[Proposal]: Discriminated Unions

[333fred]

Discriminated Unions

• Union proposals overview: https://github.com/dotnet/csharplang/blob/main/meetings/working-groups/discriminated-unions/union-proposals-overview.md
• Discussion: [LDM] - Type Unions Proposal #8313
  • Past discussion threads
    • Champion "Discriminated Unions" #8926
    • Proposal: "Closed" type hierarchies #8927
    • [LDM] - Union Types #7010
    • Proposal: Track subtype exhaustiveness for classes with only private constructors #8942

Summary

A proposal for type unions (aka discriminated unions) in C#.

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-08-31.md#discriminated-unions
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-26.md#discriminated-unions
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-07-24.md#discriminated-unions
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-01-13.md#discriminated-unions
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-13.md#unions
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-18.md#unions
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-27.md#type-value-conversion
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-27.md#union-syntax