Understanding Null Safety in Dart 3

Introduction

Dart null safety is one of the most significant language enhancements in Dart 2.12 and further refined in Dart 3. With null safety, Dart developers can write more predictable and less error-prone code by distinguishing between nullable and non-nullable types at compile time. This tutorial walks you through the essential concepts of Dart null safety, shows you how to work with nullable and non-nullable variables, and explains sound null safety guarantees introduced in Dart 3.

What Is Dart Null Safety?

Null safety in Dart enforces that non-nullable variables cannot hold a null value. This eliminates a class of runtime errors known as “null reference” or “NoSuchMethodError: The method ‘…’ was called on null.” By opting into null safety, your code becomes more robust:

• Non-nullable types (String, int, MyClass) must always contain a value.

• Nullable types (String?, int?, MyClass?) can hold either a value or null.

Dart’s static analyzer checks these annotations at compile time, preventing you from inadvertently assigning or accessing null where it’s not allowed.

Non-nullable vs Nullable Types

In Dart null safety, you explicitly mark a type as nullable by appending a question mark (?):

void main() {
  String nonNullable = 'Hello'; // Cannot be null
  String? nullableText = null;  // Can be null

  print(nonNullable.length);    // Safe
  print(nullableText?.length);  // Use safe access operator
}

Key points:

• A String variable must be initialized with a non-null string.

• A String? can be null; you must use the null-aware operators (?, ??, !) to work safely.

Using Null-aware Operators

Dart provides several operators to handle nullable types conveniently:

• ?. (null-aware access): Executes a property or method only if the object isn’t null.

• ?? (if-null operator): Provides a default when the left side is null.

• ! (null assertion): Asserts that a value isn’t null (throws at runtime if you’re wrong).

Example:

int? fetchCount() => null;

void displayCount() {
  int count = fetchCount() ?? 0;
  print('Count: $count');

  // If you’re absolutely sure fetchCount() != null:
  // int certainCount = fetchCount()!;
}

Always prefer ?? and ?. over ! to preserve soundness and avoid runtime exceptions.

Sound Null Safety in Dart 3

Dart 3 enforces sound null safety, meaning:

• Nullability is part of the type system.

• The compiler guarantees that non-nullable types never receive null.

• Optimizations can assume no runtime null checks for non-nullable values.

With sound null safety:

• Cross-module calls respect null annotations; you can’t override non-nullable parameters with nullable arguments.

• Any code compiled with null safety is more performant and reliable.

Migrating to Null Safety

If you have existing Dart code, follow these steps to migrate:

1. Upgrade SDK constraints in your pubspec.yaml:

   environment:
     sdk: '>=2.12.0 <4.0.0'

2. Run the migration tool:

   dart migrate

   This tool suggests where to add ? or ! and inserts migration comments.

3. Review and test thoroughly. Address any analyzer warnings.

4. Opt in by removing the // @dart=2.9 flags and re-running your tests.

By migrating, you gain the full benefits of Dart’s type system and reduce possible runtime null errors.

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Conclusion

Understanding null safety in Dart 3 is crucial for writing stable, maintainable Flutter and server-side applications. By differentiating between nullable and non-nullable types, using null-aware operators, and leveraging sound null safety, you’ll eliminate a large category of runtime errors.