To json mapping models with list of sub model inside

Question

I'm new to flutter dev and trying to convert a model to a json string to pass to an api call.

when i try use the RegistrationRequest.ToJson function I get this error

type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'String'

I assume it cannot convert the list of userImages properly and not quite suree how to do it. Any help would be much appreciated

here is where i call the function

var serialisedBodycom = RegistrationRequest.toJson();
      var serialisedBody = jsonEncode(serialisedBodycom);

i have a model called RegistrationRequest

    import 'UserGender.dart';
import 'UserImage.dart';

class RegistrationRequest {
  String email;
  String password;
  String msisdn;
  UserGender gender;
  UserGender matchGender;
  List<UserImage> userImages;

  RegistrationRequest(
      {this.email,
      this.password,
      this.msisdn,
      this.gender,
      this.matchGender,
      this.userImages});
  RegistrationRequest.Empty();

  RegistrationRequest.fromJson(Map<String, dynamic> json)
      : email = json['email'],
        password = json['password'],
        msisdn = json['msisdn'],
        gender = json['gender'],
        matchGender = json['matchGender'],
        userImages = json['userImages'];

  Map<String, dynamic> toJson() {
    return {
      'email': email ?? '',
      'password': password ?? '',
      'MSISDN': msisdn ?? '',
      'Gender': gender ?? UserGender.Unknown,
      'MatchGender': matchGender ?? UserGender.Unknown,
      'UserImages': userImages ?? new List<UserImage>()
    };
  }
}

which contains a list of UserImage

    class UserImage {
  String base64;
  String fileName;
  bool isMainProfilePicture;
  String contentType;

  UserImage(
      this.base64, this.fileName, this.isMainProfilePicture, this.contentType);
  UserImage.Empty();
  UserImage.fromJson(Map<dynamic, dynamic> json)
      : base64 = json['base64'],
        fileName = json['fileName'],
        isMainProfilePicture = json['isMainProfilePicture'],
        contentType = json['contentType'];

  Map<dynamic, dynamic> toJson() => {
        'base64': base64,
        'fileName': fileName,
        'isMainProfilePicture': isMainProfilePicture,
        'contentType': contentType
      };
}

Answer 1

the final answer I came up with is the following

RegistrationRequest.fromJson(Map<String, dynamic> json) => RegistrationRequest(
        email: json["email"],
        password: json["password"],
        msisdn: json["msisdn"],
        gender: json["gender"],
        matchGender: json["matchGender"],
        userImages: List<UserImage>.from(json["userImages"].map((x) => UserImage.fromJson(x))),
    );

    Map<String, dynamic> toJson() => {
        "email": email,
        "password": password,
        "msisdn": msisdn,
        "gender": gender,
        "matchGender": matchGender,
        "userImages": List<dynamic>.from(userImages.map((x) => x.toJson())),