Managed Extensibility Framework

Managed Extensibility Framework

Developer(s)	Microsoft
Stable release	V1 in .NET Framework 4.0 / April 12, 2010; 18 months ago (2010-04-12)
Preview release	V2 Preview 3 / February 27, 2011; 8 months ago (2011-02-27)
Written in	.NET Languages
Type	Web application framework
License	Ms-PL
Website	mef.codeplex.com

Managed Extensibility Framework (MEF) is a component of .NET Framework 4.0 for creating 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 not only allows extensions to be reused within applications, but across applications as well. 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.

What problems does MEF solve?

MEF presents a simple solution for 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 (another thing you won't have to worry about).
• MEF offers a set of discovery approaches for your application to locate and load available extensions.
• MEF allows tagging extensions with additional metadata which facilitates rich querying and filtering

How does MEF work?

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). 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. You're free to add composable part instances directly to the container if you wish.
• A ComposablePart returned by a catalog will likely be an extension to your 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 completely through discovery.

External links

• MEF at CodePlex
• Managed Extensibility Framework Overview
• Building Composable Apps in .NET 4 with the Managed Extensibility Framework
• MEFContrib Project
• Gentle Introduction to MEF - Part One