http post request giving type-errors

Question

In this instance, i have details of an item entered by user and need to add them to backend server. Here is my method:

Future<AddItemResponseModel> AddItem(
  String name,
  String description,
  String picture, // image encoded to base64 String
  List<String> tags,
  bool liked,
  String token,
) async {
  final String url = "API_URL";
  final response = await http.post(Uri.parse(url), body: {
    "name": name,
    "description": description,
    "picture": picture,
    "date": "date",
    "tags": tags,
    "like": liked,
    "location": "something"
  }, headers: {
    "authtype": "custom",
    "x-access-token": token
  });
  if (response.statusCode == 200) {
    final String responseString = response.body;
    print(responseString);
    return AddItemResponseModelFromJson(responseString);
  } else {
    // Error message to user
  }
}

And here is the onTap function of a GestureDetector widget that calls this method.

onTap: () async {
                                    if (checkCondition) {
                                      final AddItemResponseModel item =
                                          await AddItem(
                                        itemName, // name
                                        itemDescription, // description 
                                        _image64, // image encoded to base64 String
                                        tagNames, // List<String>
                                        true,  // liked
                                        token, // x-access-token
                                      );
                                      setState(() {
                                        _item = item;  // _item is of datatype <AddItemResponseModel> created to be accessed across this page
                                      });
                                      if (_item.success == true) {
                                        Navigator.push(
                                          context,
                                          MaterialPageRoute(
                                              builder: (context) =>
                                                  ItemAdded()),
                                        );
                                      }
                                    }
                                    setState(() {
                                      if (screen_no < 3) screen_no += 1;
                                    });
                                  }

I get the following error, but haven't been able to diagnose the cause.

Unhandled Exception: type 'List<String>' is not a subtype of type 'String' in type cast
#0      CastMap.forEach.<anonymous closure> (dart:_internal/cast.dart:288:25)
#1      _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:539:8)
#2      CastMap.forEach (dart:_internal/cast.dart:287:13)
#3      mapToQuery (package:http/src/utils.dart:17:7)
#4      Request.bodyFields= (package:http/src/request.dart:137:12)
#5      BaseClient._sendUnstreamed (package:http/src/base_client.dart:87:17)
#6      BaseClient.post (package:http/src/base_client.dart:32:7)
#7      post.<anonymous closure> (package:http/http.dart:69:16)
#8      _withClient (package:http/http.dart:164:20)
#9      post (package:http/http.dart:68:5)
#10     AddItem (package:krunch_app/Pages/AddItem/itemAddPage.dart:37:31)
#11     _ItemAddPageState.build.<anonymous closure> (package:krunch_app/Pages/AddItem/itemAddPage.dart:492:49)
#12     _ItemAddPageState.build.<anonymous closure> (package:krunc<…>

Thanks for all the help in advance.

EDIT-1:

This is my AddItemResponseModel

class AddItemResponseModel {
  AddItemResponseModel({this.success, this.error, this.id});

  bool? success;
  String? error;
  String? id;

  factory AddItemResponseModel.fromJson(Map<String, dynamic> json) =>
      AddItemResponseModel(
          success: json["success"], error: json["error"], id: json["id"]);
}

AddItem is just a function which returns a AddItemResponseModel datatype by decoding the json response

Answer 1

Your error is right in the Unhandled Exception. You are trying to cast your tags field as a List<String> type, when your API response gives you only a String. As is for now, try to change the type value like this:

String tags

Answer 2

Assuming that you're using package:http, from the documentation for http.post:

body sets the body of the request. It can be a String, a List<int> or a Map<String, String>.

You are passing a Map for body, so that Map is expected to have String keys and String values. You are attempting to include tags as a value to that Map, yet tags is a List<String>. (Similarly, liked will also produce an error since it is a bool and not a String.)

If you need to pass multiple items for a field, you must combine them and encode them to a single String first.

Also see: Dart Http POST with duplicate key form