mef

MEF stands for the Managed Extensibility Framework, a component of the .NET platform designed to help developers create extensible applications. It provides a lightweight, declarative model for composing applications from loosely coupled parts, enabling plugins or add-ins to be discovered and wired up at runtime. MEF aims to reduce the coupling between the host and its extensions while retaining flexibility for later changes.

Core concepts in MEF revolve around exports, imports, and composition. Parts can expose exports by marking classes or members with attributes such as Export, while other parts declare their dependencies with Import or ImportMany. A hosting container orchestrates the discovery and composition of parts, performing contract matching and instance creation. MEF also supports lazy resolution through Lazy<T> and on-demand creation with ExportFactory<T>. Catalogs (such as AssemblyCatalog, DirectoryCatalog, and AggregateCatalog) locate available exports, and a CompositionContainer performs the actual wiring and monitors changes for recomposition if needed.

Usage patterns typically involve a host application scanning assemblies, composing parts, and then interacting with the composed graph. This enables plugin architectures where new functionality can be added by dropping assemblies into a directory or loading them at runtime, without recompiling the host. MEF emphasizes simplicity and loose coupling, and while it can be used alongside other dependency injection containers, it is primarily a framework for discovery and composition rather than a full IoC solution.

Historically, MEF was introduced with the .NET Framework 4 to support extensibility in applications such as IDEs and office-type add-ins. A newer variant, MEF 2, provides an updated API (often via System.Composition) with different design goals and improved testability. MEF adoption varies by project type and platform, with MEF 2 more common in modern .NET Core and .NET 5+ ecosystems.

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