Dynamic MEF isn't thread safe by nature this post focus a pitfall which you should be aware in order to avoid sporadic crashes. In general most of MEF operation are operate once at the application initialization stage, and it normally happens on a single thread. this may not be true when you need to dynamically load MEF parts (using GetExportedValue or SatisfyImportsOnce ). unfortunately MEF underline is using non concurrent dictionary which is not thread safe for the above operation...
Catalogs (code cartoon) catalog are search boundary for the composition container . you can read more about MEF on MEF for beginner series, the TOC is available here תגים של Technorati: Extensibility , MEF , Composition , catalogs