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11. Plugin DI Bridge — Replace MEF Composition with DI-Based Instantiation

Last updated · View source · View changelog

Date: 2026-04-06

Status

Accepted

Context

The multi-state plugin system (ADR-0007) uses MEF (System.Composition) for both assembly loading and plugin instantiation. MEF's [ImportingConstructor] only injects other MEF-exported types, which prevents plugin constructors from receiving DI-registered services like HybridCache. As the portal's service layer grows, plugins increasingly need access to services managed by the DI container.

Decision

Replace MEF's composition and instantiation with DI-based instantiation via ActivatorUtilities, while preserving MEF's assembly-loading logic.

Key details:

  • Assembly scanning preserved. The existing logic loads assemblies from plugins-{state}/ directories and scans for types implementing IStatePlugin.
  • DI instantiation. Discovered plugin types are registered as singleton factories that use ActivatorUtilities.CreateInstance, giving plugin constructors access to any DI-registered service.
  • Health check exception. IStateHealthCheckService plugins are eagerly instantiated via a temporary ServiceProvider because ASP.NET health check registration requires concrete instances at startup. This is a known limitation — the temporary provider cannot share state with the final container.
  • MEF attributes are inert. [Export], [ExportMetadata], and [ImportingConstructor] remain on plugin classes for now but are not used. They will be removed in a future cleanup once the DI bridge is proven stable.

Consequences

  • Plugin constructors can now depend on any DI-registered service, unblocking caching and other cross-cutting concerns.
  • The temporary service provider for health checks is a documented trade-off; health check plugins should remain stateless.
  • System.Composition remains a transitive dependency until the inert attributes are removed.
  • Full design details are in the CO connector repo's specification documents.
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