HOW TO：使用 Task.WhenAll 擴充逐步解說的內容 (C# 和 Visual Basic)

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02/28/2013

您可以使用 Task.WhenAll 方法，改善非同步方案的效能在逐步解說：使用 Async 和 Await 存取 Web (C# 和 Visual Basic) 中。這個方法會以非同步方式等候多個非同步作業，表示為工作的集合。

您在本逐步解說中可能已經注意到網站下載具有不同的速率。有時一個網站的速度很慢，延遲所有剩餘的下載。當您執行您在本逐步解說中建立的非同步方案時，您可以輕易地結束程式，如果您不要等候，不過較好的選擇是同時啟動任何下載並讓較快的下載繼續執行，而不等待延遲的下載。

您將 Task.WhenAll 方法應用至工作的集合。 WhenAll 的應用程式會傳回未完成的單一工作，直到集合中的每一個工作完成。其他工作似乎平行地執行，但是沒有其他執行緒會被建立。工作可以以任何順序完成。

重要

下列程序描述副檔名對應至逐步解說：使用 Async 和 Await 存取 Web (C# 和 Visual Basic)所開發的非同步應用程式。您可以完成這個逐步解說或下載程式碼開發應用程式開發人員程式碼範例。

若要執行此範例，您必須將 Visual Studio 2012 中， Windows 桌面上的 Visual Studio Express 2012 for或者在您的電腦上安裝 .NET Framework 4.5。

將Task.WhenAll 加入至您的方案 GetURLContentsAsync

將 ProcessURLAsync 方法加入至逐步解說：使用 Async 和 Await 存取 Web (C# 和 Visual Basic)所開發的第一個應用程式。
- 如果要下載程式碼中開發人員程式碼範例，開啟 AsyncWalkthrough 專案，然後將 ProcessURLAsync 加入至 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案。
- 如果您已經藉由完成這個逐步解說開發的程式碼，請將 ProcessURLAsync 加入至 GetURLContentsAsync 方法的應用程式。這個應用程式的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案是在「完整程式碼範例會從逐步解說 > 一節中的第一個範例。
ProcessURLAsync 方法保護在 For Each 或 foreach 迴圈主體的動作在 SumPageSizesAsync 中從原始的逐步解說。方法會以非同步方式下載自指定之網站的內容做為位元組陣列，然後顯示並傳回位元組陣列的長度。
```
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;
}
```

標記為註解或刪除在 SumPageSizesAsync中的 For Each 或 foreach 迴圈，下列程式碼所示。

'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;
//}

建立工作的集合。接下來的程式碼定義 ToArray<TSource> 方法，建立工作集合下載每個網站內容的查詢。當查詢評估時，工作開始。

將下列程式碼加入至 SumPageSizesAsync 類別，放在 urlList 宣告之後：

' 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();

套用至 Task.WhenAll 工作的集合， downloadTasks。 Task.WhenAll 傳回當在工作集合中的所有工作完成後才會完成的單一工作。

在下列範例中， Await 或 await 運算式等候 WhenAll 傳回的單一工作已完成。運算式計算結果為整數的陣列，每個整數都是可下載網站的長度。將下列程式碼加入至 SumPageSizesAsync，就在您在上一個步驟中加入的程式碼之後。

' 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;

最後，請使用 Sum 方法來計算所有網站的長度的總和。將下列程式碼加入至 SumPageSizesAsync。
```
Dim total = lengths.Sum()
```
```
int total = lengths.Sum();
```

加入至 Task.WhenAll HttpClient.GetByteArrayAsync 方案

將 ProcessURLAsync 下列版本加入至逐步解說：使用 Async 和 Await 存取 Web (C# 和 Visual Basic)所開發的第二個應用程式。
- 如果要下載程式碼中開發人員程式碼範例，開啟 AsyncWalkthrough_HttpClient 專案，然後將 ProcessURLAsync 加入至 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案。
- 如果您已經藉由完成這個逐步解說開發的程式碼，請將 ProcessURLAsync 加入至使用 HttpClient.GetByteArrayAsync 方法的應用程式。這個應用程式的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案是在「完整程式碼範例會從逐步解說 > 一節的第二個範例。
ProcessURLAsync 方法保護在 For Each 或 foreach 迴圈主體的動作在 SumPageSizesAsync 中從原始的逐步解說。方法會以非同步方式下載自指定之網站的內容做為位元組陣列，然後顯示並傳回位元組陣列的長度。

ProcessURLAsync 方法的唯一差異是在上一個程序是使用 HttpClient 執行個體， 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;
}
```

標記為註解或刪除在 SumPageSizesAsync中的 For Each 或 foreach 迴圈，下列程式碼所示。

'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;
//}

接下來的程式碼定義 ToArray<TSource> 方法，建立下載每個網站內容的工作集合的查詢。當查詢評估時，工作開始。

將下列程式碼加入至方法 SumPageSizesAsync 在 client 和urlList宣告之後：

' 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();

接下來，套用至 Task.WhenAll 工作的集合， downloadTasks。 Task.WhenAll 傳回當在工作集合中的所有工作完成後才會完成的單一工作。

在下列範例中， Await 或 await 運算式等候 WhenAll 傳回的單一工作已完成。當完成時，Await 或 await 運算式計算結果為整數的陣列，每個整數都是可下載網站的長度。將下列程式碼加入至 SumPageSizesAsync，就在您在上一個步驟中加入的程式碼之後。

' 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;

最後，請使用 Sum 方法來計算所有網站的長度的總和。將下列程式碼加入至 SumPageSizesAsync。
```
Dim total = lengths.Sum()
```
```
int total = lengths.Sum();
```

測試 Task.WhenAll 方案

對於任何方案，請選取 F5 鍵執行程式，然後選取 [開始] 按鈕。輸出應類似於從非同步方案的輸出。逐步解說：使用 Async 和 Await 存取 Web (C# 和 Visual Basic)。不過，請注意網站以不同順序每次出現。

範例

下列程式碼顯示擴充對使用 GetURLContentsAsync 方法下載從 Web 內容的專案。

' 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
            {
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "https://msdn.microsoft.com/en-us/library/ee256749.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://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> 
            { 
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/library/windows/apps/br211380.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "https://msdn.microsoft.com/en-us/library/ee256749.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://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);

        }
    }
}

下列程式碼顯示副檔名對應至使用方法 HttpClient.GetByteArrayAsync 下載從網路內容的專案。

' 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",
                "https://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "https://msdn.microsoft.com/en-us/library/ee256749.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://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> 
            { 
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "https://msdn.microsoft.com/en-us/library/ee256749.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://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);
        }
    }
}

共用方式為

HOW TO：使用 Task.WhenAll 擴充逐步解說的內容 (C# 和 Visual Basic)

將Task.WhenAll 加入至您的方案 GetURLContentsAsync

加入至 Task.WhenAll HttpClient.GetByteArrayAsync 方案

測試 Task.WhenAll 方案

範例

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工作

參考

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