Dart Null-aware Operators

Summary: In this tutorial, you’ll learn about various Dart null-aware operators to handle null values.

Introduction to Dart null-aware operators #

To deal with null values, Dart uses flow analysis and type promotion. In addition, it provides you with various null-aware operators:

The if-null operator (??) #

The following example uses an if statement to check if a variable is null and assign an error message if it is:

void main() {
  String? input;
  String message;
  if (input != null) {
    message = input;
  } else {
    message = 'Error';
  }
  print(message);
}Code language: Dart (dart)

Output:

ErrorCode language: Dart (dart)

To make this code more precise, you can use the if-null operator:

value ?? value_if_nullCode language: Dart (dart)

The ?? operator returns the value if the value is not null. Otherwise, it returns the value_if_null. By using the if-null operator, you can turn the example above into a more concise code:

void main() {
  String? input;
  String message = input ?? 'Error';
  print(message);
}Code language: Dart (dart)

Output:

ErrorCode language: Dart (dart)

The null-aware assignment operator (??=) #

Sometimes, you have a single nullable variable:

String? input;Code language: Dart (dart)

And you want to assign it a value if it is null, like this:

input = input ?? 'Error';Code language: Dart (dart)

This code works well. However, it is quite redundant to reference the input variable on both sides of the expression.

To avoid this, you can use the null-aware assignment operator:

input ??= 'Error';Code language: Dart (dart)

In this syntax, if the input is null, it is assigned the string 'Error'.

The null-aware access operator (?.) #

Suppose you have a nullable string variable called input like this:

String? input;Code language: Dart (dart)

To access the property and method of the String type, you need to check if it’s not null:

void main() {
  String? input;
  if (input != null) {
    print(input.length);
    print(input.toLowerCase());
  }
}Code language: Dart (dart)

To avoid using the if statement, you can use the null-aware access operator (?.):

objectName?.propertyCode language: Dart (dart)

The null-aware access operator returns null if the objectName is null. For example:

void main() {
  String? input;
  print(input?.length); // null
  print(input?.toLowerCase()); // null
}Code language: Dart (dart)

Null assertion operator (!) #

The following code causes a compile-time error:

bool? isTextFile(String? filename) {
  if (filename != null) {
    return filename.endsWith('.txt') ? true : false;
  }
  return null;
}

void main() {
  bool result = isTextFile('readme.txt');
  print(result);
}Code language: Dart (dart)

Error:

A value of type 'bool?' can't be assigned to a variable of type 'bool'.Code language: Dart (dart)

The isTextFile() function returns a value of the bool? type not bool. Therefore, you cannot assign the result of the isTextFile() function to the result variable.

Since you’re sure that the isTextFile('readme.txt') doesn’t return a null value, you can use the null assertion operator (!) like this:

bool result = isTextFile('readme.txt')!;Code language: Dart (dart)

The program will look like the following

bool? isTextFile(String? filename) {
  if (filename != null) {
    return filename.endsWith('.txt') ? true : false;
  }
  return null;
}

void main() {
  bool result = isTextFile('readme.txt')!;
  print(result);
}Code language: Dart (dart)

Null-aware index operator #

The null-aware index operator ?[] allows you to access an element of a list when the list might be null. For example:

void main() {
  List? scores = [1, 2, 3, 4, 5];
  // somewhere in the code
  scores = null;
  print(scores?[3]); // null
}Code language: Dart (dart)

In this example, we use the null-aware index operator to access the 4th element of the scores list. Because the scores list is null, the ?[] returns a null value.

Summary #

• Use null-aware operators to deal with null values.