Flutter error: A value of type 'Object?' can't be assigned to a variable of type 'String'

Question

I am using a dropdownbuttonfield and getting this error:

A value of type 'Object?' can't be assigned to a variable of type 'String'.
Try changing the type of the variable, or casting the right-hand type to 'String'.dart(invalid_assignment)

Code :

    class SettingsForm extends StatefulWidget {
  @override
  _SettingsFormState createState() => _SettingsFormState();
}

class _SettingsFormState extends State<SettingsForm> {
  final _formKey = GlobalKey<FormState>();
  final List<String> sugars = ['0', '1', '2', '3', '4'];
  final List<int> strengths = [100, 200, 300, 400, 500, 600, 700, 800, 900];

  // form values
   String? _currentName;
   String? _currentSugars;
   int? _currentStrength;

  @override
  Widget build(BuildContext context) {
    MyUser user = Provider.of<MyUser>(context);

    return StreamBuilder<UserData>(
        stream: DatabaseService(uid: user.uid).userData,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            UserData? userData = snapshot.data;
            return Form(
              key: _formKey,
              child: Column(
                children: <Widget>[
                  Text(
                    'Update your brew settings.',
                    style: TextStyle(fontSize: 18.0),
                  ),
                  SizedBox(height: 20.0),
                  TextFormField(
                    initialValue: userData!.name,
                    decoration: textInputDecoration,
                    validator: (val) =>
                        val!.isEmpty ? 'Please enter a name' : null,
                    onChanged: (val) => setState(() => _currentName = val),
                  ),
                  SizedBox(height: 10.0),
                  DropdownButtonFormField(
                    value: _currentSugars ?? userData.sugars,
                    decoration: textInputDecoration,
                    items: sugars.map((sugar) {
                      return DropdownMenuItem(
                        value: sugar,
                        child: Text('$sugar sugars'),
                      );
                    }).toList(),
                    onChanged: (val) => setState(() => _currentSugars = val), <--Error here **val** right one
                  ),
                  SizedBox(height: 10.0),
                  Slider(
                    value: (_currentStrength ?? userData.strength).toDouble(),
                    activeColor:
                        Colors.brown[_currentStrength ?? userData.strength],
                    inactiveColor:
                        Colors.brown[_currentStrength ?? userData.strength],
                    min: 100.0,
                    max: 900.0,
                    divisions: 8,
                    onChanged: (val) =>
                        setState(() => _currentStrength = val.round()),
                  ),
                  ElevatedButton(
                      style:
                          ElevatedButton.styleFrom(primary: Colors.pink[400]),
                      child: Text(
                        'Update',
                        style: TextStyle(color: Colors.white),
                      ),
                      onPressed: () async {
                        if (_formKey.currentState!.validate()) {
                          await DatabaseService(uid: user.uid).updateUserData(
                              _currentSugars ?? snapshot.data!.sugars,
                              _currentName ?? snapshot.data!.name,
                              _currentStrength ?? snapshot.data!.strength);
                          Navigator.pop(context);
                        }
                      }),
                ],
              ),
            );
          } else {
            return Loading();
          }
        });
  }
}

Update Update 2 A value of type 'String?' can't be assigned to a variable of type 'String'. Try changing the type of the variable, or casting the right-hand type to 'String'.

Solution 1

Assign the generic type String to the DropdownButtonFormField:

DropdownButtonFormField<String>(
                    value: _currentSugars,
                    decoration: textInputDecoration,
                    items: sugars.map((sugar) {
                      return DropdownMenuItem(
                        value: sugar,
                        child: Text('$sugar sugars'),
                      );
                    }).toList(),
                    onChanged: (val) => setState(() => _currentSugars = val), 
                  ),

Unless you specify the type String dart makes the assumption with the one of the most generic types it has Object? as the generic type of the DropdownButtonFormField

Complete demo (Updated to use null safety)

class Demo extends StatefulWidget {
  Demo({Key? key}) : super(key: key);

  @override
  _DemoState createState() => _DemoState();
}

class _DemoState extends State<Demo> {
  final sugars = ['candy', 'chocolate', 'snicker'];
  String? _currentSugars = 'candy';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: DropdownButtonFormField<String>(
          value: _currentSugars,
          items: sugars.map((sugar) {
            return DropdownMenuItem(
              value: sugar,
              child: Text('$sugar sugars'),
            );
          }).toList(),
          onChanged: (val) => setState(() => _currentSugars = val),
        ),
      ),
    );
  }
}

Solution 2

Try to give val as a String, or any type you want to to become:

onChanged: (val) => setState(() => _currentSugars = val as String),

Comments

[TheAlphamerc]

pls do mention the type of _currentSugars and sugars.

[Mike Osborn]

@TheAlphamerc they are both 'Strings'

[Midhun MP]

@MikeOsborn Just set _currentSugars = val!

[Mike Osborn]

@MidhunMP the error doesn't change

[Mike Osborn]

do you want an update screenshot?

[Midhun MP]

@MikeOsborn What is the error you are getting now?

[Mike Osborn]

@MidhunMP same error I have two 'val' and they are both objects and I think they should both be 'strings' and dynamic

[Midhun MP]

@MikeOsborn My above comment is for your current screenshot and for the error 'String?' can't be assigned to a variable of type 'String'

[Mike Osborn]

no it's not the same code

[Mike Osborn]

@MidhunMP pls see the update 2 screenshot

[Midhun MP]

You need to change DropdownButtonFormField to DropdownButtonFormField<String> then use my above mentioned comment

[Mike Osborn]

Wait I didn't have to put the exclamation mark, it worked just by putting "<String> but since you solved my previous problem by "String? _currentSugars;" it now works without null check.

[Midhun MP]

@MikeOsborn Yeah, since you declared it as nullable, so you no longer need to use !.

[Mike Osborn]

@MidhunMP Can u solve this q also stackoverflow.com/questions/68460812/… Thanks in advance

[croxx5f]

It does work for me, I added a demo in which I declare both _currentSugars and sugars which are the only to unknowns to me from the code you posted

[croxx5f]

updated it. I forgot about null safety 😅

[Mike Osborn]

can you tell the difference and see what's causing the error