如何：使用 Task.WhenAll 擴充非同步逐步解說的內容 (Visual Basic)

發行項
05/22/2023

您可以使用 Task.WhenAll 方法，來提升逐步解說：使用 Async 和 Await 存取 Web (Visual Basic) 中非同步方案的效能。此方法會以非同步方式等候多個非同步作業進行，這些作業是以工作集合來表示。

您在此逐步解說中可能已注意到網站下載的速度各自不同。有時其中一個網站的速度很慢，而導致所有其餘下載延後執行。當您執行在此逐步解說中建立的非同步方案時，如果不想要等候，您可以輕鬆地結束程式；但更好的做法是同時啟動所有下載，並讓較快的下載繼續執行而不等候延遲的下載。

您可以將 Task.WhenAll 方法套用至工作集合。套用 WhenAll 會傳回未完成的單一工作，直到集合中的所有工作都完成為止。工作似乎會平行執行，但不會建立其他任何執行緒。工作可以依任何順序完成。

重要

下列程序描述在逐步解說：使用 Async 和 Await 存取 Web (Visual Basic) 中所開發非同步應用程式的擴充。您可以藉由完成此逐步解說或者從 .NET 範例瀏覽器下載範例來開發應用程式。範例程式碼位於 SerialAsyncExample 專案中。

若要執行範例，您必須在電腦上安裝 Visual Studio 2012 或更新版本。

將 Task.WhenAll 新增至您的 GetURLContentsAsync 方案

將 ProcessURLAsync 方法新增至在逐步解說：使用 Async 和 Await 存取 Web (Visual Basic) 中開發的第一個應用程式。
- 如果您已從開發人員程式碼範例下載程式碼，請開啟 AsyncWalkthrough 專案，然後將 ProcessURLAsync 新增至 MainWindow.xaml.vb 檔案。
- 如果您藉由完成此逐步解說來開發程式碼，請將 ProcessURLAsync 新增至包含 GetURLContentsAsync 方法的應用程式。此應用程式的 MainWindow.xaml.vb 檔案是＜逐步解說中完整的程式碼範例＞一節的第一個範例。
ProcessURLAsync 方法會合併原始逐步解說中 SumPageSizesAsync 之 For Each 迴圈主體內的動作。此方法會以非同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
```

將 SumPageSizesAsync 中的 For Each 迴圈註解化或刪除，如下列程式碼所示。

'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

建立工作集合。下列程式碼定義一個查詢，當 ToArray 方法執行此查詢時，會建立工作集合以下載每個網站的內容。工作會在評估查詢之後啟動。

將下列程式碼新增至 urlList 宣告後面的 SumPageSizesAsync 方法。

' 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()

將 Task.WhenAll 套用至工作集合 downloadTasks。 Task.WhenAll 會傳回當工作集合中所有工作完成後才會完成的單一工作。

在下列範例中，Await 運算式會等候 WhenAll 傳回的單一工作完成。此運算式會評估為整數陣列，其中每個整數都是所下載網站的長度。將下列程式碼新增至 SumPageSizesAsync，就在您於上一個步驟中新增的程式碼之後。
```
' 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
```
最後，使用 Sum 方法來計算所有網站的長度總和。將下列程式碼行新增至 SumPageSizesAsync。
```
Dim total = lengths.Sum()
```

將 Task.WhenAll 新增至 HttpClient.GetByteArrayAsync 方案

將下列版本的 ProcessURLAsync 新增至在逐步解說：使用 Async 和 Await 存取 Web (Visual Basic) 中開發的第二個應用程式。
- 如果您已從開發人員程式碼範例下載程式碼，請開啟 AsyncWalkthrough_HttpClient 專案，然後將 ProcessURLAsync 新增至 MainWindow.xaml.vb 檔案。
- 如果您藉由完成此逐步解說來開發程式碼，請將 ProcessURLAsync 新增至使用 HttpClient.GetByteArrayAsync 方法的應用程式。此應用程式的 MainWindow.xaml.vb 檔案是＜逐步解說中完整的程式碼範例＞一節的第二個範例。
ProcessURLAsync 方法會合併原始逐步解說中 SumPageSizesAsync 之 For Each 迴圈主體內的動作。此方法會以非同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陣列的長度。

其與上一個步驟中 ProcessURLAsync 方法的唯一差別，在於使用了 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
```

將 SumPageSizesAsync 中的 For Each 迴圈註解化或刪除，如下列程式碼所示。

'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

定義一個查詢，當 ToArray 方法執行此查詢時，會建立工作集合以下載每個網站的內容。工作會在評估查詢之後啟動。

將下列程式碼新增至 client 和 urlList 宣告後面的 SumPageSizesAsync 方法。

' 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，將 Task.WhenAll 套用至工作集合 downloadTasks。 Task.WhenAll 會傳回當工作集合中所有工作完成後才會完成的單一工作。

在下列範例中，Await 運算式會等候 WhenAll 傳回的單一工作完成。完成時，Await 運算式會評估為整數陣列，其中每個整數都是所下載網站的長度。將下列程式碼新增至 SumPageSizesAsync，就在您於上一個步驟中新增的程式碼之後。
```
' 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
```
最後，使用 Sum 方法取得所有網站的長度總和。將下列程式碼行新增至 SumPageSizesAsync。
```
Dim total = lengths.Sum()
```

測試 Task.WhenAll 方案

針對任一方案，選擇 F5 鍵以執行程式，然後選擇 [開始] 按鈕。輸出應類似於逐步解說：使用 Async 和 Await 存取 Web (Visual Basic) 中的非同步方案輸出。不過請注意，網站每次出現的順序都不同。

範例 1

下列程式碼顯示專案擴充，其使用 GetURLContentsAsync 方法從 Web 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

下列程式碼顯示專案擴充，其使用 HttpClient.GetByteArrayAsync 方法從 Web 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

        ' 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

共用方式為