The argument type 'Id?' can't be assigned to the parameter type 'Id'

Question

Don't know how to resolve it. The two lines show error. It should be resolved to run the code.

I got error on the two lines where type id and names are converted into "id" and "names".

// To parse this JSON data, do
//
//     final welcome = welcomeFromJson(jsonString);

import 'dart:convert';

Welcome welcomeFromJson(String str) => Welcome.fromJson(json.decode(str));

String welcomeToJson(Welcome data) => json.encode(data.toJson());

class Welcome {
  Welcome({
    required this.status,
    required this.totalResults,
    required this.articles,
  });

  String status;
  int totalResults;
  List<Article> articles;

  factory Welcome.fromJson(Map<String, dynamic> json) => Welcome(
        status: json["status"],
        totalResults: json["totalResults"],
        articles: List<Article>.from(
            json["articles"].map((x) => Article.fromJson(x))),
      );

  Map<String, dynamic> toJson() => {
        "status": status,
        "totalResults": totalResults,
        "articles": List<dynamic>.from(articles.map((x) => x.toJson())),
      };
}

class Article {
  Article({
    required this.source,
    required this.author,
    required this.title,
    required this.description,
    required this.url,
    required this.urlToImage,
    required this.publishedAt,
    required this.content,
  });

  Source source;
  String author;
  String title;
  String description;
  String url;
  String urlToImage;
  DateTime publishedAt;
  String content;

  factory Article.fromJson(Map<String, dynamic> json) => Article(
        source: Source.fromJson(json["source"]),
        author: json["author"] == null ? null : json["author"],
        title: json["title"],
        description: json["description"],
        url: json["url"],
        urlToImage: json["urlToImage"],
        publishedAt: DateTime.parse(json["publishedAt"]),
        content: json["content"],
      );

  Map<String, dynamic> toJson() => {
        "source": source.toJson(),
        "author": author == null ? null : author,
        "title": title,
        "description": description,
        "url": url,
        "urlToImage": urlToImage,
        "publishedAt": publishedAt.toIso8601String(),
        "content": content,
      };
}

class Source {
  Source({
    required this.id,
    required this.name,
  });

  Id id;
  Name name;

  factory Source.fromJson(Map<String, dynamic> json) => Source(
        id:  idValues.map[json["id"]],
        name: nameValues.map[json["name"]],
      );

  Map<String, dynamic> toJson() => {
        "id": idValues.reverse[id],
        "name": nameValues.reverse[name],
      };
}

enum Id { THE_WALL_STREET_JOURNAL }

final idValues =
    EnumValues({"the-wall-street-journal": Id.THE_WALL_STREET_JOURNAL});

enum Name { THE_WALL_STREET_JOURNAL }

final nameValues =
    EnumValues({"The Wall Street Journal": Name.THE_WALL_STREET_JOURNAL});

class EnumValues<T> {
  Map<String, T> map;
  late Map<T, String> reverseMap;

  EnumValues(this.map);

  Map<T, String> get reverse {
    if (reverseMap == null) {
      reverseMap = map.map((k, v) => new MapEntry(v, k));
    }
    return reverseMap;
  }
}

I think if we parse them then it can be resolved but I don't know how to parse it.

Answer 1

You have set Id to be required, yet your code does not guarantee that what you are parsing is going to actually have a Id value that is not null because you are using a Map that has dynamic values, which means they can be anything, including null. In other words: you cannot assign a nullable Id (the "?" behind "Id?" means it's nullable) to an Id that is not nullable.

Without running your code, some suggestions that might help is:

• Make your Id property nullable
• Ensure that the value is not null where you are trying to assign the id by using a "!". The "!" means that you are guaranteeing the value is not null.
• Use the "late" keyword to initialize the value after it's declaration.

Be warned though: The two latter suggestions will cause exceptions if you cannot guarantee that the value is not null, defeating the whole purpose of null safety.

Answer 2

Change these codes :

  Id id;
  Name name;

to this :

  Id? id;
  Name? name;