Explore union types in C# 15

12 hours ago

C# 15 introduces union types with the 'union' keyword, allowing a value to be exactly one of a fixed set of types.
Union types provide compiler-enforced exhaustive pattern matching, ensuring switch expressions cover all case types without a default branch.
They solve issues from previous approaches like using 'object' or inheritance, as they compose unrelated types into a closed set.
Examples include simple declarations like 'Pet(Cat, Dog, Bird)' and utility unions like 'OneOrMore<T>' for single values or collections.
Custom union types can be created using attributes for compatibility with existing libraries, with optional non-boxing access patterns.
Union types are part of a broader exhaustiveness roadmap including closed hierarchies and closed enums.
Available in .NET 11 Preview 2, with IDE support coming soon; early preview requires declaring polyfill types manually.
Feedback is encouraged to shape the final design, and features like the 'is' operator and null handling are supported.

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