Using async/await: await returns too early

Question

I have a simly Windows Forms app with just a button and a progressbar on it.

Then I have this code:

private async void buttonStart_Click(object sender, EventArgs e)
{
    progressBar.Minimum = 0;
    progressBar.Maximum = 5;
    progressBar.Step = 1;
    progressBar.Value = 0;

    await ConvertFiles();
    MessageBox.Show("ok");
}

private async Task ConvertFiles()
{
    await Task.Run(() => 
    {
        for (int i = 1; i <= 5; i++)
        {
            System.Threading.Thread.Sleep(1000);
            Invoke(new Action(() => progressBar.PerformStep()));
        }
    });
}

The await ConvertFiles(); returns too early, the ok messagebox already appears at about 80% progress.

What am I doing wrong?

If you were to run this code synchronously, would the progress bar work correctly? — Alex, Jan 07 '16 at 15:55
You will definitely want to swap your Sleep for `await Task.Delay` — spender, Jan 07 '16 at 16:03
I can reproduce that. I also tried `progressBar.Value = i`. Is it possible that the implementation of `ProgressBar` has some latency in updating the ui? I checked the sources and it uses `SendMessage` which should be synchronous....very interesting question. — René Vogt, Jan 07 '16 at 16:06
@RenéVogt With similar issues, I had to increment/decrement/increment the current value. Ollie should try and see if this workaround would produce the desired result. — Alex, Jan 07 '16 at 16:11
@Alex: You mean progressBar.Value++, then progressBar.Value--, then again progressBar.Value++? Does not help. — Oliver Kötter, Jan 07 '16 at 16:14

score 6 · Accepted Answer · edited May 23 '17 at 12:15

6

The problem you are experiencing is not related to the async/await, which you use correctly. The await is not returning too early, just the progress bar updates too late. In other words, this is a progress bar control specific problem described in a several threads - Disabling .NET progressbar animation when changing value?, Disable WinForms ProgressBar animation, The RunWorkerCompleted is triggered before the progressbar reaches 100% etc. You can use one of the workarounds provided in those threads.

edited May 23 '17 at 12:15

Community

1
1

answered Jan 07 '16 at 16:14

Ivan Stoev

195,425
15
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343

You are right. This helps: Invoke(new Action(() => { progressBar.Maximum = 101; progressBar.Value = 101; progressBar.Maximum = 100; progressBar.Value = 100; })); – Oliver Kötter Jan 07 '16 at 16:26

score 0 · Answer 2 · answered Jan 07 '16 at 16:17

0

Just to be safe why not move the

MessageBox.Show("ok");

into a Continuewith so:

            await ConvertFiles().ContinueWith((t) => { MessageBox.Show("ok"); });

this makes sure it only runs when the task is complete

answered Jan 07 '16 at 16:17

edwin the duck

95
1
8

Using async/await: await returns too early

2 Answers2