silverlight memory leak

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Silverlight4

hi,
i was creating a DependencyProperty Changed with a callback method using the following code:

public void RegisterForNotification(string propertyName, FrameworkElement element, PropertyChangedCallback callback) 
        { 
 
            Binding b = new Binding(propertyName) { Source = element }; 
            var prop = System.Windows.DependencyProperty.RegisterAttached( 
                "ListenAttached" + propertyName, 
                typeof(object), 
                typeof(UserControl), 
                new System.Windows.PropertyMetadata(callback));              
            element.SetBinding(prop, b); 
        }

The callback method caused a memory leak in my application, how can i unregister this callback method?

thanks in advance.