在完成一个异步任务后取消剩余任务（C#）

完整实例

• using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Text;
  using System.Threading.Tasks;
  using System.Windows;
  using System.Windows.Controls;
  using System.Windows.Data;
  using System.Windows.Documents;
  using System.Windows.Input;
  using System.Windows.Media;
  using System.Windows.Media.Imaging;
  using System.Windows.Navigation;
  using System.Windows.Shapes;

  // Add a using directive and a reference for System.Net.Http.
  using System.Net.Http;

  // Add the following using directive.
  using System.Threading;

  namespace CancelAfterOneTask
  {
      public partial class MainWindow : Window
      {
          // Declare a System.Threading.CancellationTokenSource.
          CancellationTokenSource cts;

  public MainWindow()
          {
              InitializeComponent();
          }

  private async void startButton_Click(object sender, RoutedEventArgs e)
          {
              // Instantiate the CancellationTokenSource.
              cts = new CancellationTokenSource();

  resultsTextBox.Clear();

  try
              {
                  await AccessTheWebAsync(cts.Token);
                  resultsTextBox.Text += "\r\nDownload complete.";
              }
              catch (OperationCanceledException)
              {
                  resultsTextBox.Text += "\r\nDownload canceled.";
              }
              catch (Exception)
              {
                  resultsTextBox.Text += "\r\nDownload failed.";
              }

  // Set the CancellationTokenSource to null when the download is complete.
              cts = null;
          }

  // You can still include a Cancel button if you want to.
          private void cancelButton_Click(object sender, RoutedEventArgs e)
          {
              if (cts != null)
              {
                  cts.Cancel();
              }
          }

  // Provide a parameter for the CancellationToken.
          async Task AccessTheWebAsync(CancellationToken ct)
          {
              HttpClient client = new HttpClient();

  // Call SetUpURLList to make a list of web addresses.
              List<string> urlList = SetUpURLList();

  // ***Comment out or delete the loop.
              //foreach (var url in urlList)
              //{
              //    // GetAsync returns a Task<HttpResponseMessage>.
              //    // Argument ct carries the message if the Cancel button is chosen.
              //    // ***Note that the Cancel button can cancel all remaining downloads.
              //    HttpResponseMessage response = await client.GetAsync(url, ct);

  //    // Retrieve the website contents from the HttpResponseMessage.
              //    byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

  //    resultsTextBox.Text +=
              //        String.Format("\r\nLength of the downloaded string: {0}.\r\n", urlContents.Length);
              //}

  // ***Create a query that, when executed, returns a collection of tasks.
              IEnumerable<Task<int>> downloadTasksQuery =
                  from url in urlList select ProcessURLAsync(url, client, ct);

  // ***Use ToArray to execute the query and start the download tasks.
              Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

  // ***Call WhenAny and then await the result. The task that finishes
              // first is assigned to firstFinishedTask.
              Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

  // ***Cancel the rest of the downloads. You just want the first one.
              cts.Cancel();

  // ***Await the first completed task and display the results.
              // Run the program several times to demonstrate that different
              // websites can finish first.
              var length = await firstFinishedTask;
              resultsTextBox.Text += String.Format("\r\nLength of the downloaded website:  {0}\r\n", length);
          }

  // ***Bundle the processing steps for a website into one async method.
          async Task<int> ProcessURLAsync(string url, HttpClient client, CancellationToken ct)
          {
              // GetAsync returns a Task<HttpResponseMessage>.
              HttpResponseMessage response = await client.GetAsync(url, ct);

  // Retrieve the website contents from the HttpResponseMessage.
              byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

  return urlContents.Length;
          }

  // Add a method that creates a list of web addresses.
          private List<string> SetUpURLList()
          {
              List<string> urls = new List<string>
              {
                  "http://msdn.microsoft.com",
                  "http://msdn.microsoft.com/en-us/library/hh290138.aspx",
                  "http://msdn.microsoft.com/en-us/library/hh290140.aspx",
                  "http://msdn.microsoft.com/en-us/library/dd470362.aspx",
                  "http://msdn.microsoft.com/en-us/library/aa578028.aspx",
                  "http://msdn.microsoft.com/en-us/library/ms404677.aspx",
                  "http://msdn.microsoft.com/en-us/library/ff730837.aspx"
              };
              return urls;
          }
      }
      // Sample output:

  // Length of the downloaded website:  158856

  // Download complete.
  }