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Differences between .NET MEF, NuGet MEF and VS MEF

VS MEF achieves very high compatibility with two prior MEF libraries that were not themselves compatible with each other. They each have a unique set of attributes, interfaces and patterns which may look similar but have contradicting default behaviors and more subtle but important differences.

MEF parts in the catalog

VS MEF models its parts catalog such that it can describe all the behaviors of both systems. It has an extensible part discovery system, with two discovery modules built-in: one that understands .NET MEF parts and another for NuGet MEF parts. Each part discovery module can contribute parts to a common VS MEF catalog, after which MEF parts no longer retain affinity to a particular MEF variety and are considered compatible with each other.

Catalog hierarchies vs. sharing boundaries

.NET MEF and NuGet MEF both had their own way to express "sub-scopes" of containers. .NET MEF's story requires custom MEF hosting code that is aware of each scope, making it impossible for a MEF part itself to define a new scope. Each scope is backed by a unique catalog. NuGet MEF's story utilizes what it calls "sharing boundaries" and gave MEF parts this freedom to create sub-scopes. The scopes are discovered automatically based on MEF attributes.

VS MEF follows NuGet MEF's model and supports sub-scopes through NuGet MEF attributes. To define a MEF part that can create new sub-scope instances, that MEF part must use NuGet MEF attributes as only they can describe sharing boundaries.

Caveats

  • A given MEF part must use a single variety of MEF attributes. Mixing a .NET MEF Export attribute with a NuGet MEF ExportMetadata attribute will result in a MEF part that exports without the export metadata, because each part discovery module produces part descriptors based on the MEF attributes it was designed to understand.
  • .NET MEF offered a CompositionContainer.Compose method which leveraged its dynamic recomposition feature to add a part to the graph after the container was instantiated. VS MEF offers no such facility. The catalog must be complete before the graph is created. VS-MEF has compatible equivalents for two common uses of CompositionContainer.Compose.