async LoadImage routine hanging

Question

I have a Windows 8 app that I'm attempting to load an image using the following code:

    private async Task<BitmapImage> LoadImage(IStorageFile storageFile)
    {
        System.Diagnostics.Debug.WriteLine("LoadImage started");

        try
        {
            // Set the image source to the selected bitmap
            BitmapImage bitmapImage = null;

            // Ensure a file was selected
            if (storageFile != null)
            {
                System.Diagnostics.Debug.WriteLine("LoadImage::OpenAsync");
                // Ensure the stream is disposed once the image is loaded
                using (IRandomAccessStream fileStream = await storageFile.OpenAsync(Windows.Storage.FileAccessMode.Read))
                {
                    System.Diagnostics.Debug.WriteLine("New Bitmap");
                    // Set the image source to the selected bitmap
                    bitmapImage = new BitmapImage();


                    System.Diagnostics.Debug.WriteLine("Set Source");
                    bitmapImage.SetSource(fileStream);
                }
            }

            return bitmapImage;
        }
        catch (Exception ex)
        {
            System.Diagnostics.Debug.WriteLine(ex.ToString());
        } // End of catch
        finally
        {
            System.Diagnostics.Debug.WriteLine("Load image finished");
        }

        return null;
    }

When I run the code it sometimes works. But other times it simply hangs and I get the following output:

LoadImage started
LoadImage::OpenAsync

Am I using storageFile.OpenAsAsync incorrectly? My storage file is the result of a call to:

        FileOpenPicker openPicker = new FileOpenPicker();
        openPicker.ViewMode = PickerViewMode.List;
        openPicker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;

        openPicker.FileTypeFilter.Add(".jpg");
        openPicker.FileTypeFilter.Add(".png");
        openPicker.FileTypeFilter.Add(".bmp");

        StorageFile file = await openPicker.PickSingleFileAsync();
        if (file != null)
        {
          var loadImageTask = LoadImage(currentStorageFile);
          loadImageTask.Wait();
          DisplayImage.Source = loadImageTask.Result;
        }

So it should not be a sandbox issue (and there is no exception).

Could anyone point me in the right path?

Do you have a call to `Task.Wait` or `Task.Result` somewhere further up your call stack? — Stephen Cleary, Apr 27 '13 at 11:52
Yes, I have: var loadImageTask = LoadImage(currentStorageFile); loadImageTask.Wait(); DisplayImage.Source = loadImageTask.Result; — Kyle, Apr 27 '13 at 12:35

score 3 · Accepted Answer · answered Apr 27 '13 at 13:05

The call to Task.Wait and Task.Result is causing the deadlock. I explain this in detail on my blog and in a recent MSDN article.

In short, when you await a Task that has not yet completed, by default it will capture a "context" and use that to resume the async method. In your case, this is the UI context. However, when you call Wait (or Result), you're blocking the UI thread, so the async method cannot complete.

To fix this, use await instead of Wait/Result, and use ConfigureAwait(false) everywhere you can.

async LoadImage routine hanging

1 Answers1