Purview.Unions 1.0.0-prerelease.3

Purview.Unions

Dependency-free C# 15 unions that do not box value-type cases. The union keyword stores one object?, so every value-type case is boxed. This package generates the language's custom union type instead: a typed field per case, a public constructor per case, and a non-boxing TryGetValue. The compiler still applies union conversions, exhaustive switch and is/null patterns.

Install

dotnet add package Purview.Unions

Quick start

using Purview.Unions;

[GenerateUnion(typeof(int), typeof(string))]
public readonly partial struct IntOrString;

IntOrString value = 42;

Console.WriteLine(value switch
{
    int number => $"int {number}",
    string text => $"string {text}",
    null => "empty",
});

The package carries the merged generator and code fixes under analyzers/dotnet/cs, so no second PackageReference is needed.

Declaration surfaces

Use B or C for a generic union: typeof(T) is not a legal attribute argument (CS0416).

Boxing

Value is part of the language's union contract and boxes. That is inherent to the contract; pattern matching does not use it.

Requirements

• .NET 11 SDK (C# 15, GA).
• net10.0 or net11.0. A net11.0 project gets C# 15 by default and needs no LangVersion; a net10.0 project sets <LangVersion>latest</LangVersion> because its default is C# 14. On net10.0 the generator emits internal [Embedded] polyfills; net11.0 uses the BCL types and never receives a duplicate.
• Do not set EnablePreviewFeatures; LangVersion=latest (only on net10.0) is enough.

Supported declarations

struct, readonly struct, class, record struct and record class, non-generic and generic, with classes, structs, records, interfaces, type parameters, nullable value types and other unions as cases.

A nullable value-type case is exposed through its underlying type: int? becomes case int, the constructor takes int?, and TryGetValue(out int) returns false for null.

Diagnostics

UNI0001–UNI0016 and UNI9000. See the Diagnostics page. Code fixes add a missing partial (UNI0001), make an inaccessible case type public (UNI0005) and, on the fallback path, the [Union] attribute (UNI0007). UNI0000, UNI9001 and UNI9002 are suppression descriptors, not rules: they suppress CA1034, CA1515 and CA1000 on a union's nested IUnionMembers provider.

Switches

Agent skills

The purview-unions-quickstart, purview-unions-modelling, purview-unions-diagnostics and purview-unions-interop agent skills cover this package. No package ships skills: install them from the git repository with the skills CLI:

npx skills add purview-dev/unions

Licence

MIT.