native_field_invalid_type

'{0}' is an unsupported type for native fields. Native fields only support pointers, arrays or numeric and compound types.

The analyzer produces this diagnostic when an @Native-annotated field has a type not supported for native fields.

Native fields support pointers, arrays, numeric types and subtypes of Compound (i.e., structs or unions). Other subtypes of NativeType, such as Handle or NativeFunction are not allowed as native fields.

Native functions should be used with external functions instead of external fields.

Handles are unsupported because there is no way to transparently load and store Dart objects into pointers.

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

The following code produces this diagnostic because the field free uses an unsupported native type, NativeFunction:

import 'dart:ffi';
​
@Native<NativeFunction<Void Function()>>()
external void Function() free;

If you meant to bind to an existing native function with a NativeFunction field, use @Native methods instead:

import 'dart:ffi';
​
@Native<Void Function(Pointer<Void>)>()
external void free(Pointer<Void> ptr);

To bind to a field storing a function pointer in C, use a pointer type for the Dart field:

import 'dart:ffi';
​
@Native()
external Pointer<NativeFunction<Void Function(Pointer<Void>)>> free;