C# – A Task’s exception(s) were not observed either by Waiting on the Task or accessing its Exception property

cerror handlingexceptiontaskwpf

These are my tasks. How should i modify them to prevent this error. I checked the other similar threads but i am using wait and continue with. So how come this error happening?

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.

    var CrawlPage = Task.Factory.StartNew(() =>
{
    return crawlPage(srNewCrawledUrl, srNewCrawledPageId, srMainSiteId);
});

var GetLinks = CrawlPage.ContinueWith(resultTask =>
{
    if (CrawlPage.Result == null)
    {
        return null;
    }
    else
    {
        return ReturnLinks(CrawlPage.Result, srNewCrawledUrl, srNewCrawledPageId, srMainSiteId);
    }
});

var InsertMainLinks = GetLinks.ContinueWith(resultTask =>
{
    if (GetLinks.Result == null)
    {

    }
    else
    {
        instertLinksDatabase(srMainSiteURL, srMainSiteId, GetLinks.Result, srNewCrawledPageId, irCrawlDepth.ToString());
    }

});

InsertMainLinks.Wait();
InsertMainLinks.Dispose();

Best Answer

You're not handling any exception.

Change this line:

InsertMainLinks.Wait();

TO:

try { 
    InsertMainLinks.Wait(); 
}
catch (AggregateException ae) { 
    /* Do what you will */ 
}

In general: to prevent the finalizer from re-throwing any unhandled exceptions originating in your worker thread, you can either:

Wait on the thread and catch System.AggregateException, or just read the exception property.

EG:

Task.Factory.StartNew((s) => {      
    throw new Exception("ooga booga");  
}, TaskCreationOptions.None).ContinueWith((Task previous) => {  
    var e=previous.Exception;
    // Do what you will with non-null exception
});

Task.Factory.StartNew((s) => {      
    throw new Exception("ooga booga");  
}, TaskCreationOptions.None).ContinueWith((Task previous) => {      
    try {
        previous.Wait();
    }
    catch (System.AggregateException ae) {
        // Do what you will
    }
});

Related Solutions

C# – 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

If you create a Task, and you don't ever call task.Wait() or try to retrieve the result of a Task<T>, when the task is collected by the garbage collector, it will tear down your application during finalization. For details, see MSDN's page on Exception Handling in the TPL.

The best option here is to "handle" the exception. This can be done via a continuation - you can attach a continuation to the task, and log/swallow/etc the exception that occurs. This provides a clean way to log task exceptions, and can be written as a simple extension method, ie:

public static void LogExceptions(this Task task)
{
    task.ContinueWith( t =>
    {
         var aggException = t.Exception.Flatten();
         foreach(var exception in aggException.InnerExceptions)
             LogException(exception);
    }, 
    TaskContinuationOptions.OnlyOnFaulted);
}

With the above, you can prevent any task from tearing down the app, and logging it, via:

Task.Factory.StartNew( () => 
   { 
       // Do your work...
   }).LogExceptions();

Alternatively, you can subscribe to the TaskScheduler.UnobservedTaskException and handle it there.

C# – How to fix “‘System.AggregateException’ occurred in mscorlib.dll”

As the message says, you have a task which threw an unhandled exception.

Turn on Break on All Exceptions (Debug, Exceptions) and rerun the program.
This will show you the original exception when it was thrown in the first place.

(comment appended): In VS2015 (or above). Select Debug > Options > Debugging > General and unselect the "Enable Just My Code" option.

Best Answer

Related Solutions

C# – 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

C# – How to fix “‘System.AggregateException’ occurred in mscorlib.dll”

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