C# 15 Closed Class Hierarchies: The closed Keyword in .NET 11 Preview 5

.NET 11 Preview 5 shipped on June 9, 2026, and tucked into the C# 15 language work is a feature that quietly fixes one of the oldest gaps in the type system: there was no way to tell the compiler “this base class has exactly these subtypes.” Now there is. The new closed modifier declares a closed class hierarchy, and switch expressions over it get full exhaustiveness checking at compile time.

This is the companion piece to type unions. Unions compose unrelated types; closed hierarchies lock down an inheritance tree you already own. Together they give C# a complete exhaustiveness story.

What closed actually does

Mark a base class closed and the compiler only allows direct subtypes that live in the same assembly. Because the set of subtypes is now known and finite, the compiler can verify that a switch handles every reachable case.

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

static string Describe(GateState state) => state switch
{
    Closed => "closed",
    Open(var percent) => $"{percent}% open"
};

No default arm, no discard pattern, no throw new InvalidOperationException("unreachable"). The compiler already knows Closed and Open are the only options. Add a third subtype later and every non-exhaustive switch lights up with a warning, which is exactly the refactoring safety net that sealed-class-plus-visitor patterns never gave you.

The rules that bite

A few constraints are worth committing to memory:

• A closed class is implicitly abstract. You cannot instantiate the base directly, and you cannot combine closed with sealed, static, or an explicit abstract modifier.
• Direct subtypes must be declared in the same assembly. Cross-assembly inheritance is blocked, which is what makes the closed set knowable.
• It applies to classes and record class, but not structs.

The one gotcha in Preview 5

Here is the part that will trip you up. The compiler emits a System.Runtime.CompilerServices.ClosedAttribute marker, but the runtime in Preview 5 does not ship that attribute yet. Until it does, every project using closed has to declare the attribute itself:

namespace System.Runtime.CompilerServices;

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

Drop that into any file in the project and the feature compiles. Expect it to disappear in a later preview once the BCL carries the type. This is the usual preview tax, so do not bake the shim into a shared library you plan to ship.

Closed hierarchies, closed enums, and union types are all gated behind <LangVersion>preview</LangVersion> in C# 15 today, heading for general availability with .NET 11 in November 2026. If you have ever written a switch with an unreachable default just to satisfy the compiler, this is the feature that finally deletes it. Full details are in the Preview 5 C# release notes.