cannot assign value to my `Map<String, String>` variable after json-decode

Question

I try to assign a value to my Map<String, String> variable after I json-encode it and then json-decode it. The following code example is a simplified reproduce;

import 'dart:convert';

String toJson(dynamic object) {
  var encoder = new JsonEncoder.withIndent("     ");
  return encoder.convert(object);
}

dynamic fromJson(String jsonString) {
  return json.decode(jsonString);
}

void main() {
  Map<String, String> data = {"hello": "world"};
  String jsonString = toJson(data);
  data = fromJson(jsonString);
  print(data);
}

When I run it, it fails with;

Unhandled exception:
type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'Map<String, String>'

And on this online editor it fails with a different error;

Uncaught exception:
TypeError: Instance of '_JsonMap': type '_JsonMap' is not a subtype of type 'Map<String, String>'

Jossef Harush Kadouri · Accepted Answer · 2019-01-14 22:00:46Z

2

Workaround it,

I don't like it but it workaround the issue. Wrap the value returned by the fromJson function with Map<String, String>.from(...)

For example:

void main() {
  Map<String, String> data = {"hello": "world"};
  String jsonString = toJson(data);
  data = Map<String, String>.from(fromJson(jsonString));
  print(data);
}

answered Jan 14, 2019 at 22:00

Jossef Harush Kadouri

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1 Comment

cora32 Over a year ago

Looks like type mismatch. It works if you replace Map<String, String> data = {"hello": "world"}; with Map<String, dynamic> data = {"hello": "world"};

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