Threading in WCF and Dependency Property (The calling thread cannot access this object because a different thread owns it exception)

Question

Any help will be very appreciated.

We have a WCF service (hosted in IIS) that is calling a method, and inside this method there is a call to a getter of a dependency property (this getter is part of of several dependency properties gathered in a single object and used for all kind of conversions).

We see sometimes, and not consistent, a crash on the WCF service side, with the following exception message:

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

I did some searching and found out a similar problem .

From the reading I understand that there is a scenario in which the getter is called on a different thread, and this is why we don't see this exception all the time.

However, I'm still puzzled about the solution. Does the WCF service has a dispatcher on which I can activate CheckAccess() and then call the Dispatcher.Invoke() method (like in WPF application)?

Can someone please advise?

Thanks,

Elad

Answer 1

ALL DependencyObjects have thread-affinity. They can only be accessed by the thread that instantiated it. Call DependencyObject.CheckAccess() to determine if on correct thread. Here's an example. Even though the code uses a Button, a Button is still a DependencyObject.

private void TryToUpdateButtonCheckAccess(object uiObject)
{
    Button theButton = uiObject as Button;

    if (theButton != null)
    {
        // Checking if this thread has access to the object
        if(theButton.CheckAccess())
        {
            // This thread has access so it can update the UI thread
            UpdateButtonUI(theButton);
        }
        else
        {
            // This thread does not have access to the UI thread
            // Pushing update method on the Dispatcher of the UI thread
            theButton.Dispatcher.BeginInvoke(DispatcherPriority.Normal,
                new UpdateUIDelegate(UpdateButtonUI), theButton);
        }
    }
}

Answer 2

What you can do is create a class in your "primary" thread which inherits from DispatcherObject . That will give you access to the Dispatcher property in WCF.

Answer 3

Since your problem is to do with accessing dependency properties across multiple threads, you could use an STA threading model in your WCF service.

This article describes the approach

http://www.netfxharmonics.com/2009/07/Accessing-WPF-Generated-Images-Via-WCF

The relevant section is near the end of the article. It sounds like he is describing your exact problem.