I'm struggling with json serialization.

It throws me na error: "Unhandled Exception: type 'String' is not a subtype of type 'num?' in type cast".

I was trying to use a custom JSON converter, but it doesn't work. It happens when I try to fetch data from API and when it starts do deserialize JSON.

Does anyone have a clue what's wrong?

The error happens when it tries to parse lat and lng values. When it parses these two values, it throws an exception that type String ist not a subtype of type num? in type cast. It happens in car_model.g.dart file when it runs these two lines:

lat: (json['lat'] as num?)?.toDouble(),
lng: (json['lng'] as num?)?.toDouble(),

[
    {
        "_id": "5e5e40c4c0ea272d00000956",
        "brand": "Tofas",
        "model": "Sahin",
        "color": "#0fc0fc",
        "registration": "WA12345",
        "year": "2005-01-01T00:00:00.000Z",
        "ownerId": "5e5e3d7fc0ea272d00000824",
        "lat": 50.754,
        "lng": 12.2145
    },
]

Entity

import 'package:equatable/equatable.dart';

abstract class Car extends Equatable {
  final String? id;
  final String? brand;
  final String? model;
  final String? color;
  final String? registration;
  final String? year;
  final String? ownerId;
  final double? lat;
  final double? lng;

  const Car({
    this.id,
    this.brand,
    this.model,
    this.color,
    this.registration,
    this.year,
    this.ownerId,
    this.lat,
    this.lng,
  });

  @override
  List<Object?> get props => [
        id,
        brand,
        model,
        color,
        registration,
        year,
        ownerId,
        lat,
        lng,
      ];
}

Model

part 'car_model.g.dart';

class CarModelList {
  final List<CarModel> carModelList;

  CarModelList({
    required this.carModelList,
  });

  factory CarModelList.fromJson(List<dynamic> parsedJson) {
    List<CarModel> carModelList = <CarModel>[];
    carModelList = parsedJson.map((i) => CarModel.fromJson(i)).toList();

    return CarModelList(carModelList: carModelList);
  }
}

@JsonSerializable()
class CarModel extends Car {
  const CarModel({
    super.id,
    super.brand,
    super.model,
    super.color,
    super.registration,
    super.year,
    super.ownerId,
    super.lat,
    super.lng,
  });

  @JsonKey(name: '_id')
  @override
  String? get id;

  factory CarModel.fromJson(Map<String, dynamic> json) =>
      _$CarModelFromJson(json);

  Map<String, dynamic> toJson() => _$CarModelToJson(this);
}

Model generated

// GENERATED CODE - DO NOT MODIFY BY HAND

part of 'car_model.dart';

// **************************************************************************
// JsonSerializableGenerator
// **************************************************************************

CarModel _$CarModelFromJson(Map<String, dynamic> json) => CarModel(
      id: json['_id'] as String?,
      brand: json['brand'] as String?,
      model: json['model'] as String?,
      color: json['color'] as String?,
      registration: json['registration'] as String?,
      year: json['year'] as String?,
      ownerId: json['ownerId'] as String?,
      lat: (json['lat'] as num?)?.toDouble(),
      lng: (json['lng'] as num?)?.toDouble(),
    );

Map<String, dynamic> _$CarModelToJson(CarModel instance) => <String, dynamic>{
      'brand': instance.brand,
      'model': instance.model,
      'color': instance.color,
      'registration': instance.registration,
      'year': instance.year,
      'ownerId': instance.ownerId,
      'lat': instance.lat,
      'lng': instance.lng,
      '_id': instance.id,
    };

CodePudding user response：

[
{
    "_id": "5e5e40c4c0ea272d00000956",
    "brand": "Tofas",
    "model": "Sahin",
    "color": "#0fc0fc",
    "registration": "WA12345",
    "year": "2005-01-01T00:00:00.000Z",
    "ownerId": "5e5e3d7fc0ea272d00000824",
    "lat": 50.754,
    "lng": 12.2145
}

]

Array has an extra comma at the end. I hope this is the problem

You can use https://app.quicktype.io/ website for from json model to dart model convert.

TOFAŞK <3

CodePudding user response：

It is because sometimes you are getting

    "lat": "50.754",
    "lng": "12.2145"

it means it is string. So you can follow below thing

  final dynamic lat;
  final dynamic lng;

then again generate model using your model generator tool.

And for auto creation of model you can use https://ashamp.github.io/jsonToDartModel/ it provide functionality for null safety too.