Have you ever attempted to write an extensible application? You know the kind. The applications where your boss wants to be able to add stuff to it without rewriting the entire application and with minimum costs. Or the type of application you want to release out to the community and provide a way for them to add their own customizations to it. We have all seen these types of applications; however, if you have ever attempted to write such, it is usually a pain to developer the core application or a pain to develop the extensions or plug-ins. Thankfully the Managed Extensibility Framework (MEF) is available to those of us that have extensibility needs inside of the application.