I recently was writing a video player in Silverlight 2 beta 2 and there was a requrement that when you hit PLAY on one player, all the others stopped. That's fine, I thought, I'll just create a static list and add each instance to the list in the constructor. Only trouble was, this didn't work because these were different instances of Silverlight on the page, so different CLR instances. What I needed was a way to call across app domains between the Silverlight instances. What I did was pretty clean so I thought I'd post about it.