Calling Async synchronously, 'marshalled for a different thread' message

Question

I have a Windows 8 store app in XAML. I have an Image control that binds to a property, but because the URL I want to display requires authentication, I have to create a webresponse and generate the bitmap that way instead of just providing a URL to my image control.

Problem is, the memorystream operations are async, and properties on an object have to be sync, not async. So I have a pretty simple setup:

public ImageSource ImageSource
{
    get { return Task.Run(() => BitmapImageUtils.ToImage(this.Upload.ThumbFile)).Result; }

and the Image control has the ImageSource property as its binding. Problem is, I'm receiving the below exception. there's multiple Image controls in a ListView and they're all binding in this way, and my guess is that the UI thread that invokes this somehow hands off control to a thread then tries to come back somehow. I'm a little new to this.

The application called an interface that was marshalled for a different thread. (Exception from HRESULT: 0x8001010E (RPC_E_WRONG_THREAD))

Any help is appreciated.

--- EDIT ToImage Method

public static async Task<BitmapImage> ToImage(byte[] byteArray)
    {
        var bitmapImage = new BitmapImage();

        var stream = new InMemoryRandomAccessStream();
        await stream.WriteAsync(byteArray.AsBuffer());
        stream.Seek(0);

        await bitmapImage.SetSourceAsync(stream);
        return bitmapImage;
    }

Exception (Note the exception is an inner exception of an aggregate exception, which I believe is pretty standard for async/await exceptions

at Windows.UI.Xaml.Media.Imaging.BitmapImage..ctor()
at Campfire.Utils.BitmapImageUtils.<ToImage>d__0.MoveNext()

Side note: property that executes network operation feels very wrong... What if it will be called more often than you expect? — Alexei Levenkov
Agreed, its not ideal. But I can't think of a way to show an image control on demand when the image is behind an authentication barrier. (requires a special header in the web request with an auth token). Plus in a listview the image isn't loaded until the item is in view, which in my case performs pretty well. — Richthofen
Side note: check out this question as it may be useful - Make WPF Image load async. — Alexei Levenkov
What is the stack trace of the exception? And the code on ToImage might be useful too. — svick

Stephen Cleary Stephen Cleary · Accepted Answer · 2013-12-30T11:50:36

The core problem is that many UI types (including BitmapImage) have UI thread affinity, and your current code is trying to create them on a background thread (in Task.Run).

I recommend that you redesign your property; you shouldn't have it blocking on an I/O operation anyway. I have a type in my AsyncEx library that allows you to essentially data-bind to the results of a Task<T>.

Your property becomes:

public INotifyTaskCompletion<ImageSource> ImageSource { get; private set; }

And when you want to start downloading, you do:

ImageSource = NotifyTaskCompletion.Create(BitmapImageUtils.ToImage(Upload.ThumbFile)));

Your databinding then changes from ImageSource to ImageSource.Result. There are other properties as well (e.g., ImageSource.IsFaulted, ImageSource.ErrorMessage) that allow you to handle other results with data binding. Note that INotifyTaskCompletion<T>.Result is not blocking; it will just return the default value until the task completes.

P.S. AggregateException is not normal in async/await. The only reason you were seeing it is because you were using Task<T>.Result; if you await the same task, you'll get an exception that is not wrapped in AggregateException.

Calling Async synchronously, 'marshalled for a different thread' message

2 Answers