Nullable types

Learn String? and nullable types.

Nullable types

In Dart 3 a type is non-nullable by default. String name; requires name to hold a value. To allow null, append ?: String? name;.

Now name is either a String or null. The question mark says 'this variable may be empty'. Assigning null to a non-nullable type is a compile error.

Nullable types describe 'maybe a value, maybe not' cases. A form field may be left blank, so its type becomes String?. The compiler then warns you about the null risk.

Assigning null

You can directly assign null to a nullable variable: String? name = null;. Leaving it unassigned also defaults to null: String? name; equals String? name = null;.

Be careful when using a nullable variable. Calling name.length throws if name is null. That is why null-aware operators and null checks matter; later lessons cover them.

Once you assign a non-null value, it is safe to use: name = 'Ayşe'; print(name.length);. Still, the compiler may warn about the nullable type; ! or a null check then helps.

Exercise Declare nullable and assign

Declare a String? name (starting as null). Then assign name = 'Ayşe' and print it with print(name).

Show example solution

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

Test yourself

Match the types

String

Text, never null
String?

Text, may be null
int

Number, never null
int?

Number, may be null
Which is nullable?
- String?
- int
- String
- double
Show answer

String? — String? is a nullable type.

Nullable declaration

void main() {
  String? ____;
  print(name);
}

Show answer

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

Next lesson: Null-aware operators

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