The operator '*' can't be unconditionally invoked because the receiver can be 'null'. Try adding a null check to the target ('!')

user_10068879 0
Viewed 3417

I used this code for responsiveness in my UI. So what this code basically does is calculate the size of the screen and I use the functions below to put the exact font size according to the design provided to me in Figma or Adobe XD. Using this method, I was able to create pixel-perfect UI.

After upgrading to Flutter 2.0.3, I am getting null safety errors. I was able to solve most of them but I am not able to solve this error. Please advice.

Complete Code

import 'package:flutter/material.dart';

class SizeConfig {
  static MediaQueryData? _mediaQueryData;
  static double? screenWidth;
  static double? screenHeight;
  static double? defaultSize;
  static Orientation? orientation;

  void init(BuildContext context) {
    _mediaQueryData = MediaQuery.of(context);
    screenWidth = _mediaQueryData!.size.width;
    screenHeight = _mediaQueryData!.size.height;
    orientation = _mediaQueryData!.orientation;
    if (orientation == Orientation.landscape) {
      defaultSize = screenHeight! * 0.024;
    } else {
      defaultSize = screenWidth! * 0.024;
    }
  }
}

double getSize(double size) {
  var defaultsSize = SizeConfig.defaultSize * size;
  return (defaultsSize / 10);
}

// Get the proportionate height as per screen size
double getProportionateScreenHeight(double inputHeight) {
  double screenHeight = SizeConfig.screenHeight!;
  // 812 is the layout height that designer use
  return (inputHeight / 812.0) * screenHeight;
}

// Get the proportionate width as per screen size
double getProportionateScreenWidth(double inputWidth) {
  double screenWidth = SizeConfig.screenWidth!;
  // 375 is the layout width that Figma provides
  return (inputWidth / 375.0) * screenWidth;
}

Error

dart-null-safety flutter-web dart flutter
3 Answers

Because SizeConfig.defaultSize is nullable, you need to make sure that its value should not be null.

You can add some assertion to notify the caller that SizeConfig should be initialized first. Then, you can change it to SizeConfig.defaultSize!.

Sample...

double getSize(double size) {
  assert(
    SizeConfig.defaultSize != null,
    "SizeConfig should be initialized (only once) before calling getSize(...). Refer to SizeConfig.init(...).",
  );
  var defaultsSize = SizeConfig.defaultSize! * size;
  return (defaultsSize / 10);
}
user_94553250

Problem:

You get this error because the object you're invoking * on can be null.

Example:

int? count = 1;

void main() {
  print(count * 2); // error: The operator '*' can't be unconditionally invoked ...
}

Solutions:

  • Use a local variable:

    int? count = 1;

void main() {
  var i = count;
  if (i != null) {
    print(i * 2); // Prints 2
  }
}

  • Use bang operator (!)

    int? count = 1;

void main() {
  print(count! * 2); // Prints 2
}
user_66186220

You have three options:

  • Use the bang operator:
int? count = 1;

void main() {
  // will throw an error if count is null
  print(count! * 2);
}
  • Use the ?? operator:
int? count = 1;

void main() {
  // safe
  print((count ?? 1) * 2);
}
  • Use an if - else statement:
int? count = 1;

void main() {
  if(count != null) {
    print(count! * 2);
  } else {
    print('count is null');
  }
}
user_163425610
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