C# task.Start() doesn't run the method

Question

I'm working on a WPF-MVVM project and I need to implement asynchronous infinite loops in some background threads. What I have done in the ViewModel is

 public TestVM()
 {
    LineIO_Task();
    //some other work
 }

and LineIO_Task is defined as

public void LineIO_Task()
{
    for (int i = 0; i < 7; i++)
    {
         Task GetP = new Task(() => { EnPost(Lines[i]); }, TaskCreationOptions.LongRunning);
         GetP.Start();

    }
}

Lines is an ObservableCollection that is initialized in TestVm. And EnPost is defined as

public async void EnPost(Line l)
{           
    int last = 0;               
    while (true)
    {
        // do the work in the loop
        int pno = l.Com.ReadPostNo();//read a serial port
        if (pno != 0 && pno != last)
        {
            log.WriteLog(pno + " to " + l.ToString());
            Dispatcher.Invoke(() =>
            {   
                // update the UI                     
                l.Posts.First(x => x.IsValid).Num = pno;
                l.Posts.First(x => x.IsValid).IsValid = false;
                LocalDb.InsertPost(l.Num, AssignedPost.ToList().Find(x => x.Num == pno));                            
            });
            pno = last;
        } 
        await Task.Delay(500);                   
    }        
}

I've tried Task.Run(() => Method()), Task.Factory.StartNew(() => Method()),，async Task EnPost() and using a System.Timers.Timer. But no matter which way I use, the EnPost method just doesn't run. I put break-points in the method. It doesn't hit there. Am I using Task wrong?

It's probably running just not stopping. You're on infinite loop. No break or return. — Boxed, Jul 31 '18 at 15:22
@Boxed yes, I need it to be not stopping. I'm reading the serial port every .5 second — user2951219, Jul 31 '18 at 15:27
But it's an infinite loop, so that await will never be reached. — Brian Rudolph, Jul 31 '18 at 15:28
@MarcGravell I'm sorry, it should be in the loop. I have edited my question. — user2951219, Jul 31 '18 at 15:29
@BrianRudolph It's a typo. I'm sorry. I have edited my question. — user2951219, Jul 31 '18 at 15:33
@user2951219 don't use task like that. They aren't threads. There's no reason to create cold tasks. Use Task.Run instead — Panagiotis Kanavos, Aug 01 '18 at 15:49
@user2951219 never use `async void` either. It's only meant for event handlers. If you want to create an asynchronous method that doesn't return anything use `async Task` — Panagiotis Kanavos, Aug 01 '18 at 15:50

Marc Gravell · Answer 1 · 2018-07-31T15:31:51.277

3

I'm guessing this is a "captured variable" issue; try:

for (int i = 0; i < 7; i++)
{
     int index = i;
     Task GetP = new Task(() => { EnPost(Lines[index]); }, TaskCreationOptions.LongRunning);
     GetP.Start();
}

(fixed by question edit) Note however that using the thread-pool for a very long lived task is not a good idea. You might want to use a full thread instead. Also; your TaskDelay may want to be inside the while loop, in which case you can ignore the previous comment, as it is no longer actually a very long lived single piece.

edited Jul 31 '18 at 15:31

answered Jul 31 '18 at 15:22

Marc Gravell

1,026,079
266
2,566
2,900

Would you explain slightly what the 'captured variable' is? – user2951219 Jul 31 '18 at 15:39
The capture issue will likely result in an exception that is likely being swallowed somewhere. If the list has 6 items, which I can infer from the `i < 7` - then each iteration will likely be executed with `i==7` which will cause an `IndexOutOfRangeException` exception. – Brian Rudolph Jul 31 '18 at 15:46
@user2951219 - https://stackoverflow.com/questions/271440/captured-variable-in-a-loop-in-c-sharp – Brian Rudolph Jul 31 '18 at 15:47
@BrianRudolph Making a copy of the variable still doesn't solve the problem. But thanks for your hint, I tried a foreach loop. It finally works. – user2951219 Aug 01 '18 at 02:41

score 0 · Accepted Answer · answered Aug 01 '18 at 02:56

0

Thanks for @Marc and @Brian's answer. They mention the"captured variable" issue, so I tried

foreach (Line l in Lines)
{ ... }

It works finally.

answered Aug 01 '18 at 02:56

user2951219

79
8

C# task.Start() doesn't run the method

2 Answers2