Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Add more clarity on member lookup #43932

Merged
merged 1 commit into from
Dec 13, 2024
Merged

Add more clarity on member lookup #43932

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
18 changes: 9 additions & 9 deletions docs/csharp/language-reference/keywords/file.md
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,17 +1,17 @@
---
description: "file modifier: Declare types whose scope is the file in which it's declared"
title: "file keyword"
ms.date: 09/15/2022
description: "The file modifier: Declare types whose visibility is the file in which it's declared"
title: "The file keyword"
ms.date: 12/10/2024
f1_keywords:
- "file_CSharpKeyword"
helpviewer_keywords:
- "file keyword [C#]"
---
# file (C# Reference)
# The file modifier

Beginning with C# 11, the `file` contextual keyword is a type modifier.

The `file` modifier restricts a top-level type's scope and visibility to the file in which it's declared. The `file` modifier will generally be applied to types written by a source generator. File-local types provide source generators with a convenient way to avoid name collisions among generated types. The `file` modifier declares a file-local type, as in this example:
The `file` modifier restricts a top-level type's visibility to the file in which it's declared. The `file` modifier is most often applied to types written by a source generator. File-local types provide source generators with a convenient way to avoid name collisions among generated types. The `file` modifier declares a file-local type, as in this example:

```csharp
file class HiddenWidget
Expand All @@ -20,11 +20,9 @@ file class HiddenWidget
}
```

Any types nested within a file-local type are also only visible within the file in which it's declared. Other types in an assembly may use the same name as a file-local type. Because the file-local type is visible only in the file where it's declared, these types don't create a naming collision.
Any types nested within a file-local type are also only visible within the file in which it's declared. Other types in an assembly can use the same name as a file-local type. Because the file-local type is visible only in the file where it's declared, these types don't create a naming collision.

A file-local type can't be the return type or parameter type of any member that is more visible than `file` scope. A file-local type can't be a field member of a type that has greater visibility than `file` scope. However, a more visible type may implicitly implement a file-local interface type. The type can also [explicitly implement](../../programming-guide/interfaces/explicit-interface-implementation.md) a file-local interface but explicit implementations can only be used within the `file` scope.

## Example
A file-local type can't be the return type or parameter type of any member declared in a non file-local type. A file-local type can't be a field member of a non-file-local. However, a more visible type can implicitly implement a file-local interface type. The type can also [explicitly implement](../../programming-guide/interfaces/explicit-interface-implementation.md) a file-local interface but explicit implementations can only be used within the same file.

The following example shows a public type that uses a file-local type to provide a worker method. In addition, the public type implements a file-local interface implicitly:

Expand All @@ -34,6 +32,8 @@ In another source file, you can declare types that have the same names as the fi

:::code language="csharp" source="./snippets/Program.cs" id="ShadowsFileScopedType":::

Member lookup prefers a file-local type declared in the same file over a non-file-local type declared in a different file. This rule ensures that a source generator can rely on member lookup resolving to a file-local type without ambiguity with other type declarations. In the preceding example, all uses of `HiddenWidget` in *File1.cs* resolve to the file-local type declared in *File1.cs*. The file-local declaration of `HiddenWidget` hides the public declaration in *File2.cs*.

## C# language specification

For more information, see [Declared accessibility](~/_csharpstandard/standard/basic-concepts.md#752-declared-accessibility) in the [C# Language Specification](~/_csharpstandard/standard/README.md), and the [C# 11 - File local types](~/_csharplang/proposals/csharp-11.0/file-local-types.md) feature specification.
Expand Down
Loading