here I do typical mapper operations from my
            //entities to my DTOs 

            //** Code omited **//

            return mappedListDto; // <-- Type: ICollection<MyDto>
         }  

    }
    catch(Exception ex)
    {
       //Log in Elmah
       //Elmah stuff code here

       //** Code omited **//

       return null;

    }





}

My Repository is standard code for make simple operations and is working fine in unit testing, and I have used my web app many times and more than 100 users at the same time without problem, but in my Logs, I have the exception of the task.

I have read this question and this one too, but I can't understand how to apply in my code, I feel stupid.

EDIT: More information: I have gotten about 500 exception of this type in the last 12 hours but the application is working fine, in the last 12 hours I have about 167 K request.

Stack Trace:

System.AggregateException:
System.NullReferenceException:
   at System.Web.ThreadContext.AssociateWithCurrentThread (System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a)
   at System.Web.HttpApplication.OnThreadEnterPrivate (System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a)
   at System.Web.HttpApplication.System.Web.Util.ISyncContext.Enter (System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a)
   at System.Web.Util.SynchronizationHelper.SafeWrapCallback (System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a)
   at System.Threading.Tasks.Task.Execute (mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089)

Any ideas? I'm totally crazy with that... Thank you so much!

Answer 1

There is probably nothing wrong. Sounds like AppInsights is being a bit paranoid.

This is most commonly caused by one of two things.

One is a "wait for any" kind of logical fork in your code. This is usually some code that uses Task.WhenAny. I recommend searching your code for "WhenAny".

The other is a "fire and forget" kind of logic. This is usually code that calls Task.Run and then ignores the returned task. The compiler is pretty good at warning you about fire-and-forget code (since it's almost always a mistake), so the first thing to check is your compiler warnings.

Since you have a unit test suite, you can hook up a handler for TaskScheduler.UnobservedTaskException, which will catch these exceptions, and then run/debug your test suite and see under which tests that gets hit. If you want to quiet down AppInsights, you can add a handler in your production code that calls UnobservedTaskExceptionEventArgs.SetObserved.