Error: The argument type 'String Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'

Question

class StudentValidationMixin {

    String validateFirstName(String value) {
      if(value.length<2){
        return 'İsim en az iki karakter olmalıdır';
    }
  }
}

The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type.

class _StudentAddState extends State with StudentValidationMixin {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Yeni öğrenci ekle"),
      ),
      body: Container(
        margin: EdgeInsets.all(20.0),
        child: Form(
          child: Column(
            children: <Widget>[
              TextFormField(
                decoration: InputDecoration(labelText: "Öğrenci Adı", hintText: "Fatih"),
                validator: validateFirstName,
              ),
            ],
          ),
        ),
      ),
    );
  }
}

The argument type 'String Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'.

These two separate bits of code are getting an error. What should I do?

score 4 · Accepted Answer · answered Jul 09 '21 at 20:13

The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type.

When you don't return anything inside a function/method, it implicitely returns null, so this

String validateFirstName(String value) {
  if(value.length<2){
    return 'İsim en az iki karakter olmalıdır';
  }
}

is equivalent to this

String validateFirstName(String value) {
  if(value.length<2){
    return 'İsim en az iki karakter olmalıdır';
  }
  return null;
}

But the return type of this method is String, a non-nullable type. To make it nullable (since you're returning null), add a question mark after it:

//    v
String? validateFirstName(String value) {
  if(value.length<2){
    return 'İsim en az iki karakter olmalıdır';
  }
}

The argument type 'String Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'.

The validator parameter of TextFormField is a function that receives a nullable type and returns a nullable type (i.e. a String? Function(String?), so just add a question mark after the value parameter:

//                              v
String? validateFirstName(String? value) {
  if (value == null) {
    // The user haven't typed anything
    return "Type something, man!";
  }
  if (value.length<2){
    return 'İsim en az iki karakter olmalıdır';
  }
}

score 2 · Answer 2 · answered Jul 09 '21 at 20:14

TextFormField validator accept a function which return String? that take a parameter String? so refactor your code like below.

class StudentValidationMixin {

  String? validateFirstName(String? value) {
    if(value == null)
      return "İsim boş olamaz!";

     if(value.length<2){
      return 'İsim en az iki karakter olmalıdır';
    }
    return null;
  }
}

score 1 · Answer 3 · edited Nov 18 '21 at 17:49

1

TextFormField(
       validator: (value) {
       controller.isNumberValid(value);

So for something like the validator for the TextFormField, you need to approach it this way to avoid that error.

edited Nov 18 '21 at 17:49

ouflak

2,458
10
44
49

answered Nov 18 '21 at 13:53

Hero Codes

11
1

score 1 · Answer 4 · answered Jan 23 '22 at 11:36

I have answered this Question already on Flutter Form validation with null safety

However I'll still drop a hint for quick reference ( read from the link for full example)

String? validatePassword(String? value) {
  if(value!.length < 6){
    return 'Invalid password, length must be more than 6';
  }

}

EDIT TEXT IS A CUSTOM TextFeild I CREATED FOR PERSONAL USE

EditText(hintText:"Email",fontSize: 12.sp,paddingLeft: 8,marginLeft:  10.h,marginRight: 10.h,
                        textController: controller.emailController,validator: validateEmail,),

Error: The argument type 'String Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'

4 Answers4