How to: Download a File in the Background

Downloading a file is a common task, and it is often useful to run this potentially time-consuming operation on a separate thread. Use the BackgroundWorker component to accomplish this task with very little code.

The following code example demonstrates how to use a BackgroundWorker component to load an XML file from a URL. When the user clicks the Download button, the Click event handler calls the RunWorkerAsync method of a BackgroundWorker component to start the download operation. The button is disabled for the duration of the download, and then enabled when the download is complete. A MessageBox displays the contents of the file.

Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Drawing
Imports System.Threading
Imports System.Windows.Forms
Imports System.Xml

Public Class Form1
    Inherits Form

    Private WithEvents downloadButton As Button
    Private WithEvents progressBar1 As ProgressBar
    Private WithEvents backgroundWorker1 As BackgroundWorker
    Private document As XmlDocument = Nothing 

    Public Sub New()
        InitializeComponent()
        Me.backgroundWorker1 = New System.ComponentModel.BackgroundWorker()
    End Sub 

    Private Sub downloadButton_Click( _
        ByVal sender As Object, _
        ByVal e As EventArgs) _
        Handles downloadButton.Click

        ' Start the download operation in the background. 
        Me.backgroundWorker1.RunWorkerAsync()

        ' Disable the button for the duration of the download. 
        Me.downloadButton.Enabled = False 

        ' Once you have started the background thread you  
        ' can exit the handler and the application will  
        ' wait until the RunWorkerCompleted event is raised. 

        ' If you want to do something else in the main thread, 
        ' such as update a progress bar, you can do so in a loop  
        ' while checking IsBusy to see if the background task is 
        ' still running. 
        While Me.backgroundWorker1.IsBusy
            progressBar1.Increment(1)
            ' Keep UI messages moving, so the form remains  
            ' responsive during the asynchronous operation.
            Application.DoEvents()
        End While 
    End Sub 

    Private Sub backgroundWorker1_DoWork( _
        ByVal sender As Object, _
        ByVal e As DoWorkEventArgs) _
        Handles backgroundWorker1.DoWork

        document = New XmlDocument()

        ' Replace this file name with a valid file name.
        document.Load("http://www.tailspintoys.com/sample.xml")

        ' Uncomment the following line to 
        ' simulate a noticeable latency. 
        'Thread.Sleep(5000); 
    End Sub 

    Private Sub backgroundWorker1_RunWorkerCompleted( _
        ByVal sender As Object, _
        ByVal e As RunWorkerCompletedEventArgs) _
        Handles backgroundWorker1.RunWorkerCompleted

        ' Set progress bar to 100% in case it isn't already there.
        progressBar1.Value = 100

        If e.Error Is Nothing Then
            MessageBox.Show(document.InnerXml, "Download Complete")
        Else
            MessageBox.Show("Failed to download file", "Download failed", MessageBoxButtons.OK, MessageBoxIcon.Error)
        End If 

        ' Enable the download button and reset the progress bar. 
        Me.downloadButton.Enabled = True
        progressBar1.Value = 0
    End Sub

#Region "Windows Form Designer generated code" 
    ' <summary> 
    ' Required designer variable. 
    ' </summary> 
    Private components As System.ComponentModel.IContainer = Nothing 

    ' <summary> 
    ' Clean up any resources being used. 
    ' </summary> 
    ' <param name="disposing">true if managed resources should be disposed; otherwise, false.</param> 
    Protected Overrides Sub Dispose(ByVal disposing As Boolean)
        If disposing AndAlso (components IsNot Nothing) Then
            components.Dispose()
        End If 
        MyBase.Dispose(disposing)
    End Sub 

    ' <summary> 
    ' Required method for Designer support - do not modify 
    ' the contents of this method with the code editor. 
    ' </summary> 
    Private Sub InitializeComponent()
        Me.downloadButton = New System.Windows.Forms.Button
        Me.progressBar1 = New System.Windows.Forms.ProgressBar
        Me.SuspendLayout()
        
        'downloadButton 
        
        Me.downloadButton.Location = New System.Drawing.Point(12, 12)
        Me.downloadButton.Name = "downloadButton" 
        Me.downloadButton.Size = New System.Drawing.Size(100, 23)
        Me.downloadButton.TabIndex = 0
        Me.downloadButton.Text = "Download file" 
        Me.downloadButton.UseVisualStyleBackColor = True 
        
        'progressBar1 
        
        Me.progressBar1.Location = New System.Drawing.Point(12, 50)
        Me.progressBar1.Name = "progressBar1" 
        Me.progressBar1.Size = New System.Drawing.Size(100, 26)
        Me.progressBar1.TabIndex = 1
        
        'Form1 
        
        Me.AutoScaleDimensions = New System.Drawing.SizeF(6.0!, 13.0!)
        Me.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font
        Me.ClientSize = New System.Drawing.Size(136, 104)
        Me.Controls.Add(Me.downloadButton)
        Me.Controls.Add(Me.progressBar1)
        Me.Name = "Form1" 
        Me.Text = "Form1" 
        Me.ResumeLayout(False)

    End Sub

#End Region
End Class 


Public Class Program

    ' <summary> 
    ' The main entry point for the application. 
    ' </summary>
    <STAThread()> _
    Shared Sub Main()
        Application.EnableVisualStyles()
        Application.Run(New Form1())
    End Sub 
End Class

Downloading the file

The file is downloaded on the BackgroundWorker component's worker thread, which runs the DoWork event handler. This thread starts when your code calls the RunWorkerAsync method.

Private Sub backgroundWorker1_DoWork( _
    ByVal sender As Object, _
    ByVal e As DoWorkEventArgs) _
    Handles backgroundWorker1.DoWork

    document = New XmlDocument()

    ' Replace this file name with a valid file name.
    document.Load("http://www.tailspintoys.com/sample.xml")

    ' Uncomment the following line to 
    ' simulate a noticeable latency. 
    'Thread.Sleep(5000); 
End Sub

Waiting for a BackgroundWorker to finish

The downloadButton_Click event handler demonstrates how to wait for a BackgroundWorker component to finish its asynchronous task.

If you only want the application to respond to events and do not want to do any work in the main thread while you wait for the background thread to complete, just exit the handler.

If you want to continue doing work in the main thread, use the IsBusy property to determine whether the BackgroundWorker thread is still running. In the example, a progress bar is updated while the download is processing. Be sure to call the Application.DoEvents method to keep the UI responsive.

Private Sub downloadButton_Click( _
    ByVal sender As Object, _
    ByVal e As EventArgs) _
    Handles downloadButton.Click

    ' Start the download operation in the background. 
    Me.backgroundWorker1.RunWorkerAsync()

    ' Disable the button for the duration of the download. 
    Me.downloadButton.Enabled = False 

    ' Once you have started the background thread you  
    ' can exit the handler and the application will  
    ' wait until the RunWorkerCompleted event is raised. 

    ' If you want to do something else in the main thread, 
    ' such as update a progress bar, you can do so in a loop  
    ' while checking IsBusy to see if the background task is 
    ' still running. 
    While Me.backgroundWorker1.IsBusy
        progressBar1.Increment(1)
        ' Keep UI messages moving, so the form remains  
        ' responsive during the asynchronous operation.
        Application.DoEvents()
    End While 
End Sub

Displaying the result

The backgroundWorker1_RunWorkerCompleted method handles the RunWorkerCompleted event and is called when the background operation is completed. This method first checks the AsyncCompletedEventArgs.Error property. If AsyncCompletedEventArgs.Error is null, then this method displays the contents of the file. It then enables the download button, which was disabled when the download began, and it resets the progress bar.

Private Sub backgroundWorker1_RunWorkerCompleted( _
    ByVal sender As Object, _
    ByVal e As RunWorkerCompletedEventArgs) _
    Handles backgroundWorker1.RunWorkerCompleted

    ' Set progress bar to 100% in case it isn't already there.
    progressBar1.Value = 100

    If e.Error Is Nothing Then
        MessageBox.Show(document.InnerXml, "Download Complete")
    Else
        MessageBox.Show("Failed to download file", "Download failed", MessageBoxButtons.OK, MessageBoxIcon.Error)
    End If 

    ' Enable the download button and reset the progress bar. 
    Me.downloadButton.Enabled = True
    progressBar1.Value = 0
End Sub

This example requires:

  • References to the System.Drawing, System.Windows.Forms, and System.Xml assemblies.

For information about building this example from the command line for Visual Basic or Visual C#, see Building from the Command Line (Visual Basic) or Command-line Building With csc.exe. You can also build this example in Visual Studio by pasting the code into a new project. Also see How to: Compile and Run a Complete Windows Forms Code Example Using Visual Studio.

Always check the AsyncCompletedEventArgs.Error property in your RunWorkerCompleted event handler before attempting to access the RunWorkerCompletedEventArgs.Result property or any other object that may have been affected by the DoWork event handler.

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