prefer-single-declaration-per-file

v1.17.0

style

configurable

pro+

Warns when a file contains more than one top-level declaration (class, mixin, extension, enum or extension type).

note

You can fix this rule's issues using the 'Move to File' assist.

⚙️ Config

Set ignore-private (default is true) to ignore private declarations (example).

Set ignore-enums (default is false) to ignore enum declarations.

analysis_options.yaml

dart_code_metrics:
  rules:
    - prefer-single-declaration-per-file:
        ignore-private: true
        ignore-enums: false

Example

❌ Bad:

some_class.dart

class SomeClass {}

// LINT: Prefer only one top-level declaration per file.
//  Try moving this declaration to a separate file or making it private.
class SomeOtherClass {}

class _Another {} // Correct, ignored by default

✅ Good:

some_class.dart

class SomeClass {}

some_other_class.dart

class SomeOtherClass {}

Example with "ignore-private"

Config

analysis_options.yaml

dart_code_metrics:
  rules:
    - prefer-single-declaration-per-file:
        ignore-private: false

❌ Bad:

some_class.dart

class SomeClass {}

// LINT: Prefer only one top-level declaration per file.
//  Try moving this declaration to a separate file or making it private.
class _Another {}

Related Rules

• prefer-single-widget-per-file