如何使用 Task.WhenAll 擴充異步教學（適用於 Visual Basic）

您可以在逐步解說：使用 Async 和 Await 存取 Web 中，透過使用 Task.WhenAll 方法來提升異步解決方案的效能。這個方法會異步地等待多個異步操作，這些操作以一組任務來表示。

您可能已在操作演示中注意到網站會以不同的速度下載。有時候其中一個網站很慢，這會延遲所有剩餘的下載。當您在逐步解說中執行建置的異步解決方案時，如果您不想等候，則可以輕鬆地結束程式，但更好的選項是同時啟動所有下載，讓更快的下載繼續，而不需要等待延遲的下載。

您將 Task.WhenAll 方法套用到任務集合。應用程式 WhenAll 傳回一個單一工作，該工作在集合中的每個工作完成之前都未完成。工作似乎會以平行方式執行，但不會建立其他線程。工作可以依任何順序完成。

這很重要

下列步驟說明針對使用 Async 和 Await 存取 Web (Visual Basic) 所開發的異步應用程式所做的擴充功能。您可以藉由完成此逐步解說，或從 .NET 範例瀏覽器下載程式碼來開發應用程式。範例程式碼位於 SerialAsyncExample 專案中。

若要執行此範例，您必須在計算機上安裝Visual Studio 2012或更新版本。

若要將 Task.WhenAll 新增至 GetURLContentsAsync 解決方案

在逐步解說ProcessURLAsync中，將方法新增至所開發的第一個應用程式。
- 如果您從開發人員程式代碼範例下載程式代碼，請開啟 AsyncWalkthrough 專案，然後將新增 ProcessURLAsync 至MainWindow.xaml.vb檔案。
- 如果您藉由完成逐步解說來開發程式代碼，請將新增 ProcessURLAsync 至包含 GetURLContentsAsync 方法的應用程式。此應用程式的 MainWindow.xaml.vb 檔案是「逐步解說中的完整程式碼範例」章節的第一個實例。
方法 ProcessURLAsync 合併了原始教學中 For Each 迴圈 SumPageSizesAsync 主體的動作。方法會以異步方式將指定網站的內容下載為位元組陣列，然後顯示並傳回位元組數位的長度。
```
Private Async Function ProcessURLAsync(url As String) As Task(Of Integer)

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

將For Each中的迴圈註解掉或刪除SumPageSizesAsync，如下列程式代碼所示。

'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

建立任務的清單。下列程式代碼定義了一個查詢，當該查詢由方法執行時，會建立一組工作來下載每個網站的內容。系統會在評估查詢時啟動工作。

在宣告SumPageSizesAsync之後，將下列程式代碼新增至方法urlList。
```
' 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()
```
套用 Task.WhenAll 至工作集合， downloadTasks。 Task.WhenAll 會傳回當工作集合中的所有工作都已完成時完成的單一工作。

在下列範例中，Await 表達式會等候 WhenAll 傳回的單一工作完成。表達式會評估為整數陣列，其中每個整數都是下載的網站長度。將下列程式代碼新增至 SumPageSizesAsync，就在您在上一個步驟中新增的程式代碼之後。
```
' 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
```
最後，使用 Sum 方法來計算所有網站的長度總和。將下列這一行新增至 SumPageSizesAsync。
```
Dim total = lengths.Sum()
```

若要在 HttpClient.GetByteArrayAsync 解決方案中新增 Task.WhenAll

請將以下版本的 ProcessURLAsync 新增至使用於逐步解說——《使用 Async 和 Await 存取 Web（Visual Basic）》中開發的第二個應用程式。
- 如果您從開發人員程式代碼範例下載程式代碼，請開啟AsyncWalkthrough_HttpClient專案，然後將新增 ProcessURLAsync 至MainWindow.xaml.vb檔案。
- 如果您藉由完成逐步解說來開發程式代碼，請將新增 ProcessURLAsync 至使用 HttpClient.GetByteArrayAsync 方法的應用程式。此應用程式的MainWindow.xaml.vb檔案是〈逐步解說中的完整程式碼範例〉章節的第二個範例。
方法 ProcessURLAsync 合併了原始教學中 For Each 迴圈 SumPageSizesAsync 主體的動作。方法會以異步方式將指定網站的內容下載為位元組陣列，然後顯示並傳回位元組數位的長度。

與上一個程式中 ProcessURLAsync 方法的唯一差異在於使用 HttpClient 實體 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
```

將For Each中的迴圈註解掉或刪除SumPageSizesAsync，如下列程式代碼所示。

'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

定義查詢，當由方法執行時，建立一個工作集合，以下載每個網站的內容。系統會在評估查詢時啟動工作。

在宣告SumPageSizesAsync和client之後，將下列程式碼新增到方法urlList。

' 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()

接下來，套用Task.WhenAll至工作集合。 downloadTasks Task.WhenAll 會傳回當工作集合中的所有工作都已完成時完成的單一工作。

在下列範例中，Await 表達式會等候 WhenAll 傳回的單一工作完成。完成後，Await 表達式會評估為一個整數陣列，其中每個整數代表下載網站的長度。將下列程式代碼新增至 SumPageSizesAsync，就在您在上一個步驟中新增的程式代碼之後。
```
' 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
```
最後，使用 Sum 方法來取得所有網站的長度總和。將下列這一行新增至 SumPageSizesAsync。
```
Dim total = lengths.Sum()
```

進行 Task.WhenAll 方法的測試

針對任一解決方案，選擇 F5 鍵來執行程式，然後選擇 [ 開始 ] 按鈕。輸出的結果應該與逐步解說：使用 Async 和 Await 存取 Web (Visual Basic)中的異步解決方案所產生的結果類似。不過，請注意，每次網站都會以不同的順序顯示。

範例 1

下列程式碼顯示專案的GetURLContentsAsync擴充功能，該專案使用GetURLContentsAsync方法從網路下載內容。

' 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
        'NextNext

        ' 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/library/hh290136.aspx",
                "https://msdn.microsoft.com/library/ee256749.aspx",
                "https://msdn.microsoft.com/library/hh290138.aspx",
                "https://msdn.microsoft.com/library/hh290140.aspx",
                "https://msdn.microsoft.com/library/dd470362.aspx",
                "https://msdn.microsoft.com/library/aa578028.aspx",
                "https://msdn.microsoft.com/library/ms404677.aspx",
                "https://msdn.microsoft.com/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 "https://".
        Dim displayURL = url.Replace("https://", "")
        resultsTextBox.Text &= String.Format(vbCrLf & "{0,-58} {1,8}", displayURL, bytes)
    End Sub

End Class

範例 2

下列程式代碼顯示使用方法 HttpClient.GetByteArrayAsync 從 Web 下載內容之專案的延伸模組。

' 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()

        ''<snippet7>
        'Dim total = 0
        'For Each url In urlList
        '    ' GetByteArrayAsync returns a task. At completion, the task
        '    ' produces a byte array.
        '    '<snippet31>
        '    Dim urlContents As Byte() = Await client.GetByteArrayAsync(url)
        '    '</snippet31>

        '    ' 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
        'NextNext

        ' 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://www.msdn.com",
                "https://msdn.microsoft.com/library/hh290136.aspx",
                "https://msdn.microsoft.com/library/ee256749.aspx",
                "https://msdn.microsoft.com/library/hh290138.aspx",
                "https://msdn.microsoft.com/library/hh290140.aspx",
                "https://msdn.microsoft.com/library/dd470362.aspx",
                "https://msdn.microsoft.com/library/aa578028.aspx",
                "https://msdn.microsoft.com/library/ms404677.aspx",
                "https://msdn.microsoft.com/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 "https://".
        Dim displayURL = url.Replace("https://", "")
        resultsTextBox.Text &= String.Format(vbCrLf & "{0,-58} {1,8}", displayURL, bytes)
    End Sub

End Class

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Last updated on 2025-06-22