实现多个异步任务的WPF应用程序中的Dispatcher

Question

在以下WPF应用程序的MSDN示例中，该示例演示了多个异步Tasks的async/await实现，显然没有使用/需要Dispatcher对象，即异步执行的Tasks似乎可以直接访问UI控件（在本例中为resultTextBox TextBox控件-请参阅行resultsTextBox.Text += String.Format("rnLength of the download: {0}", length);）。该应用程序已经过测试，性能如预期。

然而，如果该实现能够正确处理可能的竞争条件，则问题仍然存在，例如，如果等待并完成的Task尝试访问该TextBox控制，而后者仍在处理先前完成的Task的更新？在实际意义上，是否仍然需要WPF Dispatcher对象来处理async/await多任务实现中的这种潜在并发/竞争条件问题（或者，可能是在这种异步/等待编程结构中以某种方式隐式地实现了互锁功能）？

清单1。MSDN文章"启动多个异步任务并在完成时对其进行处理"(https://msdn.microsoft.com/en-us/library/jj155756.aspx)

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 ProcessTasksAsTheyFinish
{
    public partial class MainWindow : Window
    {
        // Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;
        public MainWindow()
        {
            InitializeComponent();
        }
        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Clear();
            // Instantiate the CancellationTokenSource.
            cts = new CancellationTokenSource();
            try
            {
                await AccessTheWebAsync(cts.Token);
                resultsTextBox.Text += "rnDownloads complete.";
            }
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "rnDownloads canceled.rn";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "rnDownloads failed.rn";
            }
            cts = null;
        }

        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }

        async Task AccessTheWebAsync(CancellationToken ct)
        {
            HttpClient client = new HttpClient();
            // Make a list of web addresses.
            List<string> urlList = SetUpURLList();
            // ***Create a query that, when executed, returns a collection of tasks.
            IEnumerable<Task<int>> downloadTasksQuery =
                from url in urlList select ProcessURL(url, client, ct);
            // ***Use ToList to execute the query and start the tasks. 
            List<Task<int>> downloadTasks = downloadTasksQuery.ToList();
            // ***Add a loop to process the tasks one at a time until none remain.
            while (downloadTasks.Count > 0)
            {
                    // Identify the first task that completes.
                    Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);
                    // ***Remove the selected task from the list so that you don't
                    // process it more than once.
                    downloadTasks.Remove(firstFinishedTask);
                    // Await the completed task.
                    int length = await firstFinishedTask;
                    resultsTextBox.Text += String.Format("rnLength of the download:  {0}", length);
            }
        }

        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "http://msdn.microsoft.com",
                "http://msdn.microsoft.com/library/windows/apps/br211380.aspx",
                "http://msdn.microsoft.com/en-us/library/hh290136.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;
        }

        async Task<int> ProcessURL(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;
        }
    }
}

注意：我要感谢Stephen Cleary的出色回答和相当有见地的解释，还想强调他的解决方案中概述的建议改进，即：用封装在一行代码中的相当紧凑的解决方案替换利用WhenAny的原始MSDN示例中不必要/复杂的代码块，即：await Task.WhenAll(downloadTasks);（顺便说一句，我在许多实际应用程序中使用了这种替代方案，特别是处理多个股票网络查询的在线市场数据应用程序）。非常感谢，斯蒂芬！

Answer 1

然而，问题仍然存在，这个实现是否能够正确处理可能的竞争条件，例如，如果等待和完成的任务试图访问该TextBox控件，而后者仍在处理以前完成的任务的更新？

没有比赛条件。UI线程一次只做一件事。

在实际意义上，在异步/等待多任务实现中，是否仍然需要WPF Dispatcher对象来处理这种潜在的并发/竞争条件问题（或者，可能是在这种异步/等待编程结构中以某种方式隐式地实现了互锁功能）？

是的，但您不必明确使用它。正如我在async介绍中所描述的，await关键字（默认情况下）将捕获当前上下文，并在该上下文中继续执行async方法。"上下文"为SynchronizationContext.Current（如果当前SyncCtx为null，则为TaskScheduler.Current）。

在这种情况下，它将捕获一个UI SynchronizationContext，它在后台使用WPF Dispatcher来调度UI线程上async方法的剩余部分。

顺便说一句，我不太喜欢"Task.WhenAny列表并在完成时从列表中删除"的方法。我发现如果通过添加DownloadAndUpdateAsync方法进行重构，代码会干净得多：

async Task AccessTheWebAsync(CancellationToken ct)
{
  HttpClient client = new HttpClient();
  // Make a list of web addresses.
  List<string> urlList = SetUpURLList();
  // ***Create a query that, when executed, returns a collection of tasks.
  IEnumerable<Task> downloadTasksQuery =
        from url in urlList select DownloadAndUpdateAsync(url, client, ct);
  // ***Use ToList to execute the query and start the tasks. 
  List<Task> downloadTasks = downloadTasksQuery.ToList();
  await Task.WhenAll(downloadTasks);
}
async Task DownloadAndUpdateAsync(string url, HttpClient client, CancellationToken ct)
{
  var length = await ProcessURLAsync(url, client, ct);
  resultsTextBox.Text += String.Format("rnLength of the download:  {0}", length);
}
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;
}