如何:使用 Task.WhenAll 扩展演练(C# 和 Visual Basic)

使用 Task.WhenAll 方法,可以改进"解决方案的性能。演练:使用 Async 和 Await 访问 Web(C# 和 Visual Basic) 的。 此方法异步等待多个异步操作,表示为任务的集合。

您在本演练中可能已经注意到网站下载以不同的速率。 有时一个网站非常慢,延迟所有保持下载。 当您运行在本演练中创建的异步解决方案时,可以轻松地关闭程序,如果您不希望等待,但是,一种更好的选项是启动同时下载并允许更快地下载继续运行,而不必等待延迟的脚本。

应用 Task.WhenAll 方法于集合。 WhenAll 的应用程序返回不完整的单个任务,直到集合中的每个任务完成。 任务是并行运行,但是,其他线程不会创建。 任务可以按任意顺序完成。

重要

下面的过程介绍扩展到 演练:使用 Async 和 Await 访问 Web(C# 和 Visual Basic)中开发的异步应用程序。您可以通过完成此演练或下载代码开发应用程序。开发人员代码示例

若要运行该示例,需要 Visual Studio 2012 中,Visual Studio express 2012 中的 windows 桌面或在您的计算机上安装 .NET framework 4.5。

添加 Task.WhenAll 到您的 GetURLContentsAsync 解决方案

  1. 添加 ProcessURLAsync 方法添加到 演练:使用 Async 和 Await 访问 Web(C# 和 Visual Basic)中开发的第一个应用程序。

    • 如果要下载代码中 开发人员代码示例,打开 AsyncWalkthrough 项目,然后添加 ProcessURLAsync 到 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件。

    • 如果通过完成该演练中开发的代码,请添加 ProcessURLAsync 到包含 GetURLContentsAsync 方法的应用程序。 此应用程序的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件位于“完成代码示例从演练”部分中的第一个示例。

    ProcessURLAsync 方法合并了 For Each 或 foreach 循环主体的事件。SumPageSizesAsync 中的原始演练。 方法异步下载一个指定网站的内容作为字节数组,然后显示并返回字节数组的长度。

    Private Async Function ProcessURLAsync(url As String) As Task(Of Integer)
    
        Dim byteArray = Await GetURLContentsAsync(url)
        DisplayResults(url, byteArray)
        Return byteArray.Length
    End Function
    
    private async Task<int> ProcessURLAsync(string url)
    {
        var byteArray = await GetURLContentsAsync(url);
        DisplayResults(url, byteArray);
        return byteArray.Length;
    }
    
  2. 掉或删除在 SumPageSizesAsync的 For Each 循环,或 foreach,下面的代码显示。

    'Dim total = 0
    'For Each url In urlList
    
    '    Dim urlContents As Byte() = Await GetURLContentsAsync(url)
    
    '    ' The previous line abbreviates the following two assignment statements.
    
    '    ' GetURLContentsAsync returns a task. At completion, the task
    '    ' produces a byte array.
    '    'Dim getContentsTask As Task(Of Byte()) = GetURLContentsAsync(url)
    '    'Dim urlContents As Byte() = Await getContentsTask
    
    '    DisplayResults(url, urlContents)
    
    '    ' Update the total.
    '    total += urlContents.Length
    'Next
    
    //var total = 0;
    //foreach (var url in urlList)
    //{
    //    byte[] urlContents = await GetURLContentsAsync(url);
    
    //    // The previous line abbreviates the following two assignment statements.
    //    // GetURLContentsAsync returns a Task<T>. At completion, the task
    //    // produces a byte array.
    //    //Task<byte[]> getContentsTask = GetURLContentsAsync(url);
    //    //byte[] urlContents = await getContentsTask;
    
    //    DisplayResults(url, urlContents);
    
    //    // Update the total.          
    //    total += urlContents.Length;
    //}
    
  3. 创建任务的集合。 那么,当做到 ToArray<TSource> 方法,创建任务的集合下载每个网站目录的以下代码定义 查询。 在执行查询时计算时,任务启动。

    将以下代码添加到方法 SumPageSizesAsync 在 urlList的声明之后。

    ' Create a query. 
    Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
        From url In urlList Select ProcessURLAsync(url)
    
    ' Use ToArray to execute the query and start the download tasks.
    Dim downloadTasks As Task(Of Integer)() = downloadTasksQuery.ToArray()
    
    // Create a query. 
    IEnumerable<Task<int>> downloadTasksQuery = 
        from url in urlList select ProcessURLAsync(url);
    
    // Use ToArray to execute the query and start the download tasks.
    Task<int>[] downloadTasks = downloadTasksQuery.ToArray();
    
  4. 应用 Task.WhenAll 于集合,downloadTasks。 Task.WhenAll 返回完成的单个任务,这些任务的集合的所有任务完成。

    在下面的示例中,Await 或 await 表达式等待 WhenAll 返回单个任务的完成。 该表达式计算为 integer,每个整数为一个下载网站的长度。 将以下代码添加到 SumPageSizesAsync,在上一步添加的代码后面。

    ' Await the completion of all the running tasks.
    Dim lengths As Integer() = Await Task.WhenAll(downloadTasks)
    
    '' The previous line is equivalent to the following two statements.
    'Dim whenAllTask As Task(Of Integer()) = Task.WhenAll(downloadTasks)
    'Dim lengths As Integer() = Await whenAllTask
    
    // Await the completion of all the running tasks.
    int[] lengths = await Task.WhenAll(downloadTasks);
    
    //// The previous line is equivalent to the following two statements.
    //Task<int[]> whenAllTask = Task.WhenAll(downloadTasks);
    //int[] lengths = await whenAllTask;
    
  5. 最后,使用 Sum 方法计算所有网站的长度之和。 将下行添加到 SumPageSizesAsync。

    Dim total = lengths.Sum()
    
    int total = lengths.Sum();
    

添加 Task.WhenAll 到 HttpClient.GetByteArrayAsync 解决方案

  1. 添加 ProcessURLAsync 的以下版本到 演练:使用 Async 和 Await 访问 Web(C# 和 Visual Basic)中开发的第二个应用程序。

    • 如果要下载代码中 开发人员代码示例,打开 AsyncWalkthrough_HttpClient 项目,然后添加 ProcessURLAsync 到 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件。

    • 如果通过完成该演练中开发的代码,请添加 ProcessURLAsync 到使用 HttpClient.GetByteArrayAsync 方法的应用程序。 此应用程序的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件位于“完成代码示例从演练”部分的第二个示例。

    ProcessURLAsync 方法合并了 For Each 或 foreach 循环主体的事件。SumPageSizesAsync 中的原始演练。 方法异步下载一个指定网站的内容作为字节数组,然后显示并返回字节数组的长度。

    从 ProcessURLAsync 方法的唯一区别在前面的过程是使用 HttpClient 实例,client。

    Private Async Function ProcessURLAsync(url As String, client As HttpClient) As Task(Of Integer)
    
        Dim byteArray = Await client.GetByteArrayAsync(url)
        DisplayResults(url, byteArray)
        Return byteArray.Length
    End Function
    
    async Task<int> ProcessURL(string url, HttpClient client)
    {
        byte[] byteArray = await client.GetByteArrayAsync(url);
        DisplayResults(url, byteArray);
        return byteArray.Length;
    }
    
  2. 掉或删除在 SumPageSizesAsync的 For Each 循环,或 foreach,下面的代码显示。

    'Dim total = 0
    'For Each url In urlList
    '    ' GetByteArrayAsync returns a task. At completion, the task
    '    ' produces a byte array.
    '    Dim urlContents As Byte() = Await client.GetByteArrayAsync(url)
    
    '    ' The following two lines can replace the previous assignment statement.
    '    'Dim getContentsTask As Task(Of Byte()) = client.GetByteArrayAsync(url)
    '    'Dim urlContents As Byte() = Await getContentsTask
    
    '    DisplayResults(url, urlContents)
    
    '    ' Update the total.
    '    total += urlContents.Length
    'Next
    
    //var total = 0;
    //foreach (var url in urlList)
    //{
    //    // GetByteArrayAsync returns a Task<T>. At completion, the task
    //    // produces a byte array.
    //    byte[] urlContent = await client.GetByteArrayAsync(url);
    
    //    // The previous line abbreviates the following two assignment
    //    // statements.
    //    Task<byte[]> getContentTask = client.GetByteArrayAsync(url);
    //    byte[] urlContent = await getContentTask;
    
    //    DisplayResults(url, urlContent);
    
    //    // Update the total.
    //    total += urlContent.Length;
    //}
    
  3. 定义,那么,当做到 ToArray<TSource> 方法,创建任务的集合下载每个网站内容的 查询。 在执行查询时计算时,任务启动。

    将以下代码添加到方法 SumPageSizesAsync 在 client 和 urlList的声明之后。

    ' Create a query.
    Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
        From url In urlList Select ProcessURLAsync(url, client)
    
    ' Use ToArray to execute the query and start the download tasks.
    Dim downloadTasks As Task(Of Integer)() = downloadTasksQuery.ToArray()
    
    // Create a query.
    IEnumerable<Task<int>> downloadTasksQuery = 
        from url in urlList select ProcessURL(url, client);
    
    // Use ToArray to execute the query and start the download tasks.
    Task<int>[] downloadTasks = downloadTasksQuery.ToArray();
    
  4. 接下来,将 Task.WhenAll 于集合,downloadTasks。 Task.WhenAll 返回完成的单个任务,这些任务的集合的所有任务完成。

    在下面的示例中,Await 或 await 表达式等待 WhenAll 返回单个任务的完成。 完成后,Await 或 await 表达式计算为 integer,每个整数为一个下载网站的长度。 将以下代码添加到 SumPageSizesAsync,在上一步添加的代码后面。

    ' Await the completion of all the running tasks.
    Dim lengths As Integer() = Await Task.WhenAll(downloadTasks)
    
    '' The previous line is equivalent to the following two statements.
    'Dim whenAllTask As Task(Of Integer()) = Task.WhenAll(downloadTasks)
    'Dim lengths As Integer() = Await whenAllTask
    
    // Await the completion of all the running tasks.
    int[] lengths = await Task.WhenAll(downloadTasks);
    
    //// The previous line is equivalent to the following two statements.
    //Task<int[]> whenAllTask = Task.WhenAll(downloadTasks);
    //int[] lengths = await whenAllTask;
    
  5. 最后,使用 Sum 方法获取所有网站的长度之和。 将下行添加到 SumPageSizesAsync。

    Dim total = lengths.Sum()
    
    int total = lengths.Sum();
    

测试 Task.WhenAll 解决方案

示例

下面的代码演示对使用 GetURLContentsAsync 方法下载从 web 内容的项目。

' Add the following Imports statements, and add a reference for System.Net.Http.
Imports System.Net.Http
Imports System.Net
Imports System.IO


Class MainWindow

    Async Sub startButton_Click(sender As Object, e As RoutedEventArgs) Handles startButton.Click

        resultsTextBox.Clear()

        ' One-step async call.
        Await SumPageSizesAsync()

        '' Two-step async call.
        'Dim sumTask As Task = SumPageSizesAsync()
        'Await sumTask

        resultsTextBox.Text &= vbCrLf & "Control returned to button1_Click."
    End Sub


    Private Async Function SumPageSizesAsync() As Task

        ' Make a list of web addresses.
        Dim urlList As List(Of String) = SetUpURLList()

        ' Create a query. 
        Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
            From url In urlList Select ProcessURLAsync(url)

        ' Use ToArray to execute the query and start the download tasks.
        Dim downloadTasks As Task(Of Integer)() = downloadTasksQuery.ToArray()

        ' You can do other work here before awaiting.

        ' Await the completion of all the running tasks.
        Dim lengths As Integer() = Await Task.WhenAll(downloadTasks)

        '' The previous line is equivalent to the following two statements.
        'Dim whenAllTask As Task(Of Integer()) = Task.WhenAll(downloadTasks)
        'Dim lengths As Integer() = Await whenAllTask

        Dim total = lengths.Sum()

        'Dim total = 0
        'For Each url In urlList

        '    Dim urlContents As Byte() = Await GetURLContentsAsync(url)

        '    ' The previous line abbreviates the following two assignment statements.

        '    ' GetURLContentsAsync returns a task. At completion, the task
        '    ' produces a byte array.
        '    'Dim getContentsTask As Task(Of Byte()) = GetURLContentsAsync(url)
        '    'Dim urlContents As Byte() = Await getContentsTask

        '    DisplayResults(url, urlContents)

        '    ' Update the total.
        '    total += urlContents.Length
        'Next

        ' Display the total count for all of the web addresses.
        resultsTextBox.Text &= String.Format(vbCrLf & vbCrLf &
                                             "Total bytes returned:  {0}" & vbCrLf, total)
    End Function


    Private Function SetUpURLList() As List(Of String)

        Dim urls = New List(Of String) From
            {
                "http://msdn.microsoft.com",
                "http://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "http://msdn.microsoft.com/en-us/library/ee256749.aspx",
                "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
    End Function


    ' The actions from the foreach loop are moved to this async method.
    Private Async Function ProcessURLAsync(url As String) As Task(Of Integer)

        Dim byteArray = Await GetURLContentsAsync(url)
        DisplayResults(url, byteArray)
        Return byteArray.Length
    End Function


    Private Async Function GetURLContentsAsync(url As String) As Task(Of Byte())

        ' The downloaded resource ends up in the variable named content.
        Dim content = New MemoryStream()

        ' Initialize an HttpWebRequest for the current URL.
        Dim webReq = CType(WebRequest.Create(url), HttpWebRequest)

        ' Send the request to the Internet resource and wait for
        ' the response.
        Using response As WebResponse = Await webReq.GetResponseAsync()
            ' Get the data stream that is associated with the specified URL.
            Using responseStream As Stream = response.GetResponseStream()
                ' Read the bytes in responseStream and copy them to content.  
                ' CopyToAsync returns a Task, not a Task<T>.
                Await responseStream.CopyToAsync(content)
            End Using
        End Using

        ' Return the result as a byte array.
        Return content.ToArray()
    End Function



    Private Sub DisplayResults(url As String, content As Byte())

        ' Display the length of each website. The string format 
        ' is designed to be used with a monospaced font, such as
        ' Lucida Console or Global Monospace.
        Dim bytes = content.Length
        ' Strip off the "http://".
        Dim displayURL = url.Replace("http://", "")
        resultsTextBox.Text &= String.Format(vbCrLf & "{0,-58} {1,8}", displayURL, bytes)
    End Sub

End Class
// Add the following using directives, and add a reference for System.Net.Http.
using System.Net.Http;
using System.IO;
using System.Net;

namespace AsyncExampleWPF_WhenAll
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }


        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Clear();

            // Two-step async call.
            Task sumTask = SumPageSizesAsync();
            await sumTask;

            // One-step async call.
            //await SumPageSizesAsync();

            resultsTextBox.Text += "\r\nControl returned to startButton_Click.\r\n";
        }


        private async Task SumPageSizesAsync()
        {
            // Make a list of web addresses.
            List<string> urlList = SetUpURLList();

            // Create a query. 
            IEnumerable<Task<int>> downloadTasksQuery = 
                from url in urlList select ProcessURLAsync(url);

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

            // You can do other work here before awaiting.

            // Await the completion of all the running tasks.
            int[] lengths = await Task.WhenAll(downloadTasks);

            //// The previous line is equivalent to the following two statements.
            //Task<int[]> whenAllTask = Task.WhenAll(downloadTasks);
            //int[] lengths = await whenAllTask;

            int total = lengths.Sum();

            //var total = 0;
            //foreach (var url in urlList)
            //{
            //    byte[] urlContents = await GetURLContentsAsync(url);

            //    // The previous line abbreviates the following two assignment statements.
            //    // GetURLContentsAsync returns a Task<T>. At completion, the task
            //    // produces a byte array.
            //    //Task<byte[]> getContentsTask = GetURLContentsAsync(url);
            //    //byte[] urlContents = await getContentsTask;

            //    DisplayResults(url, urlContents);

            //    // Update the total.          
            //    total += urlContents.Length;
            //}

            // Display the total count for all of the websites.
            resultsTextBox.Text +=
                string.Format("\r\n\r\nTotal bytes returned:  {0}\r\n", total);
        }


        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/ee256749.aspx",
                "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;
        }



        // The actions from the foreach loop are moved to this async method.
        private async Task<int> ProcessURLAsync(string url)
        {
            var byteArray = await GetURLContentsAsync(url);
            DisplayResults(url, byteArray);
            return byteArray.Length;
        }


        private async Task<byte[]> GetURLContentsAsync(string url)
        {
            // The downloaded resource ends up in the variable named content.
            var content = new MemoryStream();

            // Initialize an HttpWebRequest for the current URL.
            var webReq = (HttpWebRequest)WebRequest.Create(url);

            // Send the request to the Internet resource and wait for
            // the response.
            using (WebResponse response = await webReq.GetResponseAsync())
            {
                // Get the data stream that is associated with the specified url.
                using (Stream responseStream = response.GetResponseStream())
                {
                    await responseStream.CopyToAsync(content);
                }
            }

            // Return the result as a byte array.
            return content.ToArray();

        }


        private void DisplayResults(string url, byte[] content)
        {
            // Display the length of each website. The string format 
            // is designed to be used with a monospaced font, such as
            // Lucida Console or Global Monospace.
            var bytes = content.Length;
            // Strip off the "http://".
            var displayURL = url.Replace("http://", "");
            resultsTextBox.Text += string.Format("\n{0,-58} {1,8}", displayURL, bytes);

        }
    }
}

下面的代码演示对使用方法 HttpClient.GetByteArrayAsync 下载从 web 内容的项目。

' Add the following Imports statements, and add a reference for System.Net.Http.
Imports System.Net.Http
Imports System.Net
Imports System.IO

Class MainWindow

    Async Sub startButton_Click(sender As Object, e As RoutedEventArgs) Handles startButton.Click

        resultsTextBox.Clear()

        '' One-step async call.
        Await SumPageSizesAsync()

        '' Two-step async call.
        'Dim sumTask As Task = SumPageSizesAsync()
        'Await sumTask

        resultsTextBox.Text &= vbCrLf & "Control returned to button1_Click."
    End Sub


    Private Async Function SumPageSizesAsync() As Task

        ' Declare an HttpClient object and increase the buffer size. The
        ' default buffer size is 65,536.
        Dim client As HttpClient =
            New HttpClient() With {.MaxResponseContentBufferSize = 1000000}

        ' Make a list of web addresses.
        Dim urlList As List(Of String) = SetUpURLList()

        ' Create a query.
        Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
            From url In urlList Select ProcessURLAsync(url, client)

        ' Use ToArray to execute the query and start the download tasks.
        Dim downloadTasks As Task(Of Integer)() = downloadTasksQuery.ToArray()

        ' You can do other work here before awaiting.

        ' Await the completion of all the running tasks.
        Dim lengths As Integer() = Await Task.WhenAll(downloadTasks)

        '' The previous line is equivalent to the following two statements.
        'Dim whenAllTask As Task(Of Integer()) = Task.WhenAll(downloadTasks)
        'Dim lengths As Integer() = Await whenAllTask

        Dim total = lengths.Sum()

        'Dim total = 0
        'For Each url In urlList
        '    ' GetByteArrayAsync returns a task. At completion, the task
        '    ' produces a byte array.
        '    Dim urlContents As Byte() = Await client.GetByteArrayAsync(url)

        '    ' The following two lines can replace the previous assignment statement.
        '    'Dim getContentsTask As Task(Of Byte()) = client.GetByteArrayAsync(url)
        '    'Dim urlContents As Byte() = Await getContentsTask

        '    DisplayResults(url, urlContents)

        '    ' Update the total.
        '    total += urlContents.Length
        'Next

        ' Display the total count for all of the web addresses.
        resultsTextBox.Text &= String.Format(vbCrLf & vbCrLf &
                                             "Total bytes returned:  {0}" & vbCrLf, total)
    End Function


    Private Function SetUpURLList() As List(Of String)

        Dim urls = New List(Of String) From
            {
                "http://www.msdn.com",
                "http://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "http://msdn.microsoft.com/en-us/library/ee256749.aspx",
                "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
    End Function

    Private Async Function ProcessURLAsync(url As String, client As HttpClient) As Task(Of Integer)

        Dim byteArray = Await client.GetByteArrayAsync(url)
        DisplayResults(url, byteArray)
        Return byteArray.Length
    End Function


    Private Sub DisplayResults(url As String, content As Byte())

        ' Display the length of each website. The string format 
        ' is designed to be used with a monospaced font, such as
        ' Lucida Console or Global Monospace.
        Dim bytes = content.Length
        ' Strip off the "http://".
        Dim displayURL = url.Replace("http://", "")
        resultsTextBox.Text &= String.Format(vbCrLf & "{0,-58} {1,8}", displayURL, bytes)
    End Sub

End Class
// Add the following using directives, and add a reference for System.Net.Http.
using System.Net.Http;
using System.IO;
using System.Net;

namespace AsyncExampleWPF_HttpClient_WhenAll
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Clear();

            // One-step async call.
            await SumPageSizesAsync();

            // Two-step async call.
            //Task sumTask = SumPageSizesAsync();
            //await sumTask;

            resultsTextBox.Text += "\r\nControl returned to startButton_Click.\r\n";
        }


        private async Task SumPageSizesAsync()
        {
            // Make a list of web addresses.
            List<string> urlList = SetUpURLList();


            // Declare an HttpClient object and increase the buffer size. The
            // default buffer size is 65,536.
            HttpClient client = new HttpClient() { MaxResponseContentBufferSize = 1000000 };

            // Create a query.
            IEnumerable<Task<int>> downloadTasksQuery = 
                from url in urlList select ProcessURL(url, client);

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

            // You can do other work here before awaiting.

            // Await the completion of all the running tasks.
            int[] lengths = await Task.WhenAll(downloadTasks);

            //// The previous line is equivalent to the following two statements.
            //Task<int[]> whenAllTask = Task.WhenAll(downloadTasks);
            //int[] lengths = await whenAllTask;

            int total = lengths.Sum();

            //var total = 0;
            //foreach (var url in urlList)
            //{
            //    // GetByteArrayAsync returns a Task<T>. At completion, the task
            //    // produces a byte array.
            //    byte[] urlContent = await client.GetByteArrayAsync(url);

            //    // The previous line abbreviates the following two assignment
            //    // statements.
            //    Task<byte[]> getContentTask = client.GetByteArrayAsync(url);
            //    byte[] urlContent = await getContentTask;

            //    DisplayResults(url, urlContent);

            //    // Update the total.
            //    total += urlContent.Length;
            //}

            // Display the total count for all of the web addresses.
            resultsTextBox.Text +=
                string.Format("\r\n\r\nTotal bytes returned:  {0}\r\n", total);
        }


        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "http://msdn.microsoft.com",
                "http://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "http://msdn.microsoft.com/en-us/library/ee256749.aspx",
                "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;
        }


        // The actions from the foreach loop are moved to this async method.
        async Task<int> ProcessURL(string url, HttpClient client)
        {
            byte[] byteArray = await client.GetByteArrayAsync(url);
            DisplayResults(url, byteArray);
            return byteArray.Length;
        }


        private void DisplayResults(string url, byte[] content)
        {
            // Display the length of each web site. The string format 
            // is designed to be used with a monospaced font, such as
            // Lucida Console or Global Monospace.
            var bytes = content.Length;
            // Strip off the "http://".
            var displayURL = url.Replace("http://", "");
            resultsTextBox.Text += string.Format("\n{0,-58} {1,8}", displayURL, bytes);
        }
    }
}

请参见

任务

演练:使用 Async 和 Await 访问 Web(C# 和 Visual Basic)

参考

Task.WhenAll