Managed Extensibility Framework (MEF) is a component of .NET Framework 4.0 aiming to create lightweight, 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. MEF also allows extensions to be reused across applications. MEF was introduced as a part of .NET 4.0 and Silverlight 4. It is available on CodePlex along with source and as a result can be used, albeit unsupported and with limitations, on version 3.5 of the framework.
MEF presents aims to solve the runtime extensibility problem. Until now, any application that wanted to support a plugin model needed to create its own infrastructure from scratch. Those plugins would often be application-specific and could not be reused across multiple implementations.
MEF provides a standard way for the host application to expose itself and consume external extensions. Extensions, by their nature, can be reused amongst different applications. However, an extension could still be implemented in a way that is application-specific. Extensions themselves can depend on one another and MEF will make sure they are wired together in the correct order, sparing the developer from doing it manually.
MEF offers a set of discovery approaches for the application to locate and load available extensions.
MEF allows tagging extensions with additional metadata which facilitates rich querying and filtering.
Roughly speaking, MEF's core consists of a catalog and a CompositionContainer. A catalog is responsible for discovering extensions and the container coordinates creation and satisfies dependencies.
MEF's first-class citizen is a ComposablePart (see Parts). A composable part offers up one or more Exports, and may also depend on one or more externally provided services or Imports. A composable part also manages an instance, which can be an object instance of a given type (it is in the default MEF implementation)[clarification needed]. MEF, however, is extensible and additional ComposablePart implementations can be provided as long as they adhere to the Import/Export contracts.
Exports and imports each have a Contract. Contracts are the bridge between exports and imports. An export contract can consist of further metadata that can be used to filter on its discovery. For example, it might indicate a specific capability that the export offers.
MEF's container interacts with Catalogs to have access to composable parts. The container itself resolves a part's dependencies and exposes Exports to the outside world. Composable part instances may be added directly to the container.
A ComposablePart returned by a catalog will likely be an extension to the application. It might have Imports (dependencies) on components the host application offers, and it's likely to Export others.
The default MEF composable part implementation uses attribute-based metadata to declare exports and imports. This allows MEF to determine which parts, imports, and exports are available through discovery.