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After lots of wasted time trying to figure out why my MEF module in a new WPF application was not loading, I've found that if there is not a constructor that can be used, then there is a silent failure, with no errors.  I had a single constructor in my MEF module which was relying on 2 items to be imported (it is an Importing Constructor). Since one of the items I wanted to import was not created properly by the main application prior to calling this constructor, the constructor was never called.

If you have a MEF module that should load but setting a breakpoint in your constructor reveals that it is never called, then change the module to have a parameterless default constructor and see if that is called. If it is, then you can narrow down which of the Imported parameters in your real constructor is causing the problem.