[API Proposal]: ClosedAttribute

Background and motivation

As part of supporting Closed class hierarchies feature, we need a new attribute that will be used by C# compiler to designate a closed type.

API Proposal

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
    public sealed class ClosedAttribute : Attribute { }
}

Note: in the future, we may wish to add other type kinds, such as "interface" or "enum", as valid targets, as we design "closed" forms of these types. But our focus is on classes only for now. Also, the attribute is not allowed to be used explicitly in C#.

API Usage

public closed class Animal; // generates a type which has `[Closed]` attribute in metadata
public class Dog : Animal;
public class Cat : Animal;

Alternative Designs

No response

Risks

No response

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Video

Let's not waste a single-word attribute name on something no one is ever supposed to type.
- IsClosedTypeAttribute

namespace System.Runtime.CompilerServices
{
    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
    public sealed class IsClosedTypeAttribute : Attribute { }
}

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[API Proposal]: ClosedAttribute #128161

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[API Proposal]: ClosedAttribute #128161

Description

Background and motivation

API Proposal

API Usage

Alternative Designs

Risks

Activity

dotnet-policy-service commented on May 13, 2026

bartonjs commented on May 26, 2026

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