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What problems does MEF (Managed Extensibility Framework) solves that cannot be solved by existing IoC/DI containers?
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No, it is otherwise. IoC is a broad concept and DI is the design pattern to implement the core of IoC. MEF is some form of DI, but it has not all fundamental features of IoC. MEF uses composition to find out the dependencies it needs to resolve.
This article shows how to use the built-in dependency injection. The Managed Extensibility Framework (MEF) is a built-in set of elements that allows you to “export” and “import” objects across projects that allows you to not rely on hard dependencies.
For those who don't know, the Managed Extensibility Framework (MEF) is alive and well, and has been ported to . NET Core as System.
The Managed Extensibility Framework or MEF is a library for creating lightweight, and extensible applications. It allows application developers to discover and use extensions with no configuration required. It also lets extension developers easily encapsulate code and avoid fragile hard dependencies.
The principle purpose of MEF is extensibility; to serve as a 'plug-in' framework for when the author of the application and the author of the plug-in (extension) are different and have no particular knowledge of each other beyond a published interface (contract) library.
Another problem space MEF addresses that's different from the usual IoC suspects, and one of MEFs strengths, is [extension] discovery. It has a lot of, well, extensible discovery mechanisms that operate on metadata you can associate with extensions. From the MEF CodePlex site:
"MEF allows tagging extensions with additonal metadata which facilitates rich querying and filtering"
Combined with an ability to delay-load tagged extensions, being able to interrogate extension metadata prior to loading opens the door to a slew of interesting scenarios and substantially enables capabilities such as [plug-in] versioning.
MEF also has 'Contract Adapters' which allow extensions to be 'adapted' or 'transformed' (from type > to type) with complete control over the details of those transforms. Contract Adapters open up another creative front relative to just what 'discovery' means and entails.
Again, MEFs 'intent' is tightly focused on anonymous plug-in extensibility, something that very much differentiates it from other IoC containers. So while MEF can be used for composition, that's merely a small intersection of its capabilities relative to other IoCs, with which I suspect we'll be seeing a lot of incestuous interplay going forward.
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