subtype_of_ffi_class

The class '{0}' can't extend '{1}'.

The class '{0}' can't implement '{1}'.

The class '{0}' can't mix in '{1}'.

The analyzer produces this diagnostic when a class extends any FFI class other than Struct or Union, or implements or mixes in any FFI class. Struct and Union are the only FFI classes that can be subtyped, and then only by extending them.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the class C extends Double:

import 'dart:ffi';

final class C extends Double {}

If the class should extend either Struct or Union, then change the declaration of the class:

import 'dart:ffi';

final class C extends Struct {
  @Int32()
  external int i;
}

If the class shouldn't extend either Struct or Union, then remove any references to FFI classes:

final class C {}

Unless stated otherwise, the documentation on this site reflects Dart 3.8.1. Page last updated on 2025-05-08. View source or report an issue.