Use C# unions and closed hierarchies in ASP.NET Core

C# 15 and .NET 11 introduce native union types and closed class hierarchies for modeling values that can take multiple JSON shapes, using Kubernetes' maxUnavailable field and a payment-events example as motivating cases. Unions declare a fixed set of primitive, class, or interface cases with compiler-checked exhaustive switches, while closed hierarchies restrict inheritance so the compiler treats known derived types as complete. System.Text.Json serializes unions without any envelope or discriminator (writing only the active case's own JSON shape), while closed hierarchies can opt into discriminator-based polymorphism via InferClosedTypePolymorphism. Both features flow through Minimal APIs, MVC, SignalR's JsonHubProtocol, Blazor (component parameters, JS interop, persisted state, prerendering), and OpenAPI (as anyOf schemas). Limitations: unions aren't supported for query strings, route values, headers, or form fields because those bind strings without JSON parsing, leaving no way to disambiguate cases.

Source: https://devblogs.microsoft.com/dotnet/unions-and-closed-hierarchies-in-aspnetcore. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.