Question

I am attempting to have a progress bar’s progress change as the WebClient download progress changes. This code still downloads the file yet when I call startDownload() the window freezes as it downloads the file. I would like for the user to be able to see the progress change as the splash screen loads. Is there any way to fix this so that the user can see the progress of progressBar2 change?

private void startDownload()
{
    WebClient client = new WebClient();
    client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
    client.DownloadFileCompleted += new AsyncCompletedEventHandler(client_DownloadFileCompleted);
    client.DownloadFileAsync(new Uri("http://joshua-ferrara.com/luahelper/lua.syn"), @"C:\LUAHelper\Syntax Files\lua.syn");
}
void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
{
    double bytesIn = double.Parse(e.BytesReceived.ToString());
    double totalBytes = double.Parse(e.TotalBytesToReceive.ToString());
    double percentage = bytesIn / totalBytes * 100;
    label2.Text = "Downloaded " + e.BytesReceived + " of " + e.TotalBytesToReceive;
    progressBar1.Value = int.Parse(Math.Truncate(percentage).ToString());
}
void client_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
{
    label2.Text = "Completed";
}

Answer 1

You should call startDownload() from the UI thread. The whole idea of WebClient.DownloadFileAsync() is that it will spawn a worker thread for you automatically without blocking the calling thread. In startDownload(), you specified callbacks that modify controls which I assume were created by the UI thread. Thus if you call startDownload() from a background thread it will cause problems, because a thread can only modify UI elements it created.

The way it is supposed to work is you call startDownload() from the UI thread, startDownload() as you defined it sets up event call backs that are handled by the UI thread. It then starts the download asynchronously and returns immediately. The UI thread will be notified when the progress changes and the code responsible for updating the progress bar control will execute on the UI thread, and there shouldn’t be any problems.