Getting type 'List<dynamic>' is not a subtype of type 'List<...>' error in JSON

Question

I'm decoding a response body and I'm getting the error:

'List<dynamic>' is not a subtype of type 'List<Example>'

I'm parsing a JSON array of JSON objects, one of the fields is a list of objects as well and I suspect my issue stems from that. I am also using the json_serializable library. Below is my code, I omitted some fields out and changed some variable names but it represents the same code:

import 'package:json_annotation/json_annotation.dart';

part 'example_model.g.dart';

@JsonSerializable()
class Example {

  (some fields here)
  final List<Random> some_urls;
  final List<String> file_urls;


  const Example({
    (some fields here)
    this.some_urls,
    this.file_urls,

  });

  factory  Example.fromJson(Map<String, dynamic> json) =>
      _$ ExampleFromJson(json);
}

@JsonSerializable()
class Random {
  final String field_1;
  final int field_2;
  final int field_3;
  final int field_4;
  final bool field_5;

  constRandom(
      {this.field_1, this.field_2, this.field_3, this.field_4, this.field_5});

  factory Random.fromJson(Map<String, dynamic> json) => _$RandomFromJson(json);
}

from the .g dart file that json_serializable made (ommited the encoding part):

Example _$ExampleFromJson(Map<String, dynamic> json) {
  return Example(

      some_urls: (json['some_urls'] as List)
          ?.map((e) =>
      e == null ? null : Random.fromJson(e as Map<String, dynamic>))
          ?.toList(),
      file_urls: (json['file_urls'] as List)?.map((e) => e as String)?.toList(),

}

Random _$RandomFromJson(Map<String, dynamic> json) {
  return Random(
      field_1: json['field_1'] as String,
      field_2: json['field_2'] as int,
      field_3: json['field_3'] as int,
      field_4: json['field_4'] as int,
      field_5: json['field_5'] as bool);
}

This is my future function:

  Future<List<Example>> getData(int ID, String session) {
    String userID = ID.toString();
    var url = BASE_URL + ":8080/example?userid=${userID}";
    return http.get(url, headers: {
      "Cookie": "characters=${session}"
    }).then((http.Response response) {
      if (response.statusCode == 200) {
        var parsed = json.decode(response.body);
        List<Example> list = parsed.map((i) => Example.fromJson(i)).toList();
        return list;
      }
    }).catchError((e)=>print(e));
  }

KayBee · Accepted Answer · 2021-04-16 08:55:11Z

83

This code creates a List<dynamic>

parsed.map((i) => Example.fromJson(i)).toList();

You must explicitly cast List<dynamic> to List<Example> like so,

List<Example> list = List<Example>.from(parsed.map((i) => Example.fromJson(i)));

or just

var /* or final */ list = List<Example>.from(parsed.map((i) => Example.fromJson(i)));

4 Comments

DanT29 Over a year ago

I get the error type 'MappedListIterable<dynamic, dynamic>' is not a subtype of type 'Iterable<Example>'

Günter Zöchbauer Over a year ago

Try .from(...) instead of .of(...). I'm still reading up and try to understand what exactly the difference between of() and from() is.

Günter Zöchbauer Over a year ago

Seems of expects the input list to have the correct generic type already, while with from only the elements in the list need to be of the correct type.

Tyler Petrov Over a year ago

Your answer saved me from pulling out all my hair in frustration. You saved me a lot of time.

CopsOnRoad · Accepted Answer · 2020-05-26 01:58:09Z

16

Reason for Error:

You get this error when your source List is of type dynamic or Object (let's say) and you directly assign it to a specific type without casting.

List<dynamic> source = [1]; 
List<int> ints = source; // error

Solution:

You need to cast your List<dynamic> to List<int> (desired type), there are many ways of doing it. I am listing a few here:

List<int> ints = List<int>.from(source);

List<int> ints = List.castFrom<dynamic, int>(source);

```
List<int> ints = source.cast<int>();
```

List<int> ints = source.map((e) => e as int).toList();

answered May 26, 2020 at 1:58

CopsOnRoad

271k97 gold badges702 silver badges471 bronze badges

4 Comments

CopsOnRoad Over a year ago

@maheshmnj Which cast and on the basis of which Dart version you're saying that?

Mahesh Jamdade Over a year ago

please refer this github.com/dart-lang/site-www/issues/736

CopsOnRoad Over a year ago

@maheshmnj Can you please share the link of the particular thread which supports your initial statement -- cast is no longer available to use.

Mahesh Jamdade Over a year ago

please take a look at this I think I may have mis understood the statement based on the comments for this answer stackoverflow.com/a/50245597/8253662

Nat Jacobs · Accepted Answer · 2020-01-08 04:48:42Z

5

I was receiving the 'MappedListIterable<dynamic, dynamic>' is not a subtype of type 'Iterable<Example> when i tried Günter's solution.

 var parsed = json.decode(response.body);
 var list = parsed.map((i) => Example.fromJson(i)).toList();

Casting the parsed data into a List<dynamic> (rather than just letting it go to dynamic) resolved that issue for me.

 var parsed = json.decode(response.body) as List<dynamic>;
 var list = parsed.map((i) => Example.fromJson(i)).toList();

answered Jan 8, 2020 at 4:48

Nat Jacobs

2511 gold badge5 silver badges9 bronze badges

Comments

mezoni · Accepted Answer · 2021-03-14 18:09:48Z

0

Your code:

var parsed = json.decode(response.body);
List<Example> list = parsed.map((i) => Example.fromJson(i)).toList();

Can be replaced with this code:

import 'package:json_helpers/json_helpers.dart';


final examples = response.body.jsonList((e) => Example.fromJson(e));

And everything will work as you expected...

answered Mar 14, 2021 at 18:09

mezoni

11.3k4 gold badges38 silver badges58 bronze badges

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