What is null safety in Dart?

Null safety is a system in Dart that helps prevent runtime errors related to null dereferencing. The compiler analyzes the code and determines whether a variable can hold a null value. If a variable is declared as non-nullable, the compiler guarantees that it will never be assigned null.

Main aspects of null safety:

• Implicit non-nullable type: By default, types do not allow null (non-nullable).

  // int cannot be null
  int count = 0;
  

• Nullable type: To explicitly indicate that a variable can be null, a question mark ? is used:

  // String? can be null
  String? name;
  

• ! operator (non-null assertion): Used to assert that a nullable expression is not null. Use with caution, as it throws an exception if null.

  String? nullableName = 'Flutter';
  // Using !, knowing that nullableName is not null in this context
  print(nullableName!.length);
  

• ? operator (null-aware operators): Used for safe access to members or method calls on nullable expressions.
  • ?. (null-aware access):

    String? nullableString;
    // If nullableString is not null, length is called; otherwise, result is null
    int? length = nullableString?.length;
    

  • ?? (if null):

    String? serverName;
    // If serverName is not null, its value is used; otherwise, 'Guest'
    String displayName = serverName ?? 'Guest';
    

  • ??= (if null assign):

    String? cache;
    // If cache is null, assign 'Loading...'
    cache ??= 'Loading...';
    

• Flow Analysis: Dart compiler analyzes the code and determines when a nullable variable is guaranteed not to be null in a certain part of the code, allowing it to be used as non-nullable.

  String? greeting = getGreeting();
  // The compiler knows that greeting is not null after this check
  if (greeting != null) {
    print(greeting.toUpperCase()); // Safe to use toUpperCase()
  }
  

Advantages of null safety:

• Error reduction: Prevents NullReferenceException (or similar) at runtime.
• Improved readability: Clearly indicates whether a variable can be null.
• Increased performance: The compiler can perform optimizations knowing which variables are guaranteed not to be null.

Null safety was introduced in Dart 2.12.