missing_field_type_in_struct

Fields in struct classes must have an explicitly declared type of 'int', 'double' or 'Pointer'.

The analyzer produces this diagnostic when a field in a subclass of Struct or Union doesn't have a type annotation. Every field must have an explicit type, and the type must either be int, double, Pointer, or a subclass of either Struct or Union.

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

The following code produces this diagnostic because the field str doesn't have a type annotation:

import 'dart:ffi';

final class C extends Struct {
  external var str;

  @Int32()
  external int i;
}

Explicitly specify the type of the field:

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

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

  @Int32()
  external int i;
}

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.