Cannot extend or implement FFI classes other than 'Struct' or 'Union'DRT-W1626

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.

Example

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

import 'dart:ffi';

final class C extends Double {}

Common fixes

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 {}