this.reference is not working on flutter/dart

Question

I don't know what's the problem here but i get an error on [this.reference], it says "The parameter 'reference' can't have a value of 'null' because of its type, but the implicit default value is 'null'." When I add required it says "Can't have modifier 'required' here"

Here's the code for it:

import 'package:cloud_firestore/cloud_firestore.dart';

class Request {
  final String type;
  final String reason;
  DocumentReference reference;

  Request(this.reason, this.type, [this.reference]);

  String get requestId {
    return reference.id;
  }

  Map<String, dynamic> toMap() {
    return {"type": type, "reason": reason};
  }
}

add null check operator '?' , like DocumentReference? reference; — Ali Punjabi, Jan 25 '22 at 11:07
@YeasinSheikh The parameter already is optional. Curly braces make a parameter *named*. — jamesdlin, Jan 25 '22 at 17:40
@RajaEhtisham `?` is not a null check operator. A type ending with `?` is *nullable*. — jamesdlin, Jan 25 '22 at 17:41

score 1 · Accepted Answer · answered Jan 25 '22 at 11:12

class Request {
  final String type;
  final String reason;
  DocumentReference? reference;

  Request(this.reason, this.type, [this.reference]);

  String get requestId {
    return reference!.id;
  }

  Map<String, dynamic> toMap() {
    return {"type": type, "reason": reason};
  }
}

Guessing you are running on null-safety, you may want to make DocumentReference nullable

score 0 · Answer 2 · answered Jan 25 '22 at 11:16

0

Did u tried making those Nullable

final String? type;
final String? reason;

Also Try Giving default initial values

Request(String reason = '',String type = '');

answered Jan 25 '22 at 11:16

Aravind Aji

154
5

jamesdlin · Answer 3 · 2022-01-25T17:46:15.460

Square brackets around a parameter make that parameter an optional positional parameter. Therefore when you write:

Request(this.reason, this.type, [this.reference]);

You are specifying that the Request constructor take two required positional parameters (reason and type) and an optional positional parameter (reference).

If you want reference be optional, it must have a default value if the caller doesn't provide it. Your options are:

Provide a const, non-null default value:
```
Request(
   this.reason,
   this.type,
   [this.reference = DocumentReference.someConstConstructor()],
);
```
(The DocumentReference class from package:cloud_firestore does not provide a const constructor, so you can't do this in your case, but I've listed it here for completeness.)

Make reference nullable so that it can use null as an implicit default value:

class Request {
  ...
  DocumentReference? reference;
  ...
  Request(this.reason, this.type, [this.reference]);

or if you want a nullable parameter with a non-nullable member:

class Request {
  ...
  DocumentReference reference;
  ...
  Request(this.reason, this.type, [DocumentReference? reference])
    : reference = reference ?? DocumentReference();

The latter is a general technique for making parameters optional when the parameter type has no const constructors available.

Make reference non-optional. Since using square brackets around a parameter make that parameter optional, adding the required keyword would be a contradiction and would make no sense. If you want it to be a required positional parameter, simply remove the square brackets:
```
Request(this.reason, this.type, this.reference);
```
Or if you want to be a required named parameter, use curly braces with the required keyword:
```
Required(this.reason, this.type, {required this.reference});
```

I suggest reading the Functions Parameters section of the Dart Language Tour.

this.reference is not working on flutter/dart

3 Answers3