[Proposal]: Nominal Type Unions

[mattwar]

Nominal Type Unions

• Specification: Nominal Type Unions
• Working group folder: Discriminated Unions
• Discussion: [Discussion] Nominal Type Unions #9410

Summary

Named unions have a closed list of "case types":

public union IntOrString(int, string);

Fresh case types can be succinctly declared if we adopt the "Case Declarations" proposal:

public union Pet
{
    case Cat(...);
    case Dog(...);
}

Case types convert implicitly to the union type:

Pet pet = dog;

Patterns apply to the contents of a union. Switches that handle all case types are exhaustive:

var name = pet switch
{
    Cat cat => ...,
    Dog dog => ...,
    // No warning about missing cases
}

Related Proposals

• https://github.com/dotnet/csharplang/blob/main/meetings/working-groups/discriminated-unions/TypeUnions.md
• https://github.com/dotnet/csharplang/blob/main/meetings/working-groups/discriminated-unions/original-nominal-type-unions.md

LDM Notes

• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-05-28.md
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-06-25.md#unions
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-08-27.md#union-syntax