BitmapFrame in another thread

Question

I am using a WPF BackgroundWorker to create thumbnails. My worker function looks like:

private void work(object sender, DoWorkEventArgs e)
{ 
  try
  {
    var paths = e.Argument as string[];
    var boxList = new List<BoxItem>();
    foreach (string path in paths)
    {                   
       if (!string.IsNullOrEmpty(path))
       {
        FileInfo info = new FileInfo(path);
        if (info.Exists && info.Length > 0)
        {
           BitmapImage bi = new BitmapImage();
           bi.BeginInit();
           bi.DecodePixelWidth = 200;
           bi.CacheOption = BitmapCacheOption.OnLoad;
           bi.UriSource = new Uri(info.FullName);
           bi.EndInit();
           var item = new BoxItem();
           item.FilePath = path;
           MemoryStream ms = new MemoryStream();
           PngBitmapEncoder encoder = new PngBitmapEncoder();
           encoder.Frames.Add(BitmapFrame.Create(bi));  
           encoder.Save(ms);
           item.ThumbNail = ms.ToArray();
           ms.Close();
           boxList.Add(item);
        }
      }
    }
    e.Result = boxList;
  }
  catch (Exception ex)
  { 
    //nerver comes here
  }
}

When this function is finished and before the BackgroundWorker "Completed" function is started, I can see on the output window on Vs2008, that a exception is generated. It looks like:

 A first chance exception of type 'System.NotSupportedException' occurred in PresentationCore.dll

The number of exceptions generates equals the number of thumbnails to be generated.

Using "trial and error" method I have isolated the problem to: BitmapFrame.Create(bi)

Removing that line (makes my function useless) also removes the exception.

I have not found any explanation to this, or a better method to create thumbnails in a background thread.

Answer 1

Lasse, I believe the problem arises because you are performing actions outside of the UI thread that need to be done within the UI thread. Creating UI elements (BitmapImage, BitmapFrame) and adding to UI Containers, I believe, should be done on the UI thread. (Someone correct me if I'm wrong here).

There are a few ways to create those elements on the UI thread without blocking the application for an excessive period of time. The easiest is probably using the BackgroundWorker's ProgressChanged event. ProgressChanged is invoked on the UI thread, which makes it perfect for this situation.

You can use the worker's ProgressChanged event and pass it the path needed to load a thumbnail in the UserState argument.

Answer 2

Thanks for your input. It made start to look for another solutions and I come up with this.

       try
        {
            var paths = e.Argument as string[];
            var boxList = new List<BoxItem>();

            foreach (string path in paths)
            {
                using (Image photoImg = Image.FromFile(path))
                {
                    int newWidth = 200;
                    int width = newWidth;
                    int height = (photoImg.Height * newWidth) / photoImg.Width;

                    var thumbnail = new Bitmap(width, height);

                    using (Graphics g = Graphics.FromImage((System.Drawing.Image)thumbnail))
                    {
                        g.DrawImage(photoImg, 0, 0, width, height);
                        using (var ms = new System.IO.MemoryStream())
                        {
                            var item = new BoxItem();
                            item.FilePath = path;

                            thumbnail.Save(ms, System.Drawing.Imaging.ImageFormat.Png);

                            item.ThumbNail = ms.ToArray();
                            boxList.Add(item);
                        }
                    }
                }
            }
            e.Result = boxList;
        }

        catch (Exception exp)
        {
        } 

Not using any UI elements .. and works nicely. Thanks. //lasse