Question

i have created the downloader file

private void btnTestDownload_Click(object sender, EventArgs e)
{
backgroundWorker1.RunWorkerAsync();
} 

and work! but

private void btnTestDownload_Click(object sender, EventArgs e)
{
backgroundWorker1.CancelAsync();
}

the backgroundWorker dos not stop!

Answer 1

It sounds like you’ve got completely the wrong idea about the purpose of a background worker. If you want to download a single file asyncronously with the ability to cancel it, all this functionality is built into the WebClient class.

Background worker is for long running tasks which are, on the whole, processor intensive. For example, if the file you were downloading were a large text file and you needed to parse each line of the text file, you could use the background worker for that, e.g.

Downloading a File

    WebClient Client = new WebClient();

    public void TestStart()
    {
        //Handle the event for download complete
        Client.DownloadDataCompleted += Client_DownloadDataCompleted;
        //Start downloading file
        Client.DownloadDataAsync(new Uri("http://mywebsite.co.uk/myfile.txt"));
    }

    void Client_DownloadDataCompleted(object sender, DownloadDataCompletedEventArgs e)
    {
        //Remove handler as no longer needed
        Client.DownloadDataCompleted -= Client_DownloadDataCompleted;
        //Get the data of the file
        byte[] Data = e.Result;
    }
    public void TestCancel()
    {
        Client.CancelAsync();
        Client.DownloadDataCompleted -= Client_DownloadDataCompleted;
    }

Processing a File

    BackgroundWorker worker = new BackgroundWorker() { WorkerSupportsCancellation = true };

    //Take a stream reader (representation of a text file) and process it asyncronously
    public void ProcessFile(StreamReader Reader)
    {
        worker.DoWork += worker_DoWork;
        worker.RunWorkerAsync(Reader);
    }
    public void CancelProcessFile()
    {
        worker.CancelAsync();
    }

    void worker_DoWork(object sender, DoWorkEventArgs e)
    {
        //Get the reader passed as an argument
        StreamReader Reader = e.Argument as StreamReader;
        if (Reader != null)
        {
            //while not at the end of the file and cancellation not pending
            while (Reader.Peek() != -1 && !((BackgroundWorker)sender).CancellationPending)
            {
                //Read the next line
                var Line = Reader.ReadLine();
                //TODO: Process Line
            }
        }
    }