[Proposal]: Closed Hierarchies (VS 18.8, .NET 11)

Closed Hierarchies

• Specification: Closed Hierarchies
• Working group folder: Discriminated Unions
• Discussion: #8927

Summary

Allow a class to be declared closed. This prevents directly derived classes from being declared in a different assembly:

// Assembly 1
public closed record class GateState;
public record class Closed : GateState;
public record class Open(float Percent) : GateState;

// Assembly 2
public record class Locked : GateState; // ERROR - 'GateState' is a closed class

A consuming switch expression that covers all those derived classes can therefore be concluded to "exhaust" the closed class - it does not need to provide a default case to avoid warnings.

// Assembly 3
GateState state = ...;
string description = state switch
{
    Closed => "closed",
    Open(var percent) => $"{percent}% open"
    // No warning about missing cases
}; 

Design meetings

• https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-01-10.md
• 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-2025-06-25.md#unions
• https://github.com/dotnet/csharplang/blob/main/meetings/2025/LDM-2025-10-01.md#closed-hierarchies
• https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-02-09.md#closed-hierarchies-open-questions
• https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-04-20.md#closed-hierarchies-continued
• https://github.com/dotnet/csharplang/blob/main/meetings/2026/LDM-2026-05-18.md#closed-hierarchies-open-questions

Related Proposals

• https://github.com/dotnet/csharplang/issues/485
• https://github.com/dotnet/csharplang/discussions/8927