nullable_ type_ in_ with_ clause
A class or mixin can't mix in a nullable type.
Description
#
The analyzer produces this diagnostic when a class or mixin declaration has
a
with
clause, and a mixin is followed by a
?
.
It isn't valid to specify a nullable mixin because doing so would have no
meaning; it wouldn't change either the interface or implementation being
inherited by the class containing the
with
clause.
Note, however, that it
is
valid to use a nullable type as a type argument
to the mixin, such as
class A with B<C?> {}
.
Example
#
The following code produces this diagnostic because
A?
is a nullable
type, and nullable types can't be used in a
with
clause:
mixin M {}
class C with M? {}
Common fixes
#Remove the question mark from the type:
mixin M {}
class C with M {}
Unless stated otherwise, the documentation on this site reflects Dart 3.9.2. Page last updated on 2025-9-1. View source or report an issue.