title: Mef instance creation
body: <p>I am currently involved in designing an SOA application using mef. I have a doubt with instance creation. What I find is the instance can be created by two ways:
  (i) container matches the import with available export and creates instance
  (ii) Using getexportedvalue() also creates an instance.
  So when do I need to what approach. Means when the service instance lives for the whole lifetime of the application can I take 1st approach because the object is available in the container.
   And when the service is used for short time can I use the second approach and dispose it.</p>