mixin_class_declaration_extends_not_object

The class '{0}' can't be declared a mixin because it extends a class other than 'Object'.

The analyzer produces this diagnostic when a class that is marked with the mixin modifier extends a class other than Object. A mixin class can't have a superclass other than Object.

The following code produces this diagnostic because the class B, which has the modifier mixin, extends A:

class A {}

mixin class B extends A {}

If you want the class to be used as a mixin, then change the superclass to Object, either explicitly or by removing the extends clause:

class A {}

mixin class B {}

If the class needs to have a superclass other than Object, then remove the mixin modifier:

class A {}

class B extends A {}

If you need both a mixin and a subclass of a class other than Object, then move the members of the subclass to a new mixin, remove the mixin modifier from the subclass, and apply the new mixin to the subclass:

class A {}

class B extends A with M {}

mixin M {}

Depending on the members of the subclass this might require adding an on clause to the mixin.

Unless stated otherwise, the documentation on this site reflects Dart 3.8.0. Page last updated on 2025-05-08. View source or report an issue.