[Proposal]: Closed Enums

[333fred]

Closed Enums

• Specification: Closed Enums
• Working group folder: Discriminated Unions
• Discussion: #9010

Summary

Allow an enum type to be declared closed:

public closed enum Color
{
    Red,
    Green,
    Blue
}

This prevents creation of enum values other than the specified members.

A consuming switch expression that covers all its specified members can therefore be concluded to "exhaust" the closed enum - it does not need to provide a default case to avoid warnings:

Color color = ...;

string description = color switch
{
    Red => "red",
    Green => "green",
    Blue => "blue"
    // No warning about missing cases
};

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-26.md#discriminated-unions
• 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-2025-10-01.md#closed-enums

Related Proposals

• https://github.com/dotnet/csharplang/discussions/9010