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How to: Use a Background Worker

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The Silverlight BackgroundWorker class provides an easy way to run time-consuming operations on a background thread. The BackgroundWorker class enables you to check the state of the operation and it lets you cancel the operation.

When you use the BackgroundWorker class, you can indicate operation progress, completion, and cancellation in the Silverlight user interface. For example, you can check whether the background operation is completed or canceled and display a message to the user.

Procedure

To use the BackgroundWorker class

  1. At the class level, create an instance of the BackgroundWorker class.

    Dim bw As BackgroundWorker = New BackgroundWorker

    BackgroundWorker bw = new BackgroundWorker();

  2. Specify whether you want the background operation to allow cancellation and to report progress.

    bw.WorkerSupportsCancellation = True
bw.WorkerReportsProgress = True

    bw.WorkerSupportsCancellation = true;
bw.WorkerReportsProgress = true;

  3. Create an event handler for the background worker's DoWork event.

    The DoWork event handler is where you run the time-consuming operation on the background thread. Any values that are passed to the background operation are passed in the Argument property of the DoWorkEventArgs object that is passed to the event handler.

    To report progress back to the calling process, call the ReportProgress method and pass it a completion percentage from 0 to 100. Calling the ReportProgress method raises the ProgressChanged event, which you handle separately.

    NoteNote:

    If the background worker's WorkerReportsProgress property is not set to true and you call the ReportProgress method, an exception will occur.

    To determine if there is a pending request to cancel the background operation, check the CancellationPending property of the BackgroundWorker object. If the property is true, the CancelAsync method was called. Set the BackgroundWorker object's Cancel property to true and stop the operation.

    To pass data back to the calling process, set the Result property of the DoWorkEventArgs object that is passed to the event handler. This value can be read when the RunWorkerCompleted event is raised at the end of the operation.

    Private Sub bw_DoWork(ByVal sender As Object, ByVal e As DoWorkEventArgs)
    Dim worker As BackgroundWorker = CType(sender, BackgroundWorker)

    For i = 1 To 10
        If bw.CancellationPending = True Then
            e.Cancel = True
            Exit For
        Else
            ' Perform a time consuming operation and report progress.
            System.Threading.Thread.Sleep(500)
            bw.ReportProgress(i * 10)
        End If
    Next
End Sub

    private void bw_DoWork(object sender, DoWorkEventArgs e)
{
    BackgroundWorker worker = sender as BackgroundWorker;

    for (int i = 1; (i <= 10); i++)
    {
        if ((worker.CancellationPending == true))
        {
            e.Cancel = true;
            break;
        }
        else
        {
            // Perform a time consuming operation and report progress.
            System.Threading.Thread.Sleep(500);
            worker.ReportProgress((i * 10));
        }
    }
}

  4. Create an event handler for the background worker's ProgressChanged event.

    In the ProgressChanged event handler, add code to indicate the progress, such as updating the user interface.

    To determine what percentage of the operation is completed, check the ProgressPercentage property of the ProgressChangedEventArgs object that was passed to the event handler.

    Private Sub bw_ProgressChanged(ByVal sender As Object, ByVal e As ProgressChangedEventArgs)
    Me.tbProgress.Text = e.ProgressPercentage.ToString() & "%"
End Sub

    private void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    this.tbProgress.Text = (e.ProgressPercentage.ToString() + "%");
}

  5. Create an event handler for the RunWorkerCompleted event.

    The RunWorkerCompleted event is raised when the background worker has completed. Depending on whether the background operation completed successfully, encountered an error, or was canceled, update the user interface accordingly.

    To determine whether an error occurred, check the Error property of the RunWorkerCompletedEventArgs object that was passed to the event handler. If an error occurred, this property contains the exception information.

    If the background operation allows cancellation and you want to check whether the operation was canceled, check the Cancelled property of the RunWorkerCompletedEventArgs object that was passed to the event handler. If the property is true, the CancelAsync method was called.

    Private Sub bw_RunWorkerCompleted(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs)
    If e.Cancelled = True Then
        Me.tbProgress.Text = "Canceled!"
    ElseIf e.Error IsNot Nothing Then
        Me.tbProgress.Text = "Error: " & e.Error.Message
    Else
        Me.tbProgress.Text = "Done!"
    End If
End Sub

    private void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
{
    if ((e.Cancelled == true))
    {
        this.tbProgress.Text = "Canceled!";
    }

    else if (!(e.Error == null))
    {
        this.tbProgress.Text = ("Error: " + e.Error.Message);
    }

    else
    {
        this.tbProgress.Text = "Done!";
    }
}

  6. Add the event handlers to the BackgroundWorker instance's events.

    The following example shows how to add the event handlers to the DoWork, ProgressChanged, and RunWorkerCompleted events.

    AddHandler bw.DoWork, AddressOf bw_DoWork
AddHandler bw.ProgressChanged, AddressOf bw_ProgressChanged 
AddHandler bw.RunWorkerCompleted, AddressOf bw_RunWorkerCompleted

    bw.DoWork += 
    new DoWorkEventHandler(bw_DoWork);
bw.ProgressChanged += 
    new ProgressChangedEventHandler(bw_ProgressChanged);
bw.RunWorkerCompleted += 
    new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);

  7. Start running the background operation by calling the RunWorkerAsync method.

    Private Sub buttonStart_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)
    If Not bw.IsBusy = True Then
        bw.RunWorkerAsync()
    End If
End Sub

    private void buttonStart_Click(object sender, RoutedEventArgs e)
{
    if (bw.IsBusy != true)
    {
        bw.RunWorkerAsync();
    }
}

  8. Cancel the background operation by calling the CancelAsync method.

    Private Sub buttonCancel_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)
    If bw.WorkerSupportsCancellation = True Then
        bw.CancelAsync()
    End If
End Sub

    private void buttonCancel_Click(object sender, RoutedEventArgs e)
{
    if (bw.WorkerSupportsCancellation == true)
    {
        bw.CancelAsync();
    }
}

Example

The following example shows how to use the BackgroundWorker class. In the example, the background operation runs the Sleep method and reports progress to the user interface. The background worker is configured to allow cancellation.

Imports System.ComponentModel

Partial Public Class Page
    Inherits UserControl
    Private bw As BackgroundWorker = New BackgroundWorker

    Public Sub New()
        InitializeComponent()

        bw.WorkerReportsProgress = True
        bw.WorkerSupportsCancellation = True
        AddHandler bw.DoWork, AddressOf bw_DoWork
        AddHandler bw.ProgressChanged, AddressOf bw_ProgressChanged
        AddHandler bw.RunWorkerCompleted, AddressOf bw_RunWorkerCompleted

    End Sub
    Private Sub buttonStart_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)
        If Not bw.IsBusy = True Then
            bw.RunWorkerAsync()
        End If
    End Sub
    Private Sub buttonCancel_Click(ByVal sender As System.Object, ByVal e As System.Windows.RoutedEventArgs)
        If bw.WorkerSupportsCancellation = True Then
            bw.CancelAsync()
        End If
    End Sub
    Private Sub bw_DoWork(ByVal sender As Object, ByVal e As DoWorkEventArgs)
        Dim worker As BackgroundWorker = CType(sender, BackgroundWorker)

        For i = 1 To 10
            If bw.CancellationPending = True Then
                e.Cancel = True
                Exit For
            Else
                ' Perform a time consuming operation and report progress.
                System.Threading.Thread.Sleep(500)
                bw.ReportProgress(i * 10)
            End If
        Next
    End Sub
    Private Sub bw_RunWorkerCompleted(ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs)
        If e.Cancelled = True Then
            Me.tbProgress.Text = "Canceled!"
        ElseIf e.Error IsNot Nothing Then
            Me.tbProgress.Text = "Error: " & e.Error.Message
        Else
            Me.tbProgress.Text = "Done!"
        End If
    End Sub
    Private Sub bw_ProgressChanged(ByVal sender As Object, ByVal e As ProgressChangedEventArgs)
        Me.tbProgress.Text = e.ProgressPercentage.ToString() & "%"
    End Sub
End Class

using System.ComponentModel;
using System.Windows;
using System.Windows.Controls;

namespace SL_BackgroundWorker_CS
{
    public partial class Page : UserControl
    {
        private BackgroundWorker bw = new BackgroundWorker();

        public Page()
        {
            InitializeComponent();

            bw.WorkerReportsProgress = true;
            bw.WorkerSupportsCancellation = true;
            bw.DoWork += new DoWorkEventHandler(bw_DoWork);
            bw.ProgressChanged += new ProgressChangedEventHandler(bw_ProgressChanged);
            bw.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bw_RunWorkerCompleted);
        }
        private void buttonStart_Click(object sender, RoutedEventArgs e)
        {
            if (bw.IsBusy != true)
            {
                bw.RunWorkerAsync();
            }
        }
        private void buttonCancel_Click(object sender, RoutedEventArgs e)
        {
            if (bw.WorkerSupportsCancellation == true)
            {
                bw.CancelAsync();
            }
        }
        private void bw_DoWork(object sender, DoWorkEventArgs e)
        {
            BackgroundWorker worker = sender as BackgroundWorker;

            for (int i = 1; (i <= 10); i++)
            {
                if ((worker.CancellationPending == true))
                {
                    e.Cancel = true;
                    break;
                }
                else
                {
                    // Perform a time consuming operation and report progress.
                    System.Threading.Thread.Sleep(500);
                    worker.ReportProgress((i * 10));
                }
            }
        }
        private void bw_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            if ((e.Cancelled == true))
            {
                this.tbProgress.Text = "Canceled!";
            }

            else if (!(e.Error == null))
            {
                this.tbProgress.Text = ("Error: " + e.Error.Message);
            }

            else
            {
                this.tbProgress.Text = "Done!";
            }
        }
        private void bw_ProgressChanged(object sender, ProgressChangedEventArgs e)
        {
            this.tbProgress.Text = (e.ProgressPercentage.ToString() + "%");
        }
    }
}

<UserControl x:Class="SL_BackgroundWorker_CS.Page"
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml" 
    Width="400" Height="300">
    <Grid x:Name="LayoutRoot" Background="White">
        <StackPanel Height="30" Orientation="Horizontal" 
                    HorizontalAlignment="Left" VerticalAlignment="Top" 
                    Margin="10" >
            <Button x:Name="buttonStart" Content="Start" Click="buttonStart_Click"
                    Width="80" Height="30"/>
            <Button x:Name="buttonCancel" Content="Cancel" Click="buttonCancel_Click"
                    Width="80" Height="30"/>
        </StackPanel>
        <StackPanel Margin="10,50,0,0" Orientation="Horizontal">
            <TextBlock Text="Progress: "/>
            <TextBlock x:Name="tbProgress"/>
        </StackPanel>
    </Grid>
</UserControl>

<UserControl x:Class="SL_BackgroundWorker_VB.Page"
    xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml" 
    Width="400" Height="300">
    <Grid x:Name="LayoutRoot" Background="White">
        <StackPanel Height="30" Orientation="Horizontal" 
                    HorizontalAlignment="Left" VerticalAlignment="Top" 
                    Margin="10" >
            <Button x:Name="buttonStart" Content="Start" Click="buttonStart_Click"
                    Width="80" Height="30"/>
            <Button x:Name="buttonCancel" Content="Cancel" Click="buttonCancel_Click"
                    Width="80" Height="30"/>
        </StackPanel>
        <StackPanel Margin="10,50,0,0" Orientation="Horizontal">
            <TextBlock Text="Progress: " />
            <TextBlock x:Name="tbProgress" />
        </StackPanel>
    </Grid>
</UserControl>

See Also

Reference

BackgroundWorker

Other Resources

Managed Threading