Null safety

Learn null and nullable types (?) in Dart.

Null safety

Dart 3 has null safety on by default. A variable cannot be null unless you explicitly say so. This catches unexpected null errors at compile time.

You make a type nullable by adding ? at the end: String? means 'a String or null'. For example String? middleName; can be null.

With this system you get a guarantee: 'this variable is never null'. Nullable types require a null check before use; otherwise the compiler warns you.

Exercise Nullable variable

Declare a nullable name variable with String? name = null; and print it with print(name);.

Show example solution

void main() {
  String? name = null;
  print(name);
}

What is null safety for?

The biggest benefit of null safety is that it catches null errors at compile time. In older languages you could not know if a variable was null until you ran the code; Dart tells you at compile time.

This reduces famous errors like 'null pointer exception'. Before using a nullable variable you check with if (x != null) or the ? operator; we will see this in a later lesson.

The result: fewer surprises, safer code. With Dart 3 this is the default; it cannot be turned off.

Test yourself

Null safety concepts

String

cannot be null
String?

can be null
null

no value
?

nullable marker

Null value

String? email = ____;

Show answer

String? email = null;

What is the difference between String and String?
- They are the same
- String? can be null
- String is faster
- String? only in classes
Show answer

String? can be null — The correct answer: String? can be null. String can never be null.

Next lesson: Summary - Variables

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