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

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: 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....
                    }), 
                    System.Windows.Threading.DispatcherPriority.ApplicationIdle);
         }
    );
}

---

Code2: It 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;
                    }),
                    System.Windows.Threading.DispatcherPriority.ApplicationIdle);
            }
        );

---

code3: It 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;
                    }),
                    System.Windows.Threading.DispatcherPriority.ApplicationIdle);
            }
        );

Answer 1

When working with WPF be aware that if you create a UI object in one thread you can't access it from another thread. Your UI objects should (typically) be created the UI thread, and then you need the UI thread to access them later. No other thread will be able to access objects created on the UI thread.

If you need to access a UI object from another thread you need the UI thread Dispatcher, and then you can use this to invoke calls on the UI thread.

I've spent many hours in frustration of similar problems to this - believe me.. Check out this question - it gave me a lot of useful information on the subject.

Answer 2

Following code might help you solve the issue of updating a gui element from another thread :

Module level

delegate void updateCallback(string tekst);

This is the method to update your element :

private void UpdateElement(string tekst)
{
    if (element.Dispatcher.CheckAccess() == false)
    {
        updateCallback uCallBack = new updateCallback(UpdateElement);
        this.Dispatcher.Invoke(uCallBack, tekst);
    }
    else
    { 
//update your element here
    }
 }

Answer 3

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 :)