The parameter can’t have a value of ‘null’ because of its type in Dart

by

Why

The reason this happens is because with null safety enabled, your non-nullable parameter factor or key cannot be null.

In the function and the constructor, these values might be null when the function is called without the named parameter: calculate() or Foo(). However because the types (int and Key) are non-nullable, this is invalid code – they must never be null.

Solutions

There are essentially three ways of solving this:

required

This is probably the most common solution to this problem and it indicates that a variable has to be set. This means that if we have (notice the required keyword):

void calculate({required int factor}) {
  // ...
}

We indicate that the factor parameter must always be specified, which solves the problem because only calculate(factor: 42) et al. will be valid calls of the function.

Default value

Another solution is providing a default value. If our parameter has a default value, we can safely not specify the parameter when calling the function because the default value will be used instead:

void calculate({int factor = 42}) {
  // ...
}

Now, a calculate() call will use 42 as the factor, which is obviously non-null.

Nullable parameter

The third solution is something that you really want to consider, i.e. do you want to have a nullable parameter? If so, you will have to null check the parameter when using it in your function.

However, it is the way you would most commonly want to solve the Key key issue because you do not always want to provide a key to your widget in Flutter (note the nullable Key? type):

class Foo extends StatelessWidget {
  const Foo({Key? key}): super(key: key);

  // ...
}

Now, you can safely construct Foo() without providing a key.

Positional parameters

Note that the same applies to positional parameters, i.e. they can be made nullable or non-nullable, however, they cannot be annotated with required and cannot have default values as they are always required to be passed.

void foo(int param1) {} // bar(null) is invalid.

void bar(int? param1) {} // bar(null) is valid.

More Related Contents:

  1. “The operator can’t be unconditionally invoked because the receiver can be null” error after migrating to Dart null-safety
  2. Null check operator used on a null value
  3. What is Null Safety in Dart?
  4. Understanding ” ! ” Bang Operator in dart
  5. Cannot run with sound null safety because dependencies don’t support null safety
  6. The argument type ‘Function’ can’t be assigned to the parameter type ‘void Function()?’ after null safety
  7. What is a Future and how do I use it?
  8. Is there a difference in how member variables are initialized in Dart?
  9. How to get height of a Widget?
  10. Flutter: How to correctly use an Inherited Widget?
  11. Passing Data to a Stateful Widget in Flutter
  12. How to upload images and file to a server in Flutter?
  13. How do you detect the host platform from Dart code?
  14. How to import platform specific dependency in Flutter/Dart? (Combine Web with Android/iOS)
  15. How to implement Dark mode and Light Mode in flutter?
  16. How to change the application launcher icon on Flutter?
  17. Flutter StreamBuilder vs FutureBuilder
  18. Style BottomNavigationBar in Flutter
  19. TextField inside of Row causes layout exception: Unable to calculate size
  20. How to solve the “Type Null is not a subtype of type …” error?
  21. Vertical viewport was given unbounded height
  22. How can I layout widgets based on the size of the parent?
  23. Could not open settings generic class cache for settings file ‘
  24. How to run code after some delay in Flutter?
  25. How to shift focus to the next TextField in Flutter?
  26. Flutter: “RenderFlex children have non-zero flex but incoming height constraints are unbounded”
  27. Flutter: List is deprecated? [duplicate]
  28. How can a named route have URL parameters in flutter web?
  29. How to pass callback in Flutter
  30. How to save and download text file in Flutter web application

Leave a Comment