Dart Default Parameter

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Syntax and Mechanics

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Named Optional Parameters

void configureNetwork({String protocol = 'https', int timeout = 3000}) {
  print('$protocol, $timeout');
}

// Invocation
configureNetwork(); // Uses 'https' and 3000
configureNetwork(timeout: 5000); // Uses 'https' and 5000

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Positional Optional Parameters

void executeProcess(String command, [int exitCode = 0, bool force = false]) {
  print('$command, $exitCode, $force');
}

// Invocation
executeProcess('terminate'); // Uses 0 and false
executeProcess('terminate', 1); // Uses 1 and false

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Technical Constraints

1. Compile-Time Constants: Default values must be compile-time constants. You cannot use variables, instance properties, or function calls evaluated at runtime as default values.
   // INVALID: DateTime.now() is evaluated at runtime
   void logEvent({DateTime timestamp = DateTime.now()}) {} 
   
   // VALID: 'default_event' is a string literal (compile-time constant)
   void logEvent({String eventName = 'default_event'}) {}
   
2. Implicit Null Default: If an optional parameter is declared without a default value, its implicit default is null. Consequently, the parameter’s type signature must be explicitly nullable (using the ? suffix) to satisfy Dart’s sound null safety.
   // 'port' defaults to null, so it must be typed as int?
   void initializeServer({int? port}) {}
   
3. Collection Defaults: When providing a default value that is a collection (like a List, Set, or Map), the collection literal must be explicitly marked as const to satisfy the compile-time constant requirement.
   void applyFilters({List<String> filters = const ['all', 'active']}) {}
   
4. Mutual Exclusivity with required: A named parameter cannot simultaneously possess a default value and the required modifier. The required keyword mandates that the caller provides an argument, rendering a fallback default value logically dead code.
   // INVALID: Cannot be both required and have a default
   void setThreshold({required int level = 10}) {}