>>>In this case A does not seem
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>>Looks like you were cut off in mid-flow :-}
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>it was a fragment of something I started to write and then rewrote. It was scrolled off screen when I hit post.
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>I had to rush off and didn't do my usual double check. So... ooops.
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>>This is about the only time that ViewModelA and ViewModelB need to interact and, AFAICS, this is the only other communication they need so I'll leave as is for now. But, thinking about it, it would be a fairly trivial step to combine both VMs under a 'master' VM - the XAML guys would only need to know how to bind to both bits......
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>Your solution sounds fine to me too.
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>I see you've decided to go ahead with Prism.
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>Hopefully your having fun with it. <g>
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>I hooked Prism up to AvalonDock last weekend and created a basic framework/interface for my modules. MEF auto discovers the modules and they incorporate themselves into the menus and toolbars of the framework. So far it's been surprisingly easy to code and I like what I'm getting out of it.

MEF seems very cool. I saw a session on it at a user group meeting about a year ago. Not only does it auto discover, it does so dynamically at run time. The presenter threw together a demo of pictures of animals, looking in a certain folder for images. While the app was running he copied a picture of an elephant into the folder and viola, it showed up in the app.