What are the ?? double question marks in Dart?

Question

The following line of code has two question marks:

final myStringList = prefs.getStringList(\'my_string_list_key\') ?? [];

What is the mean

Answer 1

The ?? double question mark operator means "if null". Take the following expression, for example.

String a = b ?? 'hello';

This means a equals b, but if b is null then a equals 'hello'.

Another related operator is ??=. For example:

b ??= 'hello';

This means if b is null then set it equal to hello. Otherwise, don't change it.

Reference

• A Tour of the Dart Language: Operators
• Null-aware operators in Dart

Terms

The Dart 1.12 release news collectively referred to the following as null-aware operators:

• ?? -- if null operator
• ??= -- null-aware assignment
• x?.p -- null-aware access
• x?.m() -- null-aware method invocation