Task Parallel Library and exception

Question

I have an application that are executing 4 different jobs parallel. I use the parallel task library.

The code looks like this:

while (true)
        {
            var fetch = new FetcherHandler();

            var tasks = new Task[4];

            tasks[0] = Task.Factory.StartNew(fetch.GetJobs);

            tasks[1] = Task.Factory.StartNew(fetch.GetStatusNew);

            tasks[2] = Task.Factory.StartNew(fetch.GetJobsForStatus);

            tasks[3] = Task.Factory.StartNew(fetch.GetStatusOld);
            Task.WaitAll(tasks);

            Thread.Sleep(10000);
        }

The thread.Sleep above will never be reached since all these 4 tasks is in a never ending loop. Code example for the first task:

public void GetJobs()
        {
    while (true)
            {
                try
                    {
                        handler.GetNewJob();
                        }
                    catch (Exception exception)
                    {
                            var mail = new MailService();
                            mail.SendExeption("Error has accured in GetJobs", exception);
                            throw;
                        }
                    }
                }
                Thread.Sleep(15000); }}

The above code works great, it does some jobs and then it sleeps for 15 sec, and then it does it again. The problem occurres when I get an exception.

The exception is caught alright, and sends me an mail, but the thred doesn't continue in the loop after the exception, so the GetNewJob() method will never get executed again.

Any ideas what happens to the thread when exception hits, and how I can "save" the thread so it can continue?

Not related to your question, but what is the point of waiting for neverending tasks? — , Apr 16 '12 at 08:39

Tigran · Accepted Answer · 2012-04-16T08:47:07.613

2

Becasue you throw an exception, so the exception propagates on top of the stack and break execution of the source thread.

catch (Exception exception)
{
    var mail = new MailService();
    mail.SendExeption("Error has accured in GetJobs", exception);
    throw; //THIS LINE !
}

You can use an event to raise it, say, ExceptionCaught(..) and listen for that event inside your program.

edited Apr 16 '12 at 08:47

answered Apr 16 '12 at 08:37

Tigran

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Thanks! Do you know if this throw kills the whole application or only the thread? – Fore Apr 16 '12 at 08:57

score 1 · Answer 2 · edited May 23 '17 at 11:49

1

Just remove the throw; at the end of your catch clause.

From MSDN:

A throw statement can be used in a catch block to re-throw the exception that is caught by the catch statement.

So you're rethrowing the caught exception which causes the thread to interrupt, never executing again...

More information: What happens when a .NET thread throws an exception?

edited May 23 '17 at 11:49

Community

1
1

answered Apr 16 '12 at 08:36

Matten

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Task Parallel Library and exception

2 Answers2