Question

Why I can’t create CroppedBitmap in the following code? I got an exception:

The calling thread cannot access this object because a different thread owns it.

If I change the code to

CroppedBitmap cb = new CroppedBitmap(new WriteableBitmap(bf), new Int32Rect(1, 1, 5, 5));

the exception is gone? why ?

Code 1, an exception at cb.Freeze():

public MainWindow()
{
    InitializeComponent();

    ThreadPool.QueueUserWorkItem((o) =>
        {
            //load a large image file
            var bf = BitmapFrame.Create(
                new Uri("D:\\1172735642.jpg"),
                BitmapCreateOptions.None,
                BitmapCacheOption.None);
            bf.Freeze();
            Dispatcher.BeginInvoke(
                new Action(() =>
                    {
                        CroppedBitmap cb = new CroppedBitmap(bf, new Int32Rect(1,1,5,5));
                        cb.Freeze();
                        //set Image's source to cb....
                    }), 
                    DispatcherPriority.ApplicationIdle);
         }
    );
}

Code 2, works:

    ThreadPool.QueueUserWorkItem((o) =>
    {
        var bf = BitmapFrame.Create(
                new Uri("D:\\1172740755.jpg"),
                BitmapCreateOptions.None,
                //BitmapCreateOptions.DelayCreation,
                BitmapCacheOption.None);
        bf.Freeze();
        var wb = new WriteableBitmap(bf);
        wb.Freeze();
        this.Dispatcher.Invoke(
            new Action(() =>
            {
                var r = new Int32Rect(1, 1, 5, 5);
                CroppedBitmap cb = new CroppedBitmap(wb, r);
                cb.Freeze();
                //set Image's source to cb....
                Image.Source = cb;
            }),
            DispatcherPriority.ApplicationIdle);
    }
);

Code 3, works without WritableBitmap:

ThreadPool.QueueUserWorkItem((o) =>
    {
        var bf = BitmapFrame.Create(
                new Uri("D:\\1172735642.jpg"),
                BitmapCreateOptions.None,
                //BitmapCreateOptions.DelayCreation,
                BitmapCacheOption.None);
        bf.Freeze();
        var bf2 = BitmapFrame.Create(bf);
        bf2.Freeze();

        this.Dispatcher.Invoke(
            new Action(() =>
            {
                var r = new Int32Rect(1, 1, 5, 5);
                BitmapSource cb = new CroppedBitmap(bf2, r);
                cb.Freeze();
                //set Image's source to cb....
                Image.Source = cb;
            }),
            DispatcherPriority.ApplicationIdle);
    }
);

Answer 1

You can look through this classes in reflector. Exception will rise in cb.Freeze(). In

CroppedBitmap cb = new CroppedBitmap(bf, new Int32Rect(1,1,5,5));

case constructor did something like this:

this.this.Source = source;

So source wasn’t created in current thread, and so exception will rise.
In

new WriteableBitmap(bf)

case, constructor synchronize with bf object and new source is created in current thread, so, no exceptions will rise.
If you are interested in In Depth details, you can always reflect base libraries with Reflector 🙂