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Final reviews.
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‎docs/csharp/language-reference/builtin-types/record.md‎

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---
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title: "Records"
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description: Learn about the record modifier for class and struct types in C#. Records provide standard support for value based equality on instances of record types.
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ms.date: 01/14/2026
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ms.date: 06/05/2026
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f1_keywords:
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- "record_CSharpKeyword"
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helpviewer_keywords:
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### `PrintMembers` formatting in derived records
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The synthesized `PrintMembers` method of a derived record type calls the base implementation. The result is that all public properties and fields of both derived and base types are included in the `ToString` output, as shown in the following example:
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The synthesized `PrintMembers` method of a derived record type calls the base implementation. The result is that the `ToString` output includes all public properties and fields of both derived and base types, as shown in the following example:
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:::code language="csharp" source="snippets/shared/RecordType.cs" id="ToStringInheritance":::
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You can provide your own implementation of the `PrintMembers` method. If you do that, use the following signature:
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You can provide your own implementation of the `PrintMembers` method. If you do, use the following signature:
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* For a `sealed` record that derives from `object` (doesn't declare a base record): `private bool PrintMembers(StringBuilder builder)`;
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* For a `sealed` record that derives from another record (note that the enclosing type is `sealed`, so the method is effectively `sealed`): `protected override bool PrintMembers(StringBuilder builder)`;
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* For a `sealed` record that derives from another record (the enclosing type is `sealed`, so the method is effectively `sealed`): `protected override bool PrintMembers(StringBuilder builder)`;
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* For a record that isn't `sealed` and derives from object: `protected virtual bool PrintMembers(StringBuilder builder);`
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* For a record that isn't `sealed` and derives from another record: `protected override bool PrintMembers(StringBuilder builder);`
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‎docs/csharp/language-reference/keywords/abstract.md‎

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description: "abstract - C# Reference"
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title: "abstract keyword"
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ms.date: 01/21/2026
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ms.date: 06/06/2026
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f1_keywords:
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- "abstract"
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- "abstract_CSharpKeyword"

‎docs/csharp/language-reference/keywords/closed.md‎

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---
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# 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*. A closed class can only be derived from 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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[!INCLUDE[csharp-version-note](../includes/initial-version.md)]
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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
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// Assembly 1
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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]
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>`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.
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- A direct subtype of a closed class must be declared in the same assembly and module as the closed base class.
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- You must declare a direct subtype of a closed class in the same assembly and module as the closed base class.
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- 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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:::code language="csharp" source="./snippets/shared/Closed.cs" id="GenericRule":::
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```csharp
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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
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public record class Branch<T>(Tree<T> Left, Tree<T> Right) : Tree<T>; // OK: 'T' appears in the base class
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public record class Constant<U>(U Value) : Tree<int> { } // Error: 'U' isn't used in the base class
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```
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## Exhaustive switch expressions
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This rule applies whether the type parameter appears on a method or on the containing type.
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## Contextual keyword
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`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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## C# language specification
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For more information, see the [Closed hierarchies](~/_csharplang/proposals/closed-hierarchies.md) feature specification.

‎docs/csharp/language-reference/keywords/index.md‎

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description: "C# Keywords: Find the reference material for the predefined keywords and contextual keywords defined in the C# language."
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title: "C# Keywords and contextual keywords"
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‎docs/csharp/language-reference/keywords/sealed.md‎

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description: "sealed modifier - C# Reference"
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title: "sealed modifier"
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f1_keywords:
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‎docs/csharp/language-reference/keywords/snippets/shared/Closed.cs‎

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public record class Failed(string Error) : JobStatus;
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//<GenericRule>
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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
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public record class Branch<T>(Tree<T> Left, Tree<T> Right) : Tree<T>; // OK: 'T' appears in the base class
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// public record class Constant<U>(U Value) : Tree<int> { } // Error: 'U' isn't used in the base class
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//</GenericRule>
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public static class ClosedSwitchExamples

‎docs/csharp/language-reference/operators/patterns.md‎

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title: "Patterns - Pattern matching using the is and switch expressions."
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description: "Learn about the patterns supported by the `is` and `switch` expressions. Combine multiple patterns using the `and`, `or`, and `not` operators."
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:::code language="csharp" source="snippets/patterns/ClosedHierarchyPatterns.cs" id="ShapeNameCrossAssembly":::
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To restore exhaustiveness in assembly 2, add a discard arm (`_ => ...`) or make every direct descendant at least as accessible as the closed base type.
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To restore exhaustiveness in assembly 2, add a discard arm (`_ => ...`), add a base class arm (`Shape` in the preceding example), or make every direct descendant at least as accessible as the closed base type.
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When the governing type is nullable, `null` is an additional value the switch must handle. A switch over `PaymentMethod?` that omits a `null` arm isn't exhaustive even when every direct descendant is matched.
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If you want exhaustiveness to follow the hierarchy further down, declare `Car` itself `closed`. The compiler then treats `Sedan` (and any other direct descendant of `Car`) as part of an exhaustive set rooted at `Car`, so a switch arm of `Car` no longer satisfies exhaustiveness on its own when other direct descendants of `Car` exist. Marking `Car` `closed` also makes it implicitly `abstract`, which means you can no longer create instances of `Car` directly. That might not fit your design. If you need `Car` to remain instantiable, leave it open and dispatch on the specific subtypes you care about by ordering arms as shown earlier.
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If you want exhaustiveness to follow the hierarchy further down, declare `Car` itself `closed`. The compiler then treats every direct descendant of `Car` (such as `Sedan`) as the exhaustive set rooted at `Car`. A switch whose governing type is `Car` is exhaustive when it does any of the following:
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- Handle every direct descendant of `Car` with its own arm.
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- Include a `Car` arm, which covers every `Car` value (including all subtypes).
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- Include a discard arm (`_ => ...`) or an arm for a base type of `Car`, such as `Vehicle`.
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A switch whose governing type is `Vehicle` has choices: Code that handles `Car` and `Truck` is still exhaustive, because the `Car` arm covers every subtype of `Car`. Marking `Car` `closed` simply gives you a second option for that switch. You can keep the single `Car` arm, or replace it with one arm per direct descendant of `Car` (alongside the `Truck` arm) and still be exhaustive.
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Marking `Car` `closed` also makes it implicitly `abstract`, which means you can no longer create instances of `Car` directly. That might not fit your design. If you need `Car` to remain instantiable, leave it open and dispatch on the specific subtypes you care about by ordering arms as shown earlier.
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### Type parameter governing types
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‎docs/csharp/language-reference/operators/snippets/patterns/ClosedHierarchyPatterns.cs‎

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namespace Patterns.ClosedHierarchy;
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public record class Sedan(int Doors) : Car(Doors);
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public static class VehicleExamples

‎docs/csharp/whats-new/csharp-15.md‎

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title: What's new in C# 15
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description: Get an overview of the new features in C# 15. C# 15 ships with .NET 11.
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ms.topic: whats-new
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