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This page contains reference information for class modifiers.

Valid combinations

The valid combinations of class modifiers and their resulting capabilities are:

Declaration	Construct?	Extend?	Implement?	Mix in?	Exhaustive?
`class`	Yes	Yes	Yes	No	No
`base class`	Yes	Yes	No	No	No
`interface class`	Yes	No	Yes	No	No
`final class`	Yes	No	No	No	No
`sealed class`	No	No	No	No	Yes
`abstract class`	No	Yes	Yes	No	No
`abstract base class`	No	Yes	No	No	No
`abstract interface class`	No	No	Yes	No	No
`abstract final class`	No	No	No	No	No
`mixin class`	Yes	Yes	Yes	Yes	No
`base mixin class`	Yes	Yes	No	Yes	No
`abstract mixin class`	No	Yes	Yes	Yes	No
`abstract base mixin class`	No	Yes	No	Yes	No
`mixin`	No	No	Yes	Yes	No
`base mixin`	No	No	No	Yes	No

Certain combinations of modifiers aren't allowed:

Combination	Reasoning
`base`, `interface`, and `final`	All control the same two capabilities ( `extend` and `implement` ), so are mutually exclusive.
`sealed` and `abstract`	Neither can be constructed, so are redundant together.
`sealed` with `base`, `interface`, or `final`	`sealed` types already cannot be mixed in, extended or implemented from another library, so are redundant to combine with the listed modifiers.
`mixin` and `abstract`	Neither can be constructed, so are redundant together.
`mixin` and `interface`, `final`, or `sealed`	A `mixin` or `mixin class` declaration is intended to be mixed in, which the listed modifiers prevent.
`enum` and any modifiers	`enum` declarations can't be extended, implemented, mixed in, and can always be instantiated, so no modifiers apply to `enum` declarations.
`extension type` and any modifiers	`extension type` declarations can't be extended or mixed in, and can only be implemented by other `extension type` declarations.

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Unless stated otherwise, the documentation on this site reflects Dart 3.14.0. Page last updated on 2025-05-05. View source or report an issue.