Non-nullable instance field must be initialized

Question

class Foo {
  int count; // Error
  void bar() => count = 0;
}

Why I'm seeing an error when I'am already initializing it in the bar method? I could understand this error if count was marked final.

Solution 1

(Your code was fine before Dart 2.12, null safe)

With null safety, Dart has no way of knowing if you had actually assigned a variable to count. Dart can see initialization in three ways:

1. At the time of declaration:

   int count = 0;
   

2. In the initializing formals:

   Foo(this.count);
   

3. In the initializer list:

   Foo() : count = 0;
   

So, according to Dart, count was never initialized in your code and hence the error. The solution is to either initialize it in 3 ways shown above or just use the late keyword which will tell Dart that you are going to initialize the variable at some other point before using it.

4. Use the late keyword:

   class Foo {
     late int count; // No error
     void bar() => count = 0;
   }
   

5. Make variable nullable:

   class Foo {
     int? count; // No error
     void bar() => count = 0;
   }
   

Solution 2

These are new rules about Dart about null safety

class Note {
  late int _id;
  late String _title;
  late String? _description;
  late String _date;
  late int _priority;
}

make sure before your variable put late

Comments

[iDecode]

First, I have already mentioned about the late keyword, second there is no reason to put multiple variables to show the same thing.

[Musfiq Shanta]

i saw it when i was coding.that's why i put it and mention

[iDecode]

What did you see, late keyword? It's already there in my answer. Your answer isn't providing anything new. So, you can think of deleting it (in a good spirit) but the choice is always yours.

[iDecode]

I have already mentioned this in my answer. Sorry, there's nothing new in your post.

[iDecode]

So, you're basically telling me to opt out of null safety? Sorry, this isn't something I was looking for !

[Milind Gour]

your call bro ...I didn't want null safety in project amd got the same error

[Milind Gour]

for nulll safety use ? after variable like var a? and while using , use ! after variable ,like : if(a!)

[iDecode]

First, how long can you stay there without null safety? Second, I've already provided that solution in my answer.

[Milind Gour]

my error was , due to await and then in ascyn function , I was getting value from another class , which returns future , soo the error was you can't return a variable with ? because of await and then ....

[Admin]

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[Jeremy Caney]

It appears this answer was flagged as Not an Answer. It very much is an answer. It is an answer with undesirable side effects—but that's why it's been downvoted, which is the correct action for answers that aren't useful. Answers that provide a sincere effort at answering the question should not be deleted (unless, of course, they are just a link).