Home > Articles > Programming > Visual Basic

This chapter is from the book

This chapter is from the book

Cancellation and Progress

Because the Async pattern relies on the Task class, implementing cancellation is something similar to what you have already studied back in Chapter 41, thus the CancellationTokenSource and CancellationToken classes are used. In this section you see how to implement cancellation both for asynchronous methods and for Task.Run operations. Next, you learn how to report the progress of an operation, which is common in asynchronous programming and improves the user experience.

Implementing Cancellation

Let’s look at the WPF sample application created in the section “Getting Started with Async/Await.” Imagine you want to give users the ability of cancelling the download of the RSS feed from the Microsoft Channel9 website. First, make a slight modification to the user interface so that the main Grid is divided into two rows, and in the first row add a Button like this:

    <RowDefinition Height="40"/>
<Button Width="120" Height="30" Name="CancelButton"

Do not forget to add the Grid.Row="1" property assignment for the ListBox control. Double-click the new button so that you can quickly access the code editor. Declare a new CancellationTokenSource object that will listen for cancellation requests. The event handler for the new button’s Click event will invoke the Cancel method on the instance of the CancellationTokenSource:

Private tokenSource As CancellationTokenSource

Private Sub CancelButton_Click(sender As Object, e As RoutedEventArgs) _
    Handles CancelButton.Click
    'If Me.tokenSource IsNot Nothing Then
    '    Me.tokenSource.Cancel()
    'End If
End Sub

The user can now request cancellation by clicking this button. Next, you need to make a couple of edits to the QueryVideosAsync method created previously. The first edit is making this method receive a CancellationToken object as an argument. This object will handle cancellation requests during the method execution. The second edit requires replacing the WebClient class with a new class called HttpClient. The reason for this change is that the WebClient’s asynchronous methods no longer support cancellation as in the first previews of the Async library, although asynchronous methods in System.Net.Http.HttpClient do. Among the others, this class exposes a method called GetAsync that retrieves contents from the specified URL and receives the cancellation token as the second argument. The result is returned under the form of a System.Net.Http.HttpResponseMessage class. As the name implies, this class represents an HTTP response message including the status of the operation and the retrieved data. The data is represented by a property called Content, which exposes methods to convert data into a stream (ReadAsStreamAsync), into an array of bytes (ReadAsByteArrayAsync), and into a string (ReadAsStringAsync). Other than changing the code to use HttpClient and to receive the cancellation token, you only need to handle the OperationCanceledException, which is raised after the cancellation request is received by the asynchronous method. The following code demonstrates the QueryVideosAsync method:

'The following implementation with HttpClient supports Cancellation
Private Async Function QueryVideosAsync(token As CancellationToken) As  _
        Task(Of IEnumerable(Of Video))
        Dim client As New HttpClient

        'Get the feed content as an HttpResponseMessage
        Dim data = Await client.GetAsync(New Uri(Video.FeedUrl), token)

        'Parse the content into a String
        Dim actualData = Await data.Content.ReadAsStringAsync

        Dim doc = XDocument.Parse(actualData)

        Dim query = From video In doc...<item>
                    Select New Video With {
                    .Title = video.<title>.Value,
                    .Speaker = video.<dc:creator>.Value,
                    .Url = video.<link>.Value,
                    .Thumbnail = video...<media:thumbnail>.
                    .DateRecorded = String.Concat("Recorded on ",

        Return query
    Catch ex As OperationCanceledException
        MessageBox.Show("Operation was canceled by the user.")
        Return Nothing
    Catch ex As Exception
        Return Nothing
    End Try
End Function

The very last edit to the application is changing the LoadVideosAsync method to launch the query passing a cancellation token:

Private Async Function LoadVideosAsync() As Task
    Me.tokenSource = New CancellationTokenSource

    Me.DataContext = Await QueryVideosAsync(Me.tokenSource.Token)
End Function

If you now run the application, not only will the user interface remain responsive, but you will be also able to click the Cancel button to stop the query execution. Notice that in a synchronous approach, implementing cancellation has no benefits. In fact, if on one side writing code to support cancellation is legal, on the other side the user would never have a chance to click a button because the UI thread would be blocked until the completion of the task. Similarly, you can add cancellation to tasks running in a separate thread and started with Task.Run. By continuing the example shown previously about this method, you can first rewrite the SimulateIntensiveWork method as follows:

Private Function SimulateIntensiveWork(token As CancellationToken) _
         As Integer
    Dim delay As Integer = 5000


    Return delay
End Function

You should be familiar with this approach because it has been discussed in Chapter 41. The method receives the cancellation token and checks for cancellation requests. If any exist, it throws an OperationCanceledException. Next, you add support for cancellation by passing an instance of the CancellationTokenSource class to the method invocation inside Task.Run:

Private cancellationToken As CancellationTokenSource

Private Async Sub RunIntensiveWorkAsync()
    cancellationToken = New CancellationTokenSource
    'This runs on the UI thread

        'This runs on a Thread Pool thread
        Dim result As Integer = Await Task.Run(Function()
                                                   Dim workResult As Integer = _
                                                   SimulateIntensiveWork( _
                                                   Return workResult
                                               End Function)

        'This runs again on the UI thread
    Catch ex As OperationCanceledException
        Console.WriteLine("Canceled by the user.")
    Catch ex As Exception

    End Try
End Sub

To request cancellation, you should call the cancellationToken.Cancel method. At that point, the request is intercepted and an OperationCanceledException is thrown.

Reporting Progress

Reporting the progress of an asynchronous method execution is a common requirement. There is a pattern that you can use and that makes things easier. This pattern relies on the System.IProgress(Of T) interface and the System.Progress(Of T) class, which expose a ProgressChanged event that must be raised when the asynchronous operation is in progress. To provide an example that is easy to understand, imagine you still want to download the content of some feeds from the Microsoft Channel9 website and refresh the progress every time a site has been downloaded completely. The current example is based on a Console application. Consider the following code:

Private progress As Progress(Of Integer)
Private counter As Integer = 0

Sub Main()
        progress = New Progress(Of Integer)
        AddHandler progress.ProgressChanged, Sub(sender, e)
                                                 WriteLine _
                                                 ("Download progress: " & _
                                             End Sub


    Catch ex As Exception
    End Try
End Sub

You first declare an object of type Progress(Of Integer) and a counter. The first object will receive the progress value when the ProgressChanged event is raised. In this case, the code uses the Integer type to pass a simple number, but you can pass more complex information with a different or custom type. Then the code specifies a handler for the ProgressChanged event, with type inference for the lambda’s parameters. Sender is always Object, whereas e is of the same type as the generic type you assigned to Progress. So, in this case it is of type Integer. Here you are working in a Console application and are thus displaying the value as a text message. But in real-world applications, this is the value that you could assign to a ProgressBar control to report the progress in the user interface. The instance of the Progress class must be passed to the asynchronous method that performs the required tasks. The Progress class has just one method called Report; you invoke it after an Await invocation. The following code demonstrates how to report the progress of downloading a number of feeds:

Private Async Sub DownloadAllFeedsAsync(currentProgress As IProgress(Of Integer))
    Dim client As New System.Net.WebClient

    Dim feeds As New List(Of Uri) From
        {New Uri("http://channel9.msdn.com/Tags/windows+8/RSS"),
         New Uri("http://channel9.msdn.com/Tags/windows+phone"),
         New Uri("http://channel9.msdn.com/Tags/visual+basic/RSS")}
    For Each URL In feeds
        Await client.DownloadStringTaskAsync(URL)
        counter += 1
        If currentProgress IsNot Nothing Then currentProgress.Report(counter)
End Sub

Report receives as an argument an object of the same type that you assigned as the generic argument of the Progress class declaration in this case a counter of type Integer that is incremented every time a feed is downloaded. If you run this code, every time a feed is returned, the progress is also shown in the user interface, as demonstrated in Figure 42.6.


FIGURE 42.6 Reporting the progress of an asynchronous method.

This pattern makes reporting the progress of a task and in real-world applications, such as WPF and Windows apps, easy. It also makes updating controls like the ProgressBar incredibly simple by assigning to such controls the value stored in the instance of the Progress class.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020