Trimming and Native AOT

Everything Purview.Unions ships is trimming- and Native AOT-safe.

The runtime

The runtime package is dependency-free, IsAotCompatible and SupportedPlatform=browser. It contains:

• GenerateUnionAttribute — an ordinary attribute with a Type[] payload;
• Purview.Unions.IUnion — a read-only interface;
• Union<T1, T2> — typed fields, TryGetValue, Match/Switch, equality and ToString.

Union<T1, T2> uses EqualityComparer<T>.Default and two static runtime metadata checks, typeof(T).IsValueType and Nullable.GetUnderlyingType(typeof(T)), to decide whether a value needs the null test. Neither is member reflection, dynamic code or Type.GetType; both are AOT-safe and evaluated once per closed generic.

There is no System.Reflection member access, no Activator, no dynamic and no runtime type discovery.

Generated unions

A generated union is:

• one typed field per case;
• one public constructor per case;
• a switch over a small integer tag for Value and HasValue;
• one TryGetValue overload per case.

No reflection, no serialisation hooks, no dynamic code. The same members the compiler uses for pattern matching are the only ones the generator emits.

The .NET 10 polyfills

On net10.0 the generator emits internal [Embedded]/[CompilerGenerated] System.Runtime.CompilerServices.UnionAttribute and IUnion into the consumer’s compilation. They are internal, marked embedded, and not part of the public surface; trimming removes them if unused.

On net11.0 no polyfill is emitted and the BCL types are used.

Verifying a consumer

The publisher gate packs the package to an isolated feed and builds and runs consumers that reference only the package, then leak-scans their output for compiler-host assemblies:

just smoke-packed-consumers

For a trimmed or AOT application of your own:

<PropertyGroup>
  <PublishTrimmed>true</PublishTrimmed>
  <PublishAot>true</PublishAot>
  <IsAotCompatible>true</IsAotCompatible>
</PropertyGroup>

A union used as a type parameter, a field or a local is a value type with a fixed set of fields, so the trimmer has nothing to guess at. If you expose a union in a public API, the case types are still ordinary types and their own trimming annotations apply.

Blazor WebAssembly

The package’s SupportedPlatform includes browser and it uses no browser-unsupported API. A Blazor WebAssembly client can use any surface, including a generated union; Value boxes only when read.

What is deliberately absent

• No reflection-based enumeration of cases. The case list is a compile-time concept; expose it yourself if a runtime consumer needs it.
• No JSON converter. A converter would need a discriminator policy and a resolver; that belongs in a separate package.
• No ToString/Deconstruct on generated unions. They would invite treating a union as a tuple; match the active case instead.