[Solved] The argument type ‘Null Function(DataSnapshot)’ can’t be assigned to the parameter type ‘FutureOr<dynamic> Function(DatabaseEvent)’

Problem: When trying to fetch data from a Firebase Realtime database in a Flutter application, the following error is thrown:

The argument type 'Null Function(DataSnapshot)' can't be assigned to the parameter type 'FutureOr<dynamic> Function(DatabaseEvent)'.

For the following code snippet:

DatabaseReference _dbRef = FirebaseDatabase.instance.ref();
    _dbRef.once().then((DataSnapshot dataSnapshot) {
    print("read once" + dataSnapshot.value.toString());
});

Solution: Since the once() method returns a DatabaseEvent and not DatabaseSnapshot, the incorrect parameter type in the then() callback is the cause of this error. As the DatabaseSnapshot class is encapsulated in the DatabaseEvent class, we have to use its snapshot field to access the DatabaseSnapshot instance. Here is the syntactically correct version of the code snippet above:

DatabaseReference _dbRef = FirebaseDatabase.instance.ref();
    _dbRef.once().then((DatabaseEvent databaseEvent) {
    print("read once" + databaseEvent.snapshot.value.toString());
});

Conclusion

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