迁移到 Dart 空安全后出现“无法无条件调用运算符，因为接收器可以为空”错误

Question

I'm upgrading a personal package that is based on the Flutter framework. I noticed here in the Flutter Text widget source code that there is a null check:

if (textSpan != null) {
  properties.add(textSpan!.toDiagnosticsNode(name: 'textSpan', style: DiagnosticsTreeStyle.transition));
}

However, textSpan! is still using the ! operator. Shouldn't textSpan be promoted to a non-nullable type without having to use the ! operator? However, trying to remove the operator gives the following error:

 An expression whose value can be 'null' must be null-checked before it can be dereferenced. Try checking that the value isn't 'null' before dereferencing it.

Here is a self-contained example:

class MyClass {
  String? _myString;
  
  String get myString {
    if (_myString == null) {
      return '';
    }
    
    return _myString; //   <-- error here
  }
}

I get a compile-time error:

Error: A value of type 'String?'can't be returned from function 'myString' because it has a return type of 'String'.

Or if I try to get _mySting.length I get the following error:

The property 'length' can't be unconditionally accessed because the receiver can be 'null'.

I thought doing the null check would promote _myString to a non-nullable type. Why doesn't it?

My question was solved on GitHub so I'm posting an answer below.

Answer 1

Dart engineer Erik Ernst says on GitHub :

Type promotion is only applicable to local variables. ... Promotion of an instance variable is not sound, because it could be overridden by a getter that runs a computation and returns a different object each time it is invoked. Cf. dart-lang/language#1188 for discussions about a mechanism which is similar to type promotion but based on dynamic checks, with some links to related discussions.

So local type promotion works:

  String myMethod(String? myString) {
    if (myString == null) {
      return '';
    }
    
    return myString;
  }

But instance variables don't promote. For that you need to manually tell Dart that you are sure that the instance variable isn't null in this case by using the ! operator:

class MyClass {
  String? _myString;
  
  String myMethod() {
    if (_myString == null) {
      return '';
    }
    
    return _myString!;
  }
}

Answer 2

The Error:

Let's say, this is your code and you're doing a null check on the instance variable and still seeing an error:

class Foo {
  int? i = 0;

  double func() {
    if (i != null) return i.toDouble(); // <-- Error
    return -1;
  }
}

The method 'toDouble' can't be unconditionally invoked because the receiver can be 'null'.

The error you see in code like this is because Getters are not promoted to their non-nullable counterparts . Let's talk about the reason why.

---

Reason of the Error:

Let's say, there's a class Bar which extends Foo and override i variable and doesn't assign it any value (keeping it null ):

class Bar extends Foo {
  @override
  int? i;
}

So, if you could do

print(Bar().func() * 2);

You would have run into a runtime null error, which is why getters type promotion is prohibited.

---

Solutions:

We need to cast away nullability from int? . There are generally 3 ways to do this (more ways include the use of as , is , etc)

• Use local variable (Recommended)

   double bar() { var i = this.i; // <-- Use of local variable. if (i.= null) return i;toDouble(); return -1; }

• Use?. with??

   double bar() { return i?.toDouble()?? -1; // Provide some default value. }

• Use Bang operator (!)

  You should only use this solution when you're 100% sure that the variable ( i ) will never be null .

   double bar() { return i.;toDouble(). // <-- Bang operator in play. }

Answer 3

Use Ternary Operator to fix errors like this. This is an example where headline1 can be null

                    Theme
                        .of(context)
                        .textTheme
                        .headline1?.fontSize?? 32,

Answer 4

style: Theme.of(context).textTheme.headline5..copyWith(

style: Theme.of(context).textTheme.headline5!.copyWith(
                        color: Colors.white

Try making the call conditional using? or a null safety checker - !