subtype_of_struct_class

The class '{0}' can't extend '{1}' because '{1}' is a subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.

The class '{0}' can't implement '{1}' because '{1}' is a subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.

The class '{0}' can't mix in '{1}' because '{1}' is a subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.

The analyzer produces this diagnostic when a class extends, implements, or mixes in a class that extends either Struct or Union. Classes can only extend either Struct or Union directly.

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

The following code produces this diagnostic because the class C extends S, and S extends Struct:

import 'dart:ffi';

final class S extends Struct {
  external Pointer f;
}

final class C extends S {
  external Pointer g;
}

If you're trying to define a struct or union that shares some fields declared by a different struct or union, then extend Struct or Union directly and copy the shared fields:

import 'dart:ffi';

final class S extends Struct {
  external Pointer f;
}

final class C extends Struct {
  external Pointer f;

  external Pointer g;
}

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.