[Solved] flutter convert argument type Object? to parameter type Map<String, dynamic>


Asked by Pieter on December 30, 2021 (source).

I know this question has been asked multiple times, but even with the suggestions in the other posts I cannot get it solved... I have a class Score that I want to load from a Firebase backend using Flutter. Here is what the Score class looks like:

class Score {
  final String name;
  final List<String> tags;

  Score(this.name, this.tags);

  Score.fromJson(Map<String, dynamic> json)
      : name = json['name'] ?? '',
        tags = json['tags'] ?? [''];

Now I am loading this from Firebase using a Streambuilder<QuerySnapshot>, code snippet:

List<Score> scores = snapshot.data!.docs
                .map((e) => Score.fromJson(e.data()))

This leads to the error The argument type 'Object?' can't be assigned to the parameter type 'Map<String, dynamic>'. If I try to cast e.data() as Map<String, dynamic> as suggested in other posts, the whole app crashes at startup.

Could someone please help me how to convert the snapshot data to a List<Score>? Thx!


Question answered by Frank v (source).

If you know you're getting a map from the database, you can cast the Object to such a Map with:

Score.fromJson(e.data() as Map<String, dynamic>)

To convert the tags to a List<String> requires a different kind of conversion:


To learn more about why this is a different type of conversion, see Difference between List<String>.from() and as List<String> in Dart