[Proposal]: Case Declarations

[mattwar]

Case Declarations

• Specification: Case Declarations
• Working group folder: Discriminated Unions
• Discussion: [Discussion] Case Declarations #9502

Summary

A case declaration is a shorthand syntax for declaring a nested case of a closed type.
It infers much of what it is from context.

public closed record GateState
{
    case Closed;
    case Locked;
    case Open(float Percent);
}

When used within a record declaration, it is the equivalent of writing:

public closed record GateState
{
    public sealed record Closed : GateState;
    public sealed record Locked : GateState;
    public sealed record Open(float Percent) : GateState;
}

Design meetings

• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-06-25.md#unions