Error when using fromJson with multiple factories in freezed: "Could not generate _$PersonFromJson because both Person and _Person would want to generate it"

[devthigas502]

I'm using the freezed package for creating immutable classes in Dart with JSON serialization support. However, when defining multiple constructors in a sealed class, I encounter the following error during code generation with build_runner:

Could not generate _$PersonFromJson because both Person and _Person would want to generate it.
Rename one or the other, such that they don't conflict.

Here is my current code:

import 'package:freezed_annotation/freezed_annotation.dart';
part 'user.freezed.dart';
part 'user.g.dart';

@freezed
sealed class Person with _$Person {
  const factory Person({
    required String firstName,
    required String lastName,
    required int age,
  }) = _Person;

  const factory Person.two({
    required String firstName,
    required String lastName,
    required int age,
  }) = SpecialPerson;

  factory Person.fromJson(Map<String, Object?> json) => _$PersonFromJson(json);
}

I've tried using @FreezedUnionValue, unionKey, and unionValueCase, but the error persists. Additionally, I ran the following command:

flutter pub run build_runner build --delete-conflicting-outputs

Here are my dependencies:

environment: sdk: ^3.7.2

freezed: ^3.0.6

build_runner: ^2.4.15

freezed_annotation: ^3.0.0

json_annotation: ^4.9.0

json_serializable: ^6.9.4

Question: How can I define multiple constructors in a sealed class with freezed and fromJson without causing conflicts?

[rrousselGit]

Did you somehow annotate your Freezed class with @JsonSerializable?

[devthigas502]

I figured out what was causing the issue!

This code used to work fine in freezed version 2.5.8, but after updating, I started getting an error like:

Could not generate _$PersonFromJson because both Person and _Person would want to generate it.
Rename one or the other, such that they don't conflict.

It turns out the problem is that both factory constructors were trying to generate the same class name (_Person), which causes a conflict when generating the _$PersonFromJson method.

✅ What worked for me:

I changed the names of the generated classes to make them unique:

@freezed
sealed class Person with _$Person {

  const factory Person({
    required String firstName,
    required String lastName,
    required int age,
  }) = _Foo;

  const factory Person.two({
    required String firstName,
    required String lastName,
    required int age,
  }) = SpecialPerson;

  factory Person.fromJson(Map<String, Object?> json) => _$PersonFromJson(json);
}