Add closed hierarchies for C# 15, preview 5
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| --- | ||
| title: "closed modifier" | ||
| description: "Learn about the closed class modifier in C#. A closed class restricts derivation to its declaring assembly so consumers can write exhaustive switch expressions over its direct descendants." | ||
| ms.date: 06/02/2026 | ||
| f1_keywords: | ||
| - "closed" | ||
| - "closed_CSharpKeyword" | ||
| helpviewer_keywords: | ||
| - "closed keyword [C#]" | ||
| - "closed class [C#]" | ||
| - "closed hierarchy [C#]" | ||
| ai-usage: ai-assisted | ||
| --- | ||
| # closed (C# Reference) | ||
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| Starting in C# 15, you can apply the `closed` modifier to a class to declare a *closed hierarchy*. You can only derive a direct subtype from a closed class within its declaring assembly. Because the set of direct descendants is fixed, a `switch` expression that handles each direct descendant exhausts the closed base type and doesn't need a default arm. | ||
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| ```csharp | ||
| // Assembly 1 | ||
| public closed record class JobStatus; | ||
| public record class Queued : JobStatus; | ||
| public record class Running(int PercentComplete) : JobStatus; | ||
| public record class Completed(TimeSpan Elapsed) : JobStatus; | ||
| public record class Failed(string Error) : JobStatus; | ||
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| // Assembly 2 | ||
| public record class Paused : JobStatus; // Error: 'JobStatus' is a closed class | ||
| ``` | ||
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| The same-assembly restriction applies only to *direct* descendants of the closed class. A class that derives from a closed class isn't itself closed unless you also mark it `closed`. Because `Failed` in the previous example is a plain record, another assembly can derive from it: | ||
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| ```csharp | ||
| // Assembly 2 | ||
| public record class RetryableFailed(string Error, int Attempts) : Failed(Error); // OK: 'Failed' isn't sealed or closed | ||
| ``` | ||
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| If you want to prevent derivation from `Failed` as well, declare it as `sealed` or `closed`. | ||
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| [!INCLUDE[csharp-version-note](../includes/initial-version.md)] | ||
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| > [!NOTE] | ||
| > `closed` is a contextual keyword. It has special meaning only when it appears as a modifier on a class declaration. You can continue to use `closed` as an identifier in other contexts. If you need to use `closed` as an identifier in a position where the modifier would also be valid, prefix it with `@` (for example, `@closed`) to tell the compiler to treat it as an identifier rather than the modifier. | ||
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| ## Declaration rules | ||
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| The `closed` modifier is a class modifier: | ||
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| - A `closed` class is implicitly [`abstract`](abstract.md). You can't combine `closed` with `sealed`, `static`, or an explicit `abstract` modifier. | ||
| - You must declare a direct subtype of a closed class in the same assembly and module as the closed base class. | ||
| - A class that derives from a closed class isn't itself closed. Apply the `closed` modifier again if you want a derived class to also be closed. | ||
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| If a generic class directly derives from a `closed` class, every type parameter on the derived class must be used in the base class specification. This rule isn't about the `closed` modifier itself: a *closed constructed type* is a generic type whose type arguments are fully specified (such as `Tree<int>`), as opposed to an *open type* like `Tree<T>`. The rule ensures that each closed constructed type of the base class has exactly one corresponding closed constructed type among its direct descendants, so the compiler can reason about exhaustiveness. | ||
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| ```csharp | ||
| public closed record class Tree<T>; | ||
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| public record class Leaf<T>(T Value) : Tree<T>; // OK: 'T' appears in the base class | ||
| public record class Branch<T>(Tree<T> Left, Tree<T> Right) : Tree<T>; // OK: 'T' appears in the base class | ||
| public record class Constant<U>(U Value) : Tree<int> { } // Error: 'U' isn't used in the base class | ||
| ``` | ||
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| ## Exhaustive switch expressions | ||
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| When a `switch` expression handles every direct descendant of a closed class, the compiler considers the switch exhaustive and doesn't generate a non-exhaustiveness warning: | ||
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| :::code language="csharp" source="./snippets/shared/Closed.cs" id="ExhaustiveSwitch"::: | ||
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| When the switch governing expression is nullable, `null` becomes another possible value that the switch must handle. A switch over `JobStatus?` is exhaustive only when it also covers `null`: | ||
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| :::code language="csharp" source="./snippets/shared/Closed.cs" id="NullableSwitch"::: | ||
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| If you omit the `null` arm, the compiler warns that the pattern `null` isn't handled. The same rule applies whether the closed type is a class or a struct lifted to a nullable type. | ||
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| For more information about how the compiler determines exhaustiveness, including how closed hierarchies interact with generic constraints and accessibility, see [Closed hierarchy patterns](../operators/patterns.md#closed-hierarchy-patterns). | ||
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| ## Type parameters constrained to a closed type | ||
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| A type parameter constrained to a closed class is treated as that closed class for exhaustiveness checks. A `switch` expression whose governing value has such a type parameter is exhaustive when it handles every direct descendant of the closed constraint: | ||
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| :::code language="csharp" source="./snippets/shared/Closed.cs" id="TypeParameterConstrained"::: | ||
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| This rule applies whether the type parameter appears on a method or on the containing type. | ||
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| ## C# language specification | ||
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| For more information, see the [Closed hierarchies](~/_csharplang/proposals/closed-hierarchies.md) feature specification. | ||
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| ## See also | ||
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| - [Inheritance](../../fundamentals/object-oriented/inheritance.md) | ||
| - [sealed](sealed.md) | ||
| - [abstract](abstract.md) | ||
| - [Pattern matching](../operators/patterns.md) | ||
| - [Switch expression](../operators/switch-expression.md) | ||
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,48 @@ | ||
| namespace LanguageKeywords.ClosedHierarchies; | ||
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| // Setup types reused by the snippets in this file. | ||
| public closed record class JobStatus; | ||
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Check failure on line 4 in docs/csharp/language-reference/keywords/snippets/shared/Closed.cs
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| public record class Queued : JobStatus; | ||
| public record class Running(int PercentComplete) : JobStatus; | ||
| public record class Completed(TimeSpan Elapsed) : JobStatus; | ||
| public record class Failed(string Error) : JobStatus; | ||
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| //<GenericRule> | ||
| //</GenericRule> | ||
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| public static class ClosedSwitchExamples | ||
| { | ||
| //<ExhaustiveSwitch> | ||
| public static string Describe(JobStatus status) => status switch | ||
| { | ||
| Queued => "waiting to start", | ||
| Running(var percent) => $"{percent}% complete", | ||
| Completed(var elapsed) => $"finished in {elapsed.TotalSeconds:F1}s", | ||
| Failed(var error) => $"failed: {error}", | ||
| // No warning: every direct descendant of 'JobStatus' is handled. | ||
| }; | ||
| //</ExhaustiveSwitch> | ||
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| //<NullableSwitch> | ||
| public static string DescribeOrUnknown(JobStatus? status) => status switch | ||
| { | ||
| null => "unknown", | ||
| Queued => "waiting to start", | ||
| Running(var percent) => $"{percent}% complete", | ||
| Completed(var elapsed) => $"finished in {elapsed.TotalSeconds:F1}s", | ||
| Failed(var error) => $"failed: {error}", | ||
| // No warning: every direct descendant of 'JobStatus' is handled, and null is handled. | ||
| }; | ||
| //</NullableSwitch> | ||
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| //<TypeParameterConstrained> | ||
| public static string DescribeJob<X>(X status) where X : JobStatus => status switch | ||
| { | ||
| Queued => "waiting to start", | ||
| Running(var percent) => $"{percent}% complete", | ||
| Completed(var elapsed) => $"finished in {elapsed.TotalSeconds:F1}s", | ||
| Failed(var error) => $"failed: {error}", | ||
| // No warning: 'X' is constrained to a closed type, so its direct descendants exhaust the switch. | ||
| }; | ||
| //</TypeParameterConstrained> | ||
| } | ||
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,6 @@ | ||
| // In preview 5 of C# 15, the runtime doesn't yet define ClosedAttribute, | ||
| // so projects that use the 'closed' modifier must declare it. | ||
| namespace System.Runtime.CompilerServices; | ||
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| [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)] | ||
| public sealed class ClosedAttribute : Attribute { } |
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