title: How to use MEF's GetExport\ImportDefinition?
body: <p>I've got main project, and a series of dll's that have specific implementations of some interfaces as well as view\viewmodels, etc. I use {ImportMany] to get them all with some metadata, which allows me to choose one, which gives me a reference to the specific implementation of ISystem.</p>
  
  <p>I'd like to get the only specific implementation of IDisplay from whatever dll the ISystem is chosen from, which I marked with [Export("SomeDisplay", typeof(IDisplay))]
  So far, the only thing that I can find that looks like it might work is GetExports(ImportDefinition), but I don't understand how to create an ImportDefinition that would work, as it seems to want a specific contractname, which isn't known until runtime. </p>
  
  <p>Of course since I'm still a MEF n00b, it's a good bet I'm doing it wrong :D
  So, if GetExports is the best way, how can I make it work? Or is there a better way I should be using?</p>
  
  <p>Thanks!</p>