How to use multithreading in wp7?

Question

I am developing a wp7 application. I need to download some files from a server to my phone. The files are selected from a listbox. When trying to download a single item the download works perfectly. But when I am trying to download more than one item simultaneously there is an error occurs. How can I download more than one item simultaneously?

here is my code for download files.

IList<Item> selectedItems = documents.Where( p => p.IsChecked == true ).ToList();
foreach ( var item in selectedItems )
{
   FileDownload objDownload = new FileDownload();

   objDownload.FileName = item.Title;
   objDownload.Url = item.Link;
   objDownload.DocId = item.DocId;

   ThreadPool.QueueUserWorkItem( t =>
   {
     Dispatcher.BeginInvoke( () =>
     {
       objDownload.DownloadFile();
     } );
     Thread.Sleep( 0 );
   } );
}

and here is the DownloadFile() method in FileDownload class

public void DownloadFile()
{
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create( Url );
    request.AllowReadStreamBuffering = false;
    request.BeginGetResponse( new AsyncCallback( DownloadFileData ), request );
}

private void DownloadFileData( IAsyncResult result )
{
    try
    {
        AddToDownloadList();
        IsolatedStorageFile isf = IsolatedStorageFile.GetUserStoreForApplication();
        string fileName = "DownloadedFiles\\" + FileName;
        if ( !isf.DirectoryExists( "DownloadedFiles" ) )
            isf.CreateDirectory( "DownloadedFiles" );

        if ( !isf.FileExists( fileName ) )
        {
            IsolatedStorageFileStream streamToWriteTo = new IsolatedStorageFileStream( fileName, FileMode.Create, isf);
            HttpWebRequest request = (HttpWebRequest)result.AsyncState;
            HttpWebResponse response = (HttpWebResponse)request.EndGetResponse( result );
            Stream str = response.GetResponseStream();
            byte [ ] data = new byte [ 16 * 1024 ];
            int read;
            long totalValue = response.ContentLength;
            while ( ( read = str.Read( data, 0, data.Length ) ) > 0 )
            {
                streamToWriteTo.Write( data, 0, read );
            }
            string file = streamToWriteTo.ToString();
            streamToWriteTo.Close();
        }
        OfflineStatus = true;
    }
    catch ( Exception ex )
    {
        OfflineStatus = false;
    }
}

How can I use multithreading for downloading the files.

Answer 1

There are a few problem with your code:

• you have to close/dispose the store. The best pratice is to use a using statement

  using (var store = IsolatedStorageFile.GetUserStoreForApplication()) { // task }

• You won't gain any performance of downloading stuff using multiple threads* - the connection is the bottleneck not the CPU cycles per second.

• If you have a multithreaded app that does IO for the same location simple if ( !isf.DirectoryExists( "DownloadedFiles" ) ) won't work. You have a race condition. What if one thread starts creating the dir and second thread checks the dir before first one finished?

• Do not swallow exception with catch ( Exception ex ). Very often the exception has the information about the problem.

I would solve this problem using a similar solution to this one .

*Multithreaded apps tend to be indeterministic, is it possible that download will be faster, but in this case, most likely, it will be slower.

Answer 2

you are overwriting your objDownload object in your loop, destroying the previous reference. create List of FileDownload and add each on to keep separate.

Answer 3

rob is correct with regards to your objDownload, but your use of ThreadPool.QueueUserWorkItem + Dispatcher.BeginInvoke are completely unnecessary as WebRequest.BeginGetResponse is asynchronous and non-blocking. Your current code moves to a background thread, then to the UI thread, and then performs an async HTTP operation that returns on a background thread (which is pretty crazy).

Side note: are the files you are downloading large (eg. > 1mb)? If so, you should be using Background File Transfers - they will continue to download even when your application is not running.