C# How to catch exception in a Task without block UI using wait

Question

I have this code in a button click

private async void btnStart_Click(object sender, EventArgs e)
    {
        Msg.Clear();
        stopWatch.Reset();
        timer.Start();
        stopWatch.Start();
        lblTime.Text = stopWatch.Elapsed.TotalSeconds.ToString("#");
        progressBar.MarqueeAnimationSpeed = 30;
        try
        {
            await Task.Factory.StartNew(() =>
             {
                 try
                 {
                     Reprocess();
                 }
                 catch (Exception ex)
                 {
                     Msg.Add(new clsMSG(ex.Message, "Error", DateTime.Now));
                     timer.Stop();
                     stopWatch.Stop();
                     throw;
                 }
             });
        }
        catch
        {
            throw;
        }
    }

and this on the Reprocess method

private void Reprocess()
    {
        try
        {
            clsReprocess reprocess = new clsReprocess(tbBD.Text, dtpStart.Value, 50000);
            reprocess.Start(reprocess.BD);
        }
        catch
        {
            throw;
        }
    }

when the Reprocess method fails, the Task goes to catch, but the throw fails (the throw inside catch (Exception ex)) and the UI blocks until the reprocess.Start method is completed. I have two questions:

First: How can I catch the throw in the catch of my button?
Second: How can I prevent the UI blocks?

I hope you can understand me, sorry for my bad english.

Why did you insert those catch handlers that do nothing? I always wondered why people are doing this hence I'm asking. — usr
– usr, Commented Feb 9, 2016 at 22:34
I'm catching the error (on the Task) for show in a DatagridView, the problem is that the system continues doing the Reprocess method, after that catch — Abdel Posada
– Abdel Posada, Commented Feb 9, 2016 at 22:39
System catch an exception (on reprocess.Start(reprocess.BD)) — Abdel Posada
– Abdel Posada, Commented Feb 9, 2016 at 22:45
Please cleanup your code, get rid of unnecessary try .. catch { throw; } blocks, use Task.Run instead of Task.Factory.StartNew, move your catch (Exception ex) {...} block below the await, and don't EVER access any UI controls from a background thread. — Yarik
– Yarik, Commented Feb 10, 2016 at 0:02

Stephen Cleary · Accepted Answer · 2016-02-10 10:41:31Z

1

You should not use Task.Factory.StartNew; Task.Run is both safer and shorter to write.

Also, you can only access UI controls from the UI thread. This may be the cause of the problems you're seeing, if Msg is data-bound to the UI. Even if it's not, you don't want to access unprotected collections (e.g., List<clsMSG>) from multiple threads.

Applying both of these guidelines reduces the code to:

private async void btnStart_Click(object sender, EventArgs e)
{
  Msg.Clear();
  stopWatch.Reset();
  timer.Start();
  stopWatch.Start();
  lblTime.Text = stopWatch.Elapsed.TotalSeconds.ToString("#");
  progressBar.MarqueeAnimationSpeed = 30;
  try
  {
    await Task.Run(() => Reprocess());
  }
  catch (Exception ex)
  {
    Msg.Add(new clsMSG(ex.Message, "Error", DateTime.Now));
    timer.Stop();
    stopWatch.Stop();
    throw;
  }
}

If Reprocess throws an exception, that exception will be placed on the task returned from Task.Run. When your code awaits that task, that exception is re-raised and caught in the catch. At the end of the catch, the code will re-raise that exception (throw;).

answered Feb 10, 2016 at 10:41

Stephen Cleary

461k78 gold badges722 silver badges858 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Abdel Posada Over a year ago

Should I disable Just My Code? because if it's enabled when Reprocess throws the exception the catch on the button doesn't catch it, I need to press F5 to continue.

Stephen Cleary Over a year ago

@AbdelPosada: I think it's fine the way it is.

Collectives™ on Stack Overflow

C# How to catch exception in a Task without block UI using wait

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related