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How to: Extend the Async Walkthrough by Using Task.WhenAll (Visual Basic)

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You can improve the performance of the async solution in Walkthrough: Accessing the Web by Using Async and Await (Visual Basic) by using the Task.WhenAll method. This method asynchronously awaits multiple asynchronous operations, which are represented as a collection of tasks.

You might have noticed in the walkthrough that the websites download at different rates. Sometimes one of the websites is very slow, which delays all the remaining downloads. When you run the asynchronous solutions that you build in the walkthrough, you can end the program easily if you don't want to wait, but a better option would be to start all the downloads at the same time and let faster downloads continue without waiting for the one that’s delayed.

You apply the Task.WhenAll method to a collection of tasks. The application of WhenAll returns a single task that isn’t complete until every task in the collection is completed. The tasks appear to run in parallel, but no additional threads are created. The tasks can complete in any order.

Important

The following procedures describe extensions to the async applications that are developed in Walkthrough: Accessing the Web by Using Async and Await (Visual Basic). You can develop the applications by either completing the walkthrough or downloading the sample from the .NET Sample Browser. The example code is in the SerialAsyncExample project.

To run the example, you must have Visual Studio 2012 or later installed on your computer.

To add Task.WhenAll to your GetURLContentsAsync solution

  1. Add the ProcessURLAsync method to the first application that's developed in Walkthrough: Accessing the Web by Using Async and Await (Visual Basic).

    • If you downloaded the code from Developer Code Samples, open the AsyncWalkthrough project, and then add ProcessURLAsync to the MainWindow.xaml.vb file.

    • If you developed the code by completing the walkthrough, add ProcessURLAsync to the application that includes the GetURLContentsAsync method. The MainWindow.xaml.vb file for this application is the first example in the "Complete Code Examples from the Walkthrough" section.

    The ProcessURLAsync method consolidates the actions in the body of the For Each loop in SumPageSizesAsync in the original walkthrough. The method asynchronously downloads the contents of a specified website as a byte array, and then displays and returns the length of the byte array.

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

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

  2. Comment out or delete the For Each loop in SumPageSizesAsync, as the following code shows.

    '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

  3. Create a collection of tasks. The following code defines a query that, when executed by the ToArray method, creates a collection of tasks that download the contents of each website. The tasks are started when the query is evaluated.

    Add the following code to method SumPageSizesAsync after the declaration of 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()

  4. Apply Task.WhenAll to the collection of tasks, downloadTasks. Task.WhenAll returns a single task that finishes when all the tasks in the collection of tasks have completed.

    In the following example, the Await expression awaits the completion of the single task that WhenAll returns. The expression evaluates to an array of integers, where each integer is the length of a downloaded website. Add the following code to SumPageSizesAsync, just after the code that you added in the previous step.

    ' 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

  5. Finally, use the Sum method to calculate the sum of the lengths of all the websites. Add the following line to SumPageSizesAsync.

    Dim total = lengths.Sum()

To add Task.WhenAll to the HttpClient.GetByteArrayAsync solution

  1. Add the following version of ProcessURLAsync to the second application that's developed in Walkthrough: Accessing the Web by Using Async and Await (Visual Basic).

    • If you downloaded the code from Developer Code Samples, open the AsyncWalkthrough_HttpClient project, and then add ProcessURLAsync to the MainWindow.xaml.vb file.

    • If you developed the code by completing the walkthrough, add ProcessURLAsync to the application that uses the HttpClient.GetByteArrayAsync method. The MainWindow.xaml.vb file for this application is the second example in the "Complete Code Examples from the Walkthrough" section.

    The ProcessURLAsync method consolidates the actions in the body of the For Each loop in SumPageSizesAsync in the original walkthrough. The method asynchronously downloads the contents of a specified website as a byte array, and then displays and returns the length of the byte array.

    The only difference from the ProcessURLAsync method in the previous procedure is the use of the HttpClient instance, 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

  2. Comment out or delete the For Each loop in SumPageSizesAsync, as the following code shows.

    '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

  3. Define a query that, when executed by the ToArray method, creates a collection of tasks that download the contents of each website. The tasks are started when the query is evaluated.

    Add the following code to method SumPageSizesAsync after the declaration of client and 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()

  4. Next, apply Task.WhenAll to the collection of tasks, downloadTasks. Task.WhenAll returns a single task that finishes when all the tasks in the collection of tasks have completed.

    In the following example, the Await expression awaits the completion of the single task that WhenAll returns. When complete, the Await expression evaluates to an array of integers, where each integer is the length of a downloaded website. Add the following code to SumPageSizesAsync, just after the code that you added in the previous step.

    ' 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

  5. Finally, use the Sum method to get the sum of the lengths of all the websites. Add the following line to SumPageSizesAsync.

    Dim total = lengths.Sum()

To test the Task.WhenAll solutions

For either solution, choose the F5 key to run the program, and then choose the Start button. The output should resemble the output from the async solutions in Walkthrough: Accessing the Web by Using Async and Await (Visual Basic). However, notice that the websites appear in a different order each time.

Example 1

The following code shows the extensions to the project that uses the GetURLContentsAsync method to download content from the web.

' 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

Example 2

The following code shows the extensions to the project that uses method HttpClient.GetByteArrayAsync to download content from the web.

' 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

See also