enum_mixin_with_instance_variable

Mixins applied to enums can't have instance variables.

The analyzer produces this diagnostic when a mixin that's applied to an enum declares one or more instance variables. This isn't allowed because the enum values are constant, and there isn't any way for the constructor in the enum to initialize any of the mixin's fields.

The following code produces this diagnostic because the mixin M defines the instance field x:

mixin M {
  int x = 0;
}

enum E with M {
  a
}

If you need to apply the mixin, then change all instance fields into getter and setter pairs and implement them in the enum if necessary:

mixin M {
  int get x => 0;
}

enum E with M {
  a
}

If you don't need to apply the mixin, then remove it:

enum E {
  a
}

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.