Dart - how to convert Map<String, dynamic> to Map<String, String>

Question

This snippet compile successfully but fail in runtime

Map<String, dynamic> queryParameters = {"id": 3};
Map<String, String> stringQueryParameters = Map<String, String>.from(queryParameters);

Uncaught Error: TypeError: 3: type 'int' is not a subtype of type 'String'

What's the proper way to convert from Map<String, dynamic> to Map<String, String> ?

Mattia · Accepted Answer · 2020-05-23 20:35:21Z

42

You can use .map to convert your keys:

  Map<String, dynamic> queryParameters = {"id": 3};
  Map<String, String> stringQueryParameters =
      queryParameters.map((key, value) => MapEntry(key, value?.toString()));

answered May 23, 2020 at 20:35

Mattia

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2 Comments

krazy Over a year ago

Map<String, String> mapResult = queryParameters.map((key, value) => MapEntry(key, value != null ? value!.toString() : ''));

Lance Peterson Sep 17 at 18:47

In my case, map() returns a Map<dynamic, dynamic>, which is also not a subtype of Map<String, String>.

Jossef Harush Kadouri · Accepted Answer · 2020-05-23 16:43:34Z

6

Loop all items, call .toString() and store the result as the new value

Map<String, dynamic> queryParameters = {"id": 3, "name": null};
Map<String, String> stringQueryParameters = Map<String, String>();
queryParameters.forEach((key, value) => stringQueryParameters[key] = value?.toString());

// ...

answered May 23, 2020 at 16:43

Jossef Harush Kadouri

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Comments

Lance Peterson · Accepted Answer · 2025-09-17 18:55:21Z

0

@Mattia's answer didn't work for me. Calling map() returns a Map<dynamic, dynamic>, which is also not a subtype of Map<String, String>.

So in addition to calling map(), I found I needed to call cast().

Map<String, dynamic> queryParameters = {"id": 3};
Map<String, String> stringParameters = queryParameters.map(
  (key, value) => key, value?.toString())
).cast<String, String>();

answered Sep 17 at 18:55

Lance Peterson

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