Backgroundworker thread throws nullreferenceexception

Question

Hi,

I am loading the images using the BackgroundThread. I am receving "nullreferenceexception unhandled by user code" after loading all the images into the Listview. What could be the issue? Please let me know.

   private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
    {
        try
        {
            int progress = 0;

            string pname;
            Image myImage;
            max_length = files.Length;
            for (int k = 0; k < files.Length; k++)
            {
                ProgressInfo info = new ProgressInfo();
               pname = System.IO.Path.GetFullPath(files[k]);
                myImage = Image.FromFile(pname);
                info.Image = myImage;
                info.ImageIndex = k;
                backgroundWorker1.ReportProgress(progress, info);
                myImage = null;
            }
        }
        catch (Exception ex)
        {
            throw ex.InnerException;
        }
    }

    private void backgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {

            try
            {

                //Get image.
                ProgressInfo img = e.UserState as ProgressInfo;
                //Set image to ListView here.
                ImgListView.Images.Add(getThumbnaiImage(ImgListView.ImageSize.Width, img.Image));
                fname = System.IO.Path.GetFileName(files[img.ImageIndex]);
                ListViewItem lvwItem = new ListViewItem(fname, img.ImageIndex);
                lvwItem.Tag = files[img.ImageIndex];

                lstThumbNailView.Items.AddRange(new ListViewItem[] { lvwItem });

                percent = (int)((float)100 * (float)i / (float)files.Length);
                this.progressBar1.Value = (int)percent;
                 this.label1.Text = "Loading images...";

            }
            catch (Exception ex)
            {
                throw ex.InnerException;
            }
    }

Answer 1

Judging from your comments, you're seeing the error because not all exceptions have an InnerException. If InnerException is null, you will see this problem.

There are several issues at work here though. First, here is the proper try/catch method:

try
{
  // Code here
}
catch (Exception ex)
{
  // Handle your exception
  throw; // Rethrow the same exception, preserving the stack trace (optional)
}

Second, you are likely abusing the purpose of ReportProgress. You should attempt to do all your logic in your backgroundWorker_DoWork, and send the percentage (between 0 and 100) to ReportProgress to update any progress bars.

You may have used the ReportProgress in the way you did to fix a multi-threaded issue. To add items to a ListBox across threads, wrap your code in an anonymous method using the BeginInvoke function

Example:

// Note: I haven't error checked this, and this is only a rough idea of what 
// you're trying to do. I'm not even sure of the overall goal here.
this.lstThumbnailView.Invoke((Action)delegate
{
  ListViewItem lvwItem = new ListViewItem(name, img.ImageIndex);
  ivwItem.Tag = files[img.ImageIndex];
  lstThumbNailView.Items.Add(lvwItem);
});

Answer 2

Thanks for the quick response. Here are the changes I made. It seems to be working fine.

   private void backgroundWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
    {

            try
            {
                ProgressInfo img = e.UserState as ProgressInfo;
                LoadImages(img);
                this.progressBar1.Value = img.Percent;
                this.label1.Text = "Loading images...";

            }
            catch (Exception ex)
            {
               throw ex;
            }
    }

    private void LoadImages(ProgressInfo img)
    {
        ImgListView.Images.Add(getThumbnaiImage(ImgListView.ImageSize.Width, img.Image));
        this.lstThumbNailView.Invoke((Action)delegate
        {
            fname = System.IO.Path.GetFileName(files[img.ImageIndex]);
            ListViewItem lvwItem = new ListViewItem(fname, img.ImageIndex);
            lvwItem.Tag = files[img.ImageIndex]; 
            lstThumbNailView.Items.Add(lvwItem); 
        });
    }