DirectoryCatalog

DirectoryCatalog is a component of the Managed Extensibility Framework (MEF) used to discover and expose exports from assemblies located in a specified directory. It serves as a catalog that aggregates parts (types or members) found in multiple assemblies so that a composition container can compose imports and exports at runtime. DirectoryCatalog is commonly used to implement plugin or extension scenarios where new functionality is added by dropping assemblies into a folder.

How it works: DirectoryCatalog is configured with a path to a directory and, optionally, a search pattern to filter candidate assemblies (for example, a pattern like *.dll). When a composition container is created, the catalog enumerates the assemblies in the directory that match the pattern, loads them, and exposes their exports to the container. These exports can then be consumed by parts that declare corresponding imports, enabling runtime extension without recompiling the host application.

Usage considerations: DirectoryCatalog is often combined with other catalogs through an AggregateCatalog to form a unified view of multiple sources. It is typically used when plugin assemblies are independently developed and placed in a designated directory. Because assemblies are loaded into the host process, dependencies of those assemblies must be resolvable, and version conflicts can affect composition.

Limitations and maintenance: DirectoryCatalog reflects the contents of the directory at the time of catalog creation. To include new or updated assemblies, the catalog may need to be recreated or refreshed as part of the hosting application’s logic. Proper error handling is important, as missing dependencies or malformed exports can cause composition failures.