取消异步任务 C# [重复]答案

【问题标题】：Cancel an Async Task C# [duplicate]取消异步任务 C# [重复]
【发布时间】：2015-07-15 19:09:12
【问题描述】：

我希望用户能够取消/中止由异步按钮调用的任务。我看过 CancellationTokens 但我不太明白如何实现它。

“下载”按钮：

private async void dlSelected_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            if (dlDataGrid.SelectedIndex != -1)
            {
                object item = dlDataGrid.SelectedItem;
                string name = (dlDataGrid.SelectedCells[0].Column.GetCellContent(item) as TextBlock).Text;
                string dl = (dlDataGrid.SelectedCells[2].Column.GetCellContent(item) as TextBlock).Text;

                dlSelected.IsEnabled = false;
                refreshList.IsEnabled = false;
                dlPRing.IsActive = true;

                await DownloadFile2(name, dl, "./Data/Downloads/" + name);

            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
    }

“下载”任务：

private async Task DownloadFile2(string name, string url, string path)
    {

        //Set status label
        tbDLStatus.Text = "Downloading File...";

        var DLclient = new WebClient();
        DLclient.DownloadProgressChanged += (s, e) =>
        {
            //Set names for each property
            tbDLPercent.Text = e.ProgressPercentage + "%";
            tbFileName.Text = name;
            tbDir.Text = path;

            double progress = 0;
            if (e.ProgressPercentage > progress)
            {
                //Change value of progressbar to download
                pbProgress.Value = (e.ProgressPercentage / 100.0d);
                progress = e.ProgressPercentage;
            }
        };

        DLclient.DownloadFileCompleted += async (s, e) =>
        {
            if (e.Cancelled)
            {
                //Delete the file
                if (File.Exists(path))
                {
                    File.Delete(path);
                }
            }
            else if (e.Error != null)
            {
                await this.ShowMessageAsync("Error!", "An error occured while attempting to download " + name + "! \n\nPlease check your internet connection and try again in a few minutes.",
                   MessageDialogStyle.Affirmative);

                //Delete the file
                if (File.Exists(path))
                {
                    File.Delete(path);
                }
            }
            else
            {
                //Set for completed DL successfully.
                if (cbAutoRun.IsChecked == true)
                {
                    try
                    {
                        Process.Start(tbDir.Text);
                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.ToString());
                    }
                }

                //Allow open file/folder
                btnOpen.IsEnabled = true;
                btnOpenFolder.IsEnabled = true;
            }

            dlSelected.IsEnabled = true;
            pbProgress.Value = 0;
            tbDLStatus.Text = "Finished Downloading";
            dlPRing.IsActive = false;
        };

        await DLclient.DownloadFileTaskAsync((url), path);
        DLclient.Dispose();
    }

所以我的问题是，如果我正在下载一个大文件并且我想通过一个按钮取消它，我该怎么做？

谢谢，

【问题讨论】：

通过按钮取消异步任务：stackoverflow.com/a/21115961/4945059
这是 Trevor 指出的重复。您应该将下载任务切割成缓冲区，并在每个缓冲区之后检查，如果e.Cancelled。

标签： c#

【解决方案1】：

无法从外部取消任意任务。

幸运的是，WebClient 允许取消（尽管出于某种原因，它不会像应有的那样简单地采用 CancellationToken）：只要您想取消挂起的异步操作，只需调用 WebClient.CancelAsync()。

如果您想使用CancellationToken 包装此功能，您可以执行以下操作：

Task DownloadFileTaskAsync(this WebClient client, string address, string filename, 
                           CancellationToken token)
{
  token.Register(() => client.CancelAsync());

  return client.DownloadFileTaskAsync(address, filename);
}

【讨论】：