Static Constant Field

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Syntax

class ClassName {
  static const Type fieldName = compileTimeExpression;
}

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Technical Characteristics

• Compile-Time Evaluation: The right-hand side of the assignment must be a valid constant expression. This includes literals, arithmetic operations on other constants, or invocations of const constructors. It cannot rely on runtime execution (e.g., DateTime.now() or method returns).
• Class-Bound Access: Because the field is static, it is accessed strictly through the class name (ClassName.fieldName). Attempting to access it through an instance of the class will result in a compilation error.
• Deep Immutability: The const modifier enforces transitive immutability. If the static constant is a collection (such as a List, Set, or Map) or an object instantiated via a const constructor, all nested elements and fields are permanently frozen.
• Canonicalization: Dart’s compiler optimizes const values. If multiple static constants or variables across the application evaluate to the identical compile-time state, they point to the exact same memory reference.
• Mandatory Static Modifier: In Dart, an instance field cannot be const. Any field declared as const within a class must explicitly include the static modifier.
• Inline Initialization: A static constant must be initialized at the point of declaration. It cannot be initialized later in a constructor, an initializer list, or a static block.

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Code Demonstration

class SystemMetrics {
  // Primitive static constant
  static const int maxThreads = 16;

  // Computed static constant (valid because operands are constants)
  static const int threadPoolLimit = maxThreads * 2;

  // Collection static constant (deeply immutable)
  static const List<String> coreModules = ['auth', 'database', 'network'];
  
  // Object static constant (requires a const constructor)
  static const Point origin = Point(0, 0);
}

class Point {
  final int x;
  final int y;
  
  // Const constructor required for static const instantiation
  const Point(this.x, this.y); 
}

void main() {
  // Valid access via class name
  print(SystemMetrics.maxThreads);
  
  // Invalid: SystemMetrics().maxThreads (Cannot access static member via instance)
  // Invalid: SystemMetrics.maxThreads = 32 (Cannot reassign a const variable)
  // Invalid: SystemMetrics.coreModules.add('ui') (Cannot mutate a const collection)
}

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Type Inference

class Config {
  // Type explicitly defined
  static const double timeout = 5.5; 
  
  // Type inferred as double
  static const retryDelay = 2.0; 
}