The attached sample project demonstrates the error I'm getting. 

Basically, I have a UXDesktop control with a bound ItemsSource to a collection of "WindowItem" objects.  I have subclassed the UXDesktop control so I can override the GetContainerForItemOverride() method and output UXWindow objects for each bound item. 

I get an InvalidCastException: Unable to cast object of type 'SilverlightPlayground.WindowItem' to type 'Intersoft.Client.Framework.IWindow' when I try to associate a UXDesktopDock control with the TaskBar property of the UXDesktop control. 

Can anyone help me figure out how to properly associate a UXDesktopDock control to a UXDesktop control when it's ItemsSource is bound to a collection of items that are not IWindow objects?