How does the null safety system work in Dart?

Short answer

A non-nullable type such as String does not accept null, and when a value is optional we write String? and check it before use.

Explanation

Null safety lets the compiler catch most null errors before running. Use ? for a nullable type, and convert to a non-nullable type after a clear condition. The ! operator forces the compiler to trust you, but it throws a runtime error if the value really is null.

Code example

String? nickname;
if (nickname != null) {
  print(nickname.length);
}

Common mistakes

• Using ! as a permanent fix instead of checking the value or designing the type more clearly.
• Treating String? as equal to String; the former needs explicit null handling.

Follow-up questions

• When do we use late in Dart and what is its risk?

Sources

• Dart language — Sound null safety