Task Parallel Library (TPL)

Question

Hi,

I have just upgraded to .NET 4.0
One of the good things that .NET has is Task Parallel Library (TPL). It is supposed to replace ThreadPool that appeared in previous version of .NET. One of the problems though that I experience is that I cannot catch exceptions within the code that is executed using TPL.

Is there anyone who can give me more information about TPL exceptions?

Thanks in advance

Vulpes · Answer

The position as I understand it is that the TPL automatically catches all exceptions and rethrows them to the caller. This is done because it would otherwise be difficult for the caller to catch exceptions on all the threads that might be used by parallel code. A further problem is that two or more exceptions might be thrown simultaneously. To deal with this, the exception(s) are wrapped in an AggregateException whose InnerExceptions property contains details of all exceptions caught which could even include other AggregateExceptions. The AggregateException class has a couple of useful methods Flatten() and Handle(). The Flatten() method provides a simple list of all exceptions thrown including those nested in other AggregateExceptions. The Handle() method is able to filter out certain types of exception and then rethrow the remainder. For some further details see the MSDN docs ( http://msdn.microsoft.com/en-us/library/system.aggregateexception.aspx ).

Antonios Dev · Answer

I did the following and it solved my problem. The UI is not blocked anymore. I know it is not the best solution but at least it works. Thanks again a lot for your help. private void InserClient(int ClientId) { Task tsk; tsk = Task.Factory.StartNew(() => { AddClient(ClientId) }); } private void AddClient(ClientId) { try { some database code ....... } catch (Exception ex) { this.BeginInvoke((System.Action)delegate { EnableButtons(true); throw ex; }); } }

Vulpes · Answer

On second thoughts, don't use the following line to start the thread as that may lead to the same problems with the exception as before! Task.Factory.StartNew(() => InserClient(ClientId)); Instead use an ordinary Thread object: Thread t = new Thread(()=>InserClient(ClientId)); t.IsBackground = true; t.Start();

Vulpes · Answer

You can avoid explicit marshaling with tsk.ContinueWith() but only if you use the 'synchronization context scheduler' rather than the default TaskScheduler. If you declare this field in your Form class: private TaskScheduler uiScheduler; then in the constructor initialize it as follows: public Form1() { InitializeComponent(); uiScheduler = TaskScheduler.FromCurrentSynchronizationContext(); } and then change InserClient as follows: private void InserClient(int ClientId) { Task tsk; tsk = Task.Factory.StartNew(() => { AddClient(ClientId) }); try { tsk.ContinueWith(x => { LoadClients(); EnableButtons(true); }, uiScheduler ); tsk.Wait(); } catch (AggregateException ex) { EnableButtons(true); throw ex.Flatten(); } } it should avoid the need to use Invoke or BeginInvoke within the Task. However, I'm not sure whether that will cover the EnableButtons(true) statement in the 'catch' clause as that isn't really part of the Task. when you start the Task, from a Button_Click handler or wherever, to start it on a new threadpool thread, you'll now need to do: Task.Factory.StartNew(() => InserClient(ClientId));

Antonios Dev · Answer

Could you give some code for the solution you just suggested? Thanks

Antonios Dev · Answer

I wanted to avoid Invoke or BeginInvoke. I thought tsk.ContinueWith was supposed to do that or I'm mistaken?

Vulpes · Answer

Sorry, part of what I said in my previous post was nonsense. Although the Task should be running asynchronously on a background thread or threads, when Task.Wait is encountered then it will block the calling thread until the Task is completed. So, if Task Wait is being called from the UI thread, then it will block it. A possible solution would be launch a subsidiary thread from the UI thread and call your InserClient method from that. Task.Wait will then only block the subsidiary thread. However, if you're changing the state of any controls, then you'd have to marshal the changes back to the UI thread using Invoke or BeginInvoke.

Antonios Dev · Answer

If I remove Task.Wait() exception is not thrown but UI is fine (no blocking). If I add Task.Wait() I get exception but UI is blocked. This thing started to confuse me. TPL was supposed to be much easier to use that traditional Threading yet is so unclear and confusing.

Vulpes · Answer

Whether the Task is to be run synchronously or asynchronously should have been determined by the scheduler when the Task was started i.e. long before the Wait() method is called. If it's started with Start() or StartNew() then it would normally run asynchronously and so Task.Wait() should block only the background thread. Are you sure that the UI thread isn't being blocked for some other reason such as waiting for a result?

Antonios Dev · Answer

If I call the Task.Wait() the task is executed syncrhonously and not asynchronously. I noticed that because the main thread (UI) os blocked until the Task is completed.

Vulpes · Answer

What we were missing here is that you have to call the Task's Wait() method for the unhandled exceptions to be wrapped in an AggregateException and rethrown on the calling thread. So if you insert the highlighted line, then it should work: private void InserClient(int ClientId) { Task tsk; tsk = Task.Factory.StartNew(() => { AddClient(ClientId) }); try { tsk.ContinueWith(x => { LoadClients(); EnableButtons(true); }); tsk.Wait(); } catch (AggregateException ex) { EnableButtons(true); throw ex.Flatten(); } }

Antonios Dev · Answer

thank a lot for your assistance.

Vulpes · Answer

I've just been playing around with your code, Antonios, and I couldn't get it to work either :( However, the example on MSDN ( http://msdn.microsoft.com/en-us/library/dd997415.aspx ) did work and so we're probably missing something fundamental. I'll have another look later today when I've more time.

Antonios Dev · Answer

if you mean try something like this after first catch block I already tried it and it does not work. catch (Exception ex) { EnableButtons(true); throw ex.Flatten(); }

Sam Hobbs · Answer

What happens if you change the exception that is caught such that all exceptions are caught? You can do that temprarily for diganostic purposes at least. Or you could add a second catch that is a catch-all for other errors, right?

Antonios Dev · Answer

I know I marked it as answered but I m still experiencing problems. I create an exception in AddClient(Convert.toInt32("S")) but the exception is not thrown. Can you help me please private void InserClient(int ClientId) { Task tsk; tsk = Task.Factory.StartNew(() => { AddClient(ClientId) }); try { tsk.ContinueWith(x => { LoadClients(); EnableButtons(true); }); } catch (AggregateException ex) { EnableButtons(true); throw ex.Flatten(); } } private void AddClient(int ClientId) { using (CLIENTSDataContext dataContext = new CLIENTSDataContext(MyConnectionString)) { Convert.ToInt32("S"); dataContext.INSERT_Client(ClientName, ClientAddress); } }

Antonios Dev · Answer

Thanks a lot. I enclosed the result of the Task into Try/Catch which catches AggregateException and Flattens it.