Muokkaa

Jaa

Walkthrough: Accessing the Web by Using Async and Await (Visual Basic)

  • Artikkeli

You can write asynchronous programs more easily and intuitively by using async/await features. You can write asynchronous code that looks like synchronous code and let the compiler handle the difficult callback functions and continuations that asynchronous code usually entails.

For more information about the Async feature, see Asynchronous Programming with Async and Await (Visual Basic).

This walkthrough starts with a synchronous Windows Presentation Foundation (WPF) application that sums the number of bytes in a list of websites. The walkthrough then converts the application to an asynchronous solution by using the new features.

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.

In this walkthrough, you complete the following tasks:

See the Example section for the complete asynchronous example.

Prerequisites

Visual Studio 2012 or later must be installed on your computer. For more information, see the Visual Studio Downloads page.

Create a WPF application

  1. Start Visual Studio.

  2. On the menu bar, choose File, New, Project.

    The New Project dialog box opens.

  3. In the Installed Templates pane, choose Visual Basic, and then choose WPF Application from the list of project types.

  4. In the Name text box, enter AsyncExampleWPF, and then choose the OK button.

    The new project appears in Solution Explorer.

Design a simple WPF MainWindow

  1. In the Visual Studio Code Editor, choose the MainWindow.xaml tab.

  2. If the Toolbox window isn’t visible, open the View menu, and then choose Toolbox.

  3. Add a Button control and a TextBox control to the MainWindow window.

  4. Highlight the TextBox control and, in the Properties window, set the following values:

    • Set the Name property to resultsTextBox.

    • Set the Height property to 250.

    • Set the Width property to 500.

    • On the Text tab, specify a monospaced font, such as Lucida Console or Global Monospace.

  5. Highlight the Button control and, in the Properties window, set the following values:

    • Set the Name property to startButton.

    • Change the value of the Content property from Button to Start.

  6. Position the text box and the button so that both appear in the MainWindow window.

    For more information about the WPF XAML Designer, see Creating a UI by using XAML Designer.

Add a reference

  1. In Solution Explorer, highlight your project's name.

  2. On the menu bar, choose Project, Add Reference.

    The Reference Manager dialog box appears.

  3. At the top of the dialog box, verify that your project is targeting the .NET Framework 4.5 or higher.

  4. In the Assemblies area, choose Framework if it isn’t already chosen.

  5. In the list of names, select the System.Net.Http check box.

  6. Choose the OK button to close the dialog box.

Add necessary Imports statements

  1. In Solution Explorer, open the shortcut menu for MainWindow.xaml.vb, and then choose View Code.

  2. Add the following Imports statements at the top of the code file if they’re not already present.

    Imports System.Net.Http
Imports System.Net
Imports System.IO

Create a synchronous application

  1. In the design window, MainWindow.xaml, double-click the Start button to create the startButton_Click event handler in MainWindow.xaml.vb.

  2. In MainWindow.xaml.vb, copy the following code into the body of startButton_Click:

    resultsTextBox.Clear()
SumPageSizes()
resultsTextBox.Text &= vbCrLf & "Control returned to startButton_Click."

    The code calls the method that drives the application, SumPageSizes, and displays a message when control returns to startButton_Click.

  3. The code for the synchronous solution contains the following four methods:

    • SumPageSizes, which gets a list of webpage URLs from SetUpURLList and then calls GetURLContents and DisplayResults to process each URL.

    • SetUpURLList, which makes and returns a list of web addresses.

    • GetURLContents, which downloads the contents of each website and returns the contents as a byte array.

    • DisplayResults, which displays the number of bytes in the byte array for each URL.

    Copy the following four methods, and then paste them under the startButton_Click event handler in MainWindow.xaml.vb:

    Private Sub SumPageSizes()

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

    Dim total = 0
    For Each url In urlList
        ' GetURLContents returns the contents of url as a byte array.
        Dim urlContents As Byte() = GetURLContents(url)

        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 Sub

Private Function SetUpURLList() As List(Of String)

    Dim urls = New List(Of String) From
        {
            "https://msdn.microsoft.com/library/windows/apps/br211380.aspx",
            "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

Private Function GetURLContents(url As String) As 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.
    ' Note: you can't use HttpWebRequest.GetResponse in a Windows Store app.
    Using response As WebResponse = webReq.GetResponse()
        ' 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.
            responseStream.CopyTo(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

Test the synchronous solution

  1. Choose the F5 key to run the program, and then choose the Start button.

    Output that resembles the following list should appear:

    msdn.microsoft.com/library/windows/apps/br211380.aspx        383832
msdn.microsoft.com                                            33964
msdn.microsoft.com/library/hh290136.aspx               225793
msdn.microsoft.com/library/ee256749.aspx               143577
msdn.microsoft.com/library/hh290138.aspx               237372
msdn.microsoft.com/library/hh290140.aspx               128279
msdn.microsoft.com/library/dd470362.aspx               157649
msdn.microsoft.com/library/aa578028.aspx               204457
msdn.microsoft.com/library/ms404677.aspx               176405
msdn.microsoft.com/library/ff730837.aspx               143474

Total bytes returned:  1834802

Control returned to startButton_Click.

    Notice that it takes a few seconds to display the counts. During that time, the UI thread is blocked while it waits for requested resources to download. As a result, you can't move, maximize, minimize, or even close the display window after you choose the Start button. These efforts fail until the byte counts start to appear. If a website isn’t responding, you have no indication of which site failed. It is difficult even to stop waiting and close the program.

Convert GetURLContents to an asynchronous method

  1. To convert the synchronous solution to an asynchronous solution, the best place to start is in GetURLContents because the calls to the HttpWebRequest.GetResponse method and to the Stream.CopyTo method are where the application accesses the web. The .NET Framework makes the conversion easy by supplying asynchronous versions of both methods.

    For more information about the methods that are used in GetURLContents, see WebRequest.

    Note

    As you follow the steps in this walkthrough, several compiler errors appear. You can ignore them and continue with the walkthrough.

    Change the method that's called in the third line of GetURLContents from GetResponse to the asynchronous, task-based GetResponseAsync method.

    Using response As WebResponse = webReq.GetResponseAsync()

  2. GetResponseAsync returns a Task<TResult>. In this case, the task return variable, TResult, has type WebResponse. The task is a promise to produce an actual WebResponse object after the requested data has been downloaded and the task has run to completion.

    To retrieve the WebResponse value from the task, apply an Await operator to the call to GetResponseAsync, as the following code shows.

    Using response As WebResponse = Await webReq.GetResponseAsync()

    The Await operator suspends the execution of the current method, GetURLContents, until the awaited task is complete. In the meantime, control returns to the caller of the current method. In this example, the current method is GetURLContents, and the caller is SumPageSizes. When the task is finished, the promised WebResponse object is produced as the value of the awaited task and assigned to the variable response.

    The previous statement can be separated into the following two statements to clarify what happens.

    Dim responseTask As Task(Of WebResponse) = webReq.GetResponseAsync()
Using response As WebResponse = Await responseTask

    The call to webReq.GetResponseAsync returns a Task(Of WebResponse) or Task<WebResponse>. Then an Await operator is applied to the task to retrieve the WebResponse value.

    If your async method has work to do that doesn’t depend on the completion of the task, the method can continue with that work between these two statements, after the call to the async method and before the await operator is applied. For examples, see How to: Make Multiple Web Requests in Parallel by Using Async and Await (Visual Basic) and How to: Extend the Async Walkthrough by Using Task.WhenAll (Visual Basic).

  3. Because you added the Await operator in the previous step, a compiler error occurs. The operator can be used only in methods that are marked with the Async modifier. Ignore the error while you repeat the conversion steps to replace the call to CopyTo with a call to CopyToAsync.

    • Change the name of the method that’s called to CopyToAsync.

    • The CopyTo or CopyToAsync method copies bytes to its argument, content, and doesn’t return a meaningful value. In the synchronous version, the call to CopyTo is a simple statement that doesn't return a value. The asynchronous version, CopyToAsync, returns a Task. The task functions like "Task(void)" and enables the method to be awaited. Apply Await or await to the call to CopyToAsync, as the following code shows.

      Await responseStream.CopyToAsync(content)

      The previous statement abbreviates the following two lines of code.

      ' CopyToAsync returns a Task, not a Task<T>.
Dim copyTask As Task = responseStream.CopyToAsync(content)

' When copyTask is completed, content contains a copy of
' responseStream.
Await copyTask

  4. All that remains to be done in GetURLContents is to adjust the method signature. You can use the Await operator only in methods that are marked with the Async modifier. Add the modifier to mark the method as an async method, as the following code shows.

    Private Async Function GetURLContents(url As String) As Byte()

  5. The return type of an async method can only be Task, Task<TResult>. In Visual Basic, the method must be a Function that returns a Task or a Task(Of T), or the method must be a Sub. Typically, a Sub method is used only in an async event handler, where Sub is required. In other cases, you use Task(T) if the completed method has a Return statement that returns a value of type T, and you use Task if the completed method doesn’t return a meaningful value.

    For more information, see Async Return Types (Visual Basic).

    Method GetURLContents has a return statement, and the statement returns a byte array. Therefore, the return type of the async version is Task(T), where T is a byte array. Make the following changes in the method signature:

    • Change the return type to Task(Of Byte()).

    • By convention, asynchronous methods have names that end in "Async," so rename the method GetURLContentsAsync.

    The following code shows these changes.

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

    With those few changes, the conversion of GetURLContents to an asynchronous method is complete.

Convert SumPageSizes to an asynchronous method

  1. Repeat the steps from the previous procedure for SumPageSizes. First, change the call to GetURLContents to an asynchronous call.

    • Change the name of the method that’s called from GetURLContents to GetURLContentsAsync, if you haven't already done so.

    • Apply Await to the task that GetURLContentsAsync returns to obtain the byte array value.

    The following code shows these changes.

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

    The previous assignment abbreviates the following two lines of code.

    ' 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

  2. Make the following changes in the method's signature:

    • Mark the method with the Async modifier.

    • Add "Async" to the method name.

    • There is no task return variable, T, this time because SumPageSizesAsync doesn’t return a value for T. (The method has no Return statement.) However, the method must return a Task to be awaitable. Therefore, change the method type from Sub to Function. The return type of the function is Task.

    The following code shows these changes.

    Private Async Function SumPageSizesAsync() As Task

    The conversion of SumPageSizes to SumPageSizesAsync is complete.

Convert startButton_Click to an asynchronous method

  1. In the event handler, change the name of the called method from SumPageSizes to SumPageSizesAsync, if you haven’t already done so.

  2. Because SumPageSizesAsync is an async method, change the code in the event handler to await the result.

    The call to SumPageSizesAsync mirrors the call to CopyToAsync in GetURLContentsAsync. The call returns a Task, not a Task(T).

    As in previous procedures, you can convert the call by using one statement or two statements. The following code shows these changes.

    ' One-step async call.
Await SumPageSizesAsync()

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

  3. To prevent accidentally reentering the operation, add the following statement at the top of startButton_Click to disable the Start button.

    ' Disable the button until the operation is complete.
startButton.IsEnabled = False

    You can reenable the button at the end of the event handler.

    ' Reenable the button in case you want to run the operation again.
startButton.IsEnabled = True

    For more information about reentrancy, see Handling Reentrancy in Async Apps (Visual Basic).

  4. Finally, add the Async modifier to the declaration so that the event handler can await SumPagSizesAsync.

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

    Typically, the names of event handlers aren’t changed. The return type isn’t changed to Task because event handlers must be Sub procedures in Visual Basic.

    The conversion of the project from synchronous to asynchronous processing is complete.

Test the asynchronous solution

  1. Choose the F5 key to run the program, and then choose the Start button.

  2. Output that resembles the output of the synchronous solution should appear. However, notice the following differences.

    • The results don’t all occur at the same time, after the processing is complete. For example, both programs contain a line in startButton_Click that clears the text box. The intent is to clear the text box between runs if you choose the Start button for a second time, after one set of results has appeared. In the synchronous version, the text box is cleared just before the counts appear for the second time, when the downloads are completed and the UI thread is free to do other work. In the asynchronous version, the text box clears immediately after you choose the Start button.

    • Most importantly, the UI thread isn’t blocked during the downloads. You can move or resize the window while the web resources are being downloaded, counted, and displayed. If one of the websites is slow or not responding, you can cancel the operation by choosing the Close button (the x in the red field in the upper-right corner).

Replace the GetURLContentsAsync method with a .NET Framework method

  1. The .NET Framework provides many async methods that you can use. One of them, the HttpClient.GetByteArrayAsync(String) method, does just what you need for this walkthrough. You can use it instead of the GetURLContentsAsync method that you created in an earlier procedure.

    The first step is to create an HttpClient object in the SumPageSizesAsync method. Add the following declaration at the start of the method.

    ' 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}

  2. In SumPageSizesAsync, replace the call to your GetURLContentsAsync method with a call to the HttpClient method.

    Dim urlContents As Byte() = Await client.GetByteArrayAsync(url)

  3. Remove or comment out the GetURLContentsAsync method that you wrote.

  4. Choose the F5 key to run the program, and then choose the Start button.

    The behavior of this version of the project should match the behavior that the "To test the asynchronous solution" procedure describes but with even less effort from you.

Example

The following is the full example of the converted asynchronous solution that uses the asynchronous GetURLContentsAsync method. Notice that it strongly resembles the original, synchronous solution.

' 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

        ' Disable the button until the operation is complete.
        startButton.IsEnabled = False

        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."

        ' Reenable the button in case you want to run the operation again.
        startButton.IsEnabled = True
    End Sub

    Private Async Function SumPageSizesAsync() As Task

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

        Dim total = 0
        For Each url In urlList
            Dim urlContents As Byte() = Await GetURLContentsAsync(url)

            ' The previous line abbreviates the following two assignment statements.

            '//<snippet21>
            ' 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 websites.
        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/library/windows/apps/br211380.aspx",
                "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

    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()

            ' The previous statement abbreviates the following two statements.

            'Dim responseTask As Task(Of WebResponse) = webReq.GetResponseAsync()
            'Using response As WebResponse = Await responseTask

            ' 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.
                Await responseStream.CopyToAsync(content)

                ' The previous statement abbreviates the following two statements.

                ' CopyToAsync returns a Task, not a Task<T>.
                'Dim copyTask As Task = responseStream.CopyToAsync(content)

                ' When copyTask is completed, content contains a copy of
                ' responseStream.
                'Await copyTask
            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

The following code contains the full example of the solution that uses the HttpClient method, GetByteArrayAsync.

' 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()

        ' Disable the button until the operation is complete.
        startButton.IsEnabled = False

        ' 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."

        ' Reenable the button in case you want to run the operation again.
        startButton.IsEnabled = True
    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()

        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 websites.
        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/library/windows/apps/br211380.aspx",
                "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

    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