CompositionContainer

CompositionContainer is a central component of the Managed Extensibility Framework (MEF) in the .NET ecosystem. It acts as the runtime engine that performs the composition of parts by matching imports and exports declared across a set of catalogs. Parts are classes that expose capabilities via exports and declare dependencies via imports. The container resolves these dependencies and wires parts together so that consumers can obtain the required services.

Catalogs provide the universe of available exports. Common types include AssemblyCatalog (discovering parts in a single assembly), DirectoryCatalog (discovering parts from assemblies in a directory), TypeCatalog (explicitly listing types), and AggregateCatalog (combining multiple catalogs). A container is typically created with a catalog, or with an AggregateCatalog, and is then used to compose parts.

Usage patterns include creating a container and calling ComposeParts or SatisfyImportsOnce to satisfy the imports of a given instance. Exports can be retrieved directly from the container using GetExportedValue<T> or GetExportedValues<T>. A CompositionBatch can be used to add parts, exports, and imports in a single operation. The container also supports lazy and metadata-wrapped exports, which allow for deferred resolution and additional information about exports.

CompositionContainer implements IDisposable and releases resources when disposed. It supports extending or customizing the hosting environment and is designed for lightweight dependency resolution without explicit wiring by the consumer. While MEF emphasizes convention-based composition, it is well suited for modular applications, plug-in systems, and runtime extensibility within .NET applications.