Type 'List<Map<String, String?>>' is not a subtype of type 'String' in type cast

Question

Context: I want to register new customer to my API server(with package:http/http.dart).

Problem: This error was thrown:

flutter: type 'List<Map<String, String?>>' is not a subtype of type 'String' in type cast

I think flutter is complaining about the type of metadata property, but I have no clue on how to resolve it.

I tried to create a Map<dynamic, dynamic> object but it does not help as well.

var response = await _httpClient.post(
        createCustomerUrl,
        body: {
          'email': customerModel.email,
          'first_name': customerModel.firstName,
          'last_name': customerModel.lastName,
          'password': customerModel.password,
          'username': customerModel.username,
          'meta_data': [
            {'key': 'birthdate', 'value': customerModel.birthdate},
          ],
        },
      );

username_jj · Accepted Answer · 2021-08-24 13:02:28Z

@Abbasihsn answer was almost there. JSON.encode + 'Content-Type': 'application/json' in the headers solve the problem.

var response = await _httpClient.post(
        createCustomerUrl,
        body: json.encode({
          'email': customerModel.email,
          'first_name': customerModel.firstName,
          'last_name': customerModel.lastName,
          'password': customerModel.password,
          'username': customerModel.username,
          'meta_data': [
            {'key': 'birthdate', 'value': customerModel.birthdate},
          ],
        }),
        headers: { 'Content-Type': 'application/json' }
      );

The error message is due to http package internally perform type check and trying to cast the body to Map<String, String>. Here is the code snippet extracted from the http package that causes the error:

  /// Sends a non-streaming [Request] and returns a non-streaming [Response].
  Future<Response> _sendUnstreamed(
      String method, Uri url, Map<String, String>? headers,
      [body, Encoding? encoding]) async {
    var request = Request(method, url);

    if (headers != null) request.headers.addAll(headers);
    if (encoding != null) request.encoding = encoding;
    if (body != null) {
      if (body is String) {
        request.body = body;
      } else if (body is List) {
        request.bodyBytes = body.cast<int>();
      } else if (body is Map) {
        request.bodyFields = body.cast<String, String>();  // <-- this line
      } else {
        throw ArgumentError('Invalid request body "$body".');
      }
    }

    return Response.fromStream(await send(request));
  }

Abbasihsn · Accepted Answer · 2021-08-24 10:25:40Z

1

try encoding your body to a JSON, like this:

var response = await _httpClient.post(
        createCustomerUrl,
        body: json.encode({
          'email': customerModel.email,
          'first_name': customerModel.firstName,
          'last_name': customerModel.lastName,
          'password': customerModel.password,
          'username': customerModel.username,
          'meta_data': [
            {'key': 'birthdate', 'value': customerModel.birthdate},
          ],
        }),
      );

answered Aug 24, 2021 at 10:25

Abbasihsn

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4 Comments

username_jj Over a year ago

Sadly not working, json.encode return a stringified version of the request body, but the server is expecting the request body to be key-value pair json, when passing the string to server, server complain missing params.

Abbasihsn Over a year ago

ok, have you tried json.encode only for meta_data': [...], section?

username_jj Over a year ago

I have solved, you are almost there, the solution is json.encode with explicitly add Content-Type: application/json to the headers. Thank you for helping.

Abbasihsn Over a year ago

your welcome. if my answer was helpful, I would be grateful if you vote it up or set it as an acceptable solution.

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