invalid_field_type_in_struct

Fields in struct classes can't have the type '{0}'. They can only be declared as 'int', 'double', 'Array', 'Pointer', or subtype of 'Struct' or 'Union'.

The analyzer produces this diagnostic when a field in a subclass of Struct has a type other than int, double, Array, Pointer, or subtype of Struct or Union.

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

The following code produces this diagnostic because the field str has the type String, which isn't one of the allowed types for fields in a subclass of Struct:

import 'dart:ffi';

final class C extends Struct {
  external String s;

  @Int32()
  external int i;
}

Use one of the allowed types for the field:

import 'dart:ffi';
import 'package:ffi/ffi.dart';

final class C extends Struct {
  external Pointer<Utf8> s;

  @Int32()
  external int i;
}

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.