Async/await for long-running API methods with progress/cancelation

Question

Edit I suppose the proper way of forcing await to invoke the worker asynchronously is with a Task.Run, like this:

await Task.Run(() => buil         


        

Answer 1

As noted on the top of that blog post, the information in that post is outdated. You should use the new IProgress<T> API provided in .NET 4.5.

If you're using blocking I/O, then make your core method blocking:

public void Build(string sInputFileName, CancellationToken cancel, IProgress<int> progress)
{
  using (StreamReader reader = new StreamReader(sInputFileName))
  {
    int nLine = 0;
    int nTotalLines = CountLines(sInputFileName);

    while ((sLine = reader.ReadLine()) != null)
    {
      nLine++;
      // do something here...
      cancel.ThrowIfCancellationRequested();
      if (progress != null) progress.Report(nLine * 100 / nTotalLines);
    }

    return nLine;
  }
}

and then wrap it in Task.Run when you call it:

private async void OnDoSomethingClick(object sender, RoutedEventArgs e)
{
  OpenFileDialog dlg = new OpenFileDialog { Filter = "Text Files (*.txt)|*.txt" };
  if (dlg.ShowDialog() == false) return;

  // show the job progress UI...

  CancellationTokenSource cts = new CancellationTokenSource();
  DummyWorker worker = new DummyWorker();
  var progress = new Progress<int>((_, value) => { _progress.Value = value; });
  await Task.Run(() => builder.Build(dlg.FileName, cts.Token, progress);

  // hide the progress UI...
}

Alternatively, you could rewrite Build to use asynchronous APIs and then just call it directly from the event handler without wrapping it in Task.Run.