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Posted 22 Apr 2009


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VB.NET Background File Downloader

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4.80/5 (39 votes)
20 May 2009GPL33 min read
A multithreaded file downloader with progress details, speed info and more


This class enables you to easily download multiple files in the background (via a separate thread), and will provide information about the amount of downloaded data, the percentage of completion, and the download speed. On top of this, you can cancel downloading, pause it, and of course also resume. Note that there is also a C# implementation of this class available. 


I started working on this class after someone on a programming help forum asked how to best download files in the background. I got a lot of ideas and based part of my code on the Downloading Files in .NET with all Information article, by Carmine_XX. This class adds multiple file support, logic/layout abstraction and is designed to be easy to use.

Using the Code 

Once you added the class to your project, you should be able to access it via the namespace Bn.Classes. You can of course adapt the namespace to your own preferences or delete it altogether.  

The first thing you need to do when using this class is (logically) create a new instance, and then add the files you want to download. You'll also need to set the local directory to which you want to download. This is pretty straight forward.

' Creating a new instance of a FileDownloader
Private WithEvents downloader As New FileDownloader
' A simple implementation of setting the directory path, 
' adding files from a textbox and starting the download
Private Sub btnStart_Click(ByVal sender As System.Object, _
	ByVal e As System.EventArgs) Handles btnStart.Click
    Dim openFolderDialog As New FolderBrowserDialog
    With downloader
        If openFolderDialog.ShowDialog() = Windows.Forms.DialogResult.OK Then
            .LocalDirectory = openFolderDialog.SelectedPath
            For Each path As String In txtFilesToDownload.Lines
           ' The FileInfo structure will parse your path, 
	  ' and split it to the path itself and the file name, 
	  ' which will both be available for you
                .Files.Add(New FileDownloader.FileInfo(path))
        End If
    End With
End Sub

The code needed to then pause, resume or cancel the downloads couldn't be simpler:

Private Sub btnPauze_Click(ByVal sender As System.Object, _
		ByVal e As System.EventArgs) Handles btnPauze.Click
    ' Pause the downloader
End Sub
Private Sub btnResume_Click(ByVal sender As System.Object, _
		ByVal e As System.EventArgs) Handles btnResume.Click
    ' Resume the downloader
End Sub 
Private Sub btnStop_Click(ByVal sender As System.Object, _
		ByVal e As System.EventArgs) Handles btnStop.Click
    ' Stop the downloader
    ' Note: This will not be instantaneous - the current requests need to 
    ' be closed down, and the downloaded files need to be deleted
End Sub 

The downloader also provides a few properties to indicate its current state: IsBusy, IsPaused, CanStart, CanStop, CanPause and CanResume (all booleans). Here you have an example of how to use these to set your interface:

' This event is fired every time the paused or busy state is changed, 
' and used here to set the controls of the interface
Private Sub downloader_StateChanged() _
    Handles downloader.StateChanged 'This is equal to: Handles 
                       'downloader.IsBusyChanged, downloader.IsPausedChanged
    ' Setting the buttons
    btnStart.Enabled = downloader.CanStart
    btnStop.Enabled = downloader.CanStop
    btnPauze.Enabled = downloader.CanPause
    btnResume.Enabled = downloader.CanResume

    ' Enabling or disabling the setting controls
    txtFilesToDownload.ReadOnly = downloader.IsBusy
    cbUseProgress.Enabled = Not downloader.IsBusy
End Sub	

This is the demo code to display the progress information:

' Occurs every time a block of data has been downloaded, 
' and can be used to display the progress with
' Note that you can also create a timer, and display the progress 
' every certain interval
' Also note that the progress properties return a size in bytes, 
' which is not really user friendly to display
' The FileDownloader class provides shared functions to format these 
' byte amounts to a more readable format, either in binary or decimal notation
Private Sub downloader_ProgressChanged() Handles downloader.ProgressChanged
    pBarFileProgress.Value = CInt(downloader.CurrentFilePercentage)
    lblFileProgressDetails.Text = String.Format("Downloaded {0} of {1} ({2}%)", _
		FileDownloader.FormatSizeBinary(downloader.CurrentFileProgress), _
		FileDownloader.FormatSizeBinary(downloader.CurrentFileSize), _
		downloader.CurrentFilePercentage) & String.Format(" - {0}/s", _
    If downloader.SupportsProgress Then
        pBarTotalProgress.Value = CInt(downloader.TotalPercentage)
        lblTotalProgressDetails.Text = _
		String.Format("Downloaded {0} of {1} ({2}%)", _
		FileDownloader.FormatSizeBinary(downloader.TotalProgress), _
		FileDownloader.FormatSizeBinary(downloader.TotalSize), _
    End If
End Sub 

Another noteworthy snippet of code is how to set the SupportsProgress property.

' Setting the SupportsProgress property - 
' if set to false, no total progress data will be available!
Private Sub cbUseProgress_CheckedChanged(ByVal sender As System.Object, _
		ByVal e As System.EventArgs) Handles cbUseProgress.CheckedChanged
    downloader.SupportsProgress = cbUseProgress.Checked
End Sub 

When the SupportProgress property is set to true, the file sizes will be calculated before any download is started. This can take a while, definitely when you have a large number of files. The FileDownloader class fires an event every time it starts checking the size of a file, which can be used to display the progress.

' Show the progress of file size calculation
Private Sub downloader_CalculationFileSize(ByVal sender As Object, _
	ByVal fileNumber As Int32) Handles downloader.CalculatingFileSize
    lblStatus.Text = String.Format("Calculating file sizes - _
		file {0} of {1}", fileNumber, downloader.Files.Count)
End Sub
Image 2

Points of Interest  

When I started working on this class, I assumed it would be ready after working on it for a day. I ran into some problems though, involving a wrong assumption I had about multithreading, and the fact that you can only have two open HttpWebResponses at the same time. Before I created this class, I was oblivious to the fact that objects get locked and can't always be set via all threads without the proper extra code. I also never used the ReportProgress event of the BackGroundWorker. After creating this class, I did some more reading on multithreading to get a better grip on it, and I can recommend the following series of articles: Beginners Guide To Threading In .NET Part 1 of n. I hope this article will both help other people out, and receive some critic feedback on how to improve the class itself.

I'm hoping to implement some more features soon, including cancellation without deleting the files and the option to resume downloading afterwards, and the ability to download multiple files simultaneously, on separate threads. 


  • May 20th 2009: Published version 1.0.3 and updated this article 
  • April 30th 2009: Published version 1.0.2 on my forums (added PackageSize and StopWatchCyclesAmount properties) 
  • April 22nd 2009: Published the first version of this article 
  • April 21st 2009: Published the class
    • For the code this is based upon (can be seen as an older version), see this article.


  • Dutch support for this class can be found here.
  • If I have only slightly updated code, I won't update this article and only publish it here.


This article, along with any associated source code and files, is licensed under The GNU General Public License (GPLv3)

Written By
Software Developer
Belgium Belgium
I am a free and open source software enthusiast and freelance software developer with multiple years of experience in both web and desktop development. Currently my primarily focus is on MediaWiki and Semantic MediaWiki work. I'm in the all time top 10 MediaWiki comitters and am one of the WikiWorks consultants. You can contact me at jeroendedauw at gmail for development jobs and questions related to my work.

More info can be found on my website [0] and my blog [1]. You can also follow me on twitter [2] and [3].


Comments and Discussions

QuestionFileDownloader for multiple local directories Pin
Member 421445729-Feb-16 4:46
Member 421445729-Feb-16 4:46 
QuestionAn exception of type 'System.IO.DirectoryNotFoundException' occurred in mscorlib.dll but was not handled in user code Additional information: Could not find a part of the path 'C:\Users\Joseph\Desktop\P Pin
Member 1028596914-Jun-15 23:13
professionalMember 1028596914-Jun-15 23:13 
QuestionAbout Software code Pin
Member 1028596914-Jun-15 20:59
professionalMember 1028596914-Jun-15 20:59 
QuestionCan the code be used for resuming downloads at a later date? Pin
Soumya Kanti Sar22-Apr-15 18:32
Soumya Kanti Sar22-Apr-15 18:32 
QuestionTHANKS!!!! Pin
Member 1050394123-May-14 6:12
Member 1050394123-May-14 6:12 
Questionrestart Pin
mikehood20066-Apr-14 9:38
mikehood20066-Apr-14 9:38 
Questionhow can download a file from password protected host ? Pin
light2536025-Mar-13 12:53
light2536025-Mar-13 12:53 
QuestionInfinity % Pin
Synaps320-Feb-13 16:34
Synaps320-Feb-13 16:34 
QuestionProblem with the ProgressChanged Event Pin
Synaps38-Feb-13 10:27
Synaps38-Feb-13 10:27 
AnswerRe: Problem with the ProgressChanged Event Pin
Synaps320-Feb-13 16:32
Synaps320-Feb-13 16:32 
BugHuge problem with THE SPEED Pin
Member 847446029-Sep-12 13:03
Member 847446029-Sep-12 13:03 
GeneralRe: Huge problem with THE SPEED Pin
webflashing11-Oct-12 14:57
webflashing11-Oct-12 14:57 
GeneralRe: Huge problem with THE SPEED Pin
Member 847446018-Oct-12 6:49
Member 847446018-Oct-12 6:49 
GeneralRe: Huge problem with THE SPEED Pin
revno19-Jan-13 3:49
revno19-Jan-13 3:49 
GeneralRe: Huge problem with THE SPEED Pin
Member 1028596914-Jun-15 21:16
professionalMember 1028596914-Jun-15 21:16 
QuestionSimpleDownloadFile.suo Pin
Trizarian11-Sep-12 14:13
Trizarian11-Sep-12 14:13 
Generalnot working Pin
stefanz9227-Jun-12 2:26
stefanz9227-Jun-12 2:26 
GeneralRe: not working Pin
FusionOz19-Aug-12 16:13
FusionOz19-Aug-12 16:13 
QuestionWith File Size Pin
udnsofyan22-Feb-12 19:15
udnsofyan22-Feb-12 19:15 
Questionif use datagridview? Pin
featrick14-Nov-11 1:25
featrick14-Nov-11 1:25 
GeneralMy vote of 5 Pin
Аslam Iqbal18-Jan-11 0:48
professionalАslam Iqbal18-Jan-11 0:48 
QuestionAdding a class Pin
James Milligan16-Aug-10 22:25
James Milligan16-Aug-10 22:25 
AnswerRe: Adding a class Pin
James Milligan16-Aug-10 22:28
James Milligan16-Aug-10 22:28 
AnswerRe: Adding a class Pin
FusionOz19-Aug-12 14:50
FusionOz19-Aug-12 14:50 
GeneralGood article Pin
LloydA11110-Aug-10 23:54
LloydA11110-Aug-10 23:54 

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