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Downloader Component

, 14 Jan 2007
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A component to download files over the network with support for proxies, SSL and resume.
Sample Image - Downloader.jpg

Introduction

Downloading files over the network could be a cumbersome task. Trying to implement other features like auto-resume and support for proxy servers, etc. makes it even harder. Using this component can make things a lot easier for yourself. You can download files over proxies, password protected sites or untrusted SSL connections simply by setting a few properties. The example that is provided shows how to do the job and how to handle events over a simple GUI.

Since the download operation takes place in a worker thread asynchronously, you don't need to worry about downloading large files or blocking the user interface. There are some points you need to have in mind though.

Aborting the Worker Thread

Stopping a download operation that is running is tricky. You could just call the DownloadThread.Abort() method on the downloader thread to stop the work, but this is not the recommended way. The main reason for this is that Thread.Abort throws an exception which can occur at any time during your work and leaves your running state inconsistent. The easier way is to check for a boolean field, and terminate the operation when the field is set, or run the job on an entirely different process. To read more about this idea, check Ian Griffiths blog post here.

Updating GUI

When running a download job, the Downloader component provides its internal progress and error state through events and custom eventargs. Since the eventargs are constructed on the worker thread, trying to update the user-interface from the worker thread throws a Cross-Thread exception. The workaround to update the user interface is to use the Invoke method (inherited from Control), a Delegate and call to the necessary method(s), like this:

private delegate void ParamMethodInvoker
    (long fileSize, long progressValue, string message);

/// <summary>
/// Call UpdateProgress method using a delegate and provide method parameters
/// </summary>
private void OnDownloadProgressChanged
    (object sender, DownloadProgressEventArgs e)
{
    Invoke(new ParamMethodInvoker(UpdateProgress), new object[] 
        { e.BytesRead, e.TotalBytes, e.Message });
}

/// <summary>
/// Normally update the user interface
/// </summary>
private void UpdateProgress(long bytesRead, long totalBytes, string message)
{
    int kbRead = Convert.ToInt32(bytesRead) / 1024;
    int kbTotal = Convert.ToInt32(totalBytes) / 1024;
    int percent = Convert.ToInt32((bytesRead * 100) / totalBytes);

    progress.Value = percent;
    lblDownloadMessage.Text = 
        string.Format("{0:#,###} of {1:#,###} KBytes ({2}%)", 
        kbRead, kbTotal, percent);
}

Resuming a Download

To resume a download job, we can simply continue where we left off. When restarting a download job, the downloader component first checks for the existing file at the specified location (through DownloadPath property). If the file exists, it reads the Length of the existing file and sets the HttpWebRequest's range using AddRange method. Overloads of the AddRange method could be used to handle multi-part downloading of a single file.

long startingPoint = 0;

if(File.Exists(DownloadPath))
{
    startingPoint = new FileInfo(DownloadPath).Length;
}

HttpWebRequest _Request = (HttpWebRequest)HttpWebRequest.Create(url);
_Request.AddRange(startingPoint);

Points of Interest

Implementing other features such as multi-part downloading (like download accelerator applications) and FTP downloads seems like a good idea. Any suggestions/comments/feedback are highly appreciated.

History

Version 1

  • Provided the component with base features and events
  • Provided a basic form to show features of the component

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

About the Author

Hadi Eskandari
Software Developer (Senior) Readify
Australia Australia
Working on both Java and .NET Technology, I have developed various enterprise level applications on both platforms. Currently, I am working as a Senior Software Developer at Readify which is a leading company on .NET technology in Australia.
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Comments and Discussions

 
GeneralMy vote of 5 PinmvpMichael Haephrati7-Mar-13 11:03 
QuestionMy vote of 5 PinmemberPouriya.GH30-Apr-12 3:27 
GeneralMy vote of 5 PinmemberShahin Khorshidnia3-Apr-11 19:18 
GeneralError Downloading file greater than 2GB [modified] Pinmemberakshayoh12-Feb-11 8:43 
GeneralRe: Error Downloading file greater than 2GB [modified] PinmemberKOS_void28-Feb-12 4:39 
Generaldoc required Pinmemberprudhvi1234516-Mar-10 9:03 
GeneralImplementing Speed Limiting (aka Throttling/Shaping) PinmemberTrendyTim14-Sep-07 16:30 
Generalpoint of interest Pinmemberjugomkd7510-Aug-07 8:25 
GeneralRe: point of interest Pinmemberjugomkd7510-Aug-07 21:20 
QuestionHow about FTP ?? Pinmember_Thurein_9-May-07 17:09 
AnswerRe: How about FTP ?? Pinmemberh.eskandari9-May-07 19:52 
GeneralRe: How about FTP ?? Pinmember_Thurein_9-May-07 21:36 
GeneralRe: How about FTP ?? Pinmemberh.eskandari9-May-07 22:41 
GeneralRe: How about FTP ?? PinmemberGaara [BTK]12-Oct-07 0:28 
Questionwhat if from a webdav server Pinmemberroychoo16-Mar-07 4:11 
AnswerRe: what if from a webdav server PinmemberHadi Eskandari16-Mar-07 18:51 
GeneralTest for file size if resuming PinmemberJeffrey Scott Flesher23-Jan-07 18:14 
GeneralVery similar... PinmemberScott Dorman15-Jan-07 6:30 
GeneralRe: Very similar... PinmemberHadi Eskandari16-Jan-07 19:22 
GeneralRe: Very similar... PinmemberScott Dorman18-Jan-07 4:09 
GeneralTnx For This Article Pinmemberyunas14-Jan-07 2:36 
GeneralJust a little problem PinmemberVertyg014-Jan-07 1:34 
GeneralRe: Just a little problem PinmemberHadi Eskandari14-Jan-07 19:14 
GeneralRe: Just a little problem PinmemberHadi Eskandari11-Feb-07 20:50 
GeneralRe: Just a little problem PinmemberVertyg011-Feb-07 21:42 
GeneralRe: Just a little problem PinmemberHadi Eskandari12-Feb-07 19:39 
GeneralRe: Just a little problem PinmemberVertyg012-Feb-07 20:13 
GeneralRe: Just a little problem Pinmemberdeerchao8-Aug-07 22:38 

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