UI blocked when using await to run a time consuming task

Question

I want to build a folder cleaner program. It is expected to report deleted files to a TextBox control at real-time. So I use await Task.Run(() => CleanFolder(folderPath, progress)) function in my button click event. But the UI blocked when running. After a while when the CheanFolder() method run complete, all the deleted files are showed at one time.

namespace FolderCleaner
{
    public partial class MainWindow : Window
    {
        string folderPath;
        string matchPattern;

        private void ButtonOpen_Click(object sender, RoutedEventArgs e)
        {
            FolderBrowserDialog fbd = new FolderBrowserDialog() { Description = "Select a folder" };
            if (fbd.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                folderPath = fbd.SelectedPath;
                textBoxPath.Text = folderPath;
                buttonClean.IsEnabled = true;
                textBoxList.Text = "Folder path: " + folderPath + "\n";
            }
        }

        private async void ButtonClean_Click(object sender, RoutedEventArgs e)
        {
            matchPattern = textBoxPattern.Text;
            buttonOpen.IsEnabled = false;
            buttonClean.IsEnabled = false;
            Progress<string> progress = new Progress<string>(msg =>
            {
                textBoxList.AppendText("File deleted: " + msg + "\n");
                textBoxList.CaretIndex = textBoxList.Text.Length;
                textBoxList.ScrollToEnd();
            });

            try
            {
                await Task.Run(() => CleanFolder(folderPath, progress));

                textBoxList.AppendText("Mission complete!");
                textBoxList.CaretIndex = textBoxList.Text.Length;
                textBoxList.ScrollToEnd();
            }
            catch
            {
                System.Windows.MessageBox.Show("Error!");
            }
            finally
            {
                buttonOpen.IsEnabled = true;
            }
        }

        private void CleanFolder(string path, IProgress<string> progress)
        {
            var filePaths = Directory.EnumerateFiles(path, "*.*", System.IO.SearchOption.AllDirectories);
            foreach (var filePath in filePaths)
            {
                var matchResult = Regex.Match(filePath, matchPattern);
                if (matchResult.Success)
                {
                    File.Delete(filePath);
                    progress.Report(filePath);
                }
            }
        }
    }
}

Answer 1

Thanks for everyone. Thanks to the book C# 6.0 in a nutshell

I have figured out the solution and have a better understanding of async/await.

First of all, Dispatcher.Invoke is not recommended to use since .Net Framework 4.5, task-based asynchrony has become the dominant pattern (using async/awit).

Second, there are a few principles of using async/await:

• The expression after await must be a Task or Task<TResult> object

• If you use async modifier to a method, then the method dont need to return aTaskmethod manually. The compile will wrap the method as aTask` object.

• If you use a method like async Task Foo(), you must use an await keyword in it.

• If there is nothing to await, then remove the async modifier, return a Task object by using return Task.Run(() => { Do Something });. Now you can use await Foo() in the method that calling Foo().

• Task Foo() can not operate UI, but async Task Foo() can.

Answer 2

GUI can`t be controlled from another thread.

But i think, that real problem is that concatenating of string and output to a TextBox is a very inefficient operation.

In your case it is better to show progress of removal in a single line or by using the progress bar.

Here is my solution of your problem (i`ve changed 2 methods):

    private async void ButtonClean_Click(object sender, RoutedEventArgs e)
    {
        matchPattern = textBoxPattern.Text;
        buttonOpen.IsEnabled = false;
        buttonClean.IsEnabled = false;

        await Task.Run(() => CleanFolder(folderPath));

        textBoxList.Text += "Mission complete!";
        buttonOpen.IsEnabled = true;
    }

    private void CleanFolder(string path)
    {
        var filePaths = Directory.EnumerateFiles(path, "*.*", SearchOption.AllDirectories);
        foreach (var filePath in filePaths)
        {
            var matchResult = Regex.Match(filePath, matchPattern);
            if (matchResult.Success)
            {
                File.Delete(filePath);
                System.Windows.Application.Current.Dispatcher.Invoke(delegate
                {
                    // this working fast
                    textBoxList.Text  = "File deleted: " + filePath + "\n";
                    // this working slow and slower over time
                  //textBoxList.Text += "File deleted: " + filePath + "\n";
                    textBoxList.ScrollToEnd();
                });
            }
        }
    }

I hope this will help.