c# - 取消异步任务C#

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Canceling a task
                                
                                    （1个答案）
                                
                        
                                4年前关闭。
            
                    
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，但是我不太了解如何实现它。

“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，如果我正在下载大文件，并且想通过按钮取消它，该怎么办？

谢谢，

最佳答案

无法从外部取消任意任务。

幸运的是，WebClient允许取消（尽管由于某些原因，它并不能简单地采用CancellationToken应有的方式）：只要想取消挂起的异步操作，就只需调用WebClient.CancelAsync()即可。

如果要使用CancellationToken包装此功能，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);
}

关于c# - 取消异步任务C#，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/31428580/