C# is expected to add language-level discriminated unions, and we should prepare to support them end-to-end once the feature lands.

Impacted frameworks/tech: Minimal APIs, Blazor, SignalR, MVC, JSON Patch, OpenAPI

Key Areas to Address

• Serialization handling
  Ensure discriminated unions work correctly as both input and result types. This includes model binding, JSON serialization, and any polymorphic scenarios.

• Polymorphism support
  Verify that discriminated unions interoperate smoothly with existing polymorphic type handling in the framework and serializers.

• OpenAPI integration
  Update OpenAPI generation so that discriminated union types are accurately represented in generated specs.

• Source-generated & non-source-generated parity
  Confirm consistent behavior across both source-generated Minimal APIs and the non-source-generated RDF.

Goals

Achieve seamless usage of discriminated unions across the API surface so developers can use them naturally in Minimal APIs without workarounds.