The instance member can't be accessed in an initializer

Problem Statement

When working with Dart classes, you may encounter the error "The instance member 'X' can't be accessed in an initializer." This typically occurs when you try to use one instance variable to initialize another variable directly in the class body. Here's a common example:

class LevelUp extends GetxController {
  Map<String, String> params = Get.arguments;
  
  // Error: The instance member 'params' can't be accessed in an initializer
  var myTest = params['comLevel']; 
}

This error happens because Dart's initialization order requires that instance variables be initialized before they can be referenced by other instance variables. The compiler cannot guarantee that params will be initialized before myTest tries to access it.

Solutions

There are several approaches to resolve this issue, each with different use cases and considerations.

1. Using the late keyword (Recommended)

With Dart 2.12 and null safety, the late keyword allows you to defer initialization until the variable is first used:

class LevelUp extends GetxController {
  Map<String, String> params = Get.arguments;
  
  // Late initialization - no error
  late var myTest = params['comLevel'];
}

TIP

The late keyword is generally the cleanest solution for modern Dart code as it clearly communicates that initialization is deferred.

2. Constructor initialization

Move the initialization to the class constructor:

class LevelUp extends GetxController {
  Map<String, String> params = Get.arguments;
  late String myTest;
  
  LevelUp() {
    myTest = params['comLevel'];
  }
}

3. Build method initialization (For Flutter widgets)

For variables that depend on widget properties, initialize them in the build method:

class LevelUp extends GetxController {
  Map<String, String> params = Get.arguments;
  
  Widget build(BuildContext context) {
    var myTest = params['comLevel'];
    // Use myTest in your widget tree
  }
}

4. Getter method

Use a getter to compute the value on demand:

class LevelUp extends GetxController {
  Map<String, String> params = Get.arguments;
  
  String get myTest => params['comLevel'];
}

5. Static variables (Limited use case)

If the variable doesn't need to be instance-specific, you can make it static:

class LevelUp extends GetxController {
  static Map<String, String> params = Get.arguments;
  
  // Now works because params is static
  var myTest = params['comLevel'];
}

WARNING

Use static variables only when appropriate, as they are shared across all instances of the class.

Common Scenarios and Solutions

StatefulWidget initialization

When working with StatefulWidgets, you might encounter this error when trying to access widget.property:

class _CategoryScrollViewState extends State<CategoryScrollView> {
  // Error: The instance member 'widget' can't be accessed in an initializer
  int selectedCategory = widget.defaultSelection;
  
  @override
  Widget build(BuildContext context) { ... }
}

This should be resolved using initState():

class _CategoryScrollViewState extends State<CategoryScrollView> {
  int selectedCategory;
  
  @override
  void initState() {
    selectedCategory = widget.defaultSelection;
    super.initState();
  }
  
  @override
  Widget build(BuildContext context) { ... }
}

Complex object initialization

When you need to initialize one object with another:

// Wrong approach - causes error
final A _a = A();
final B _b = B(_a); // Error

// Correct approach
final A _a = A();
late final B _b;

MyClass() {
  _b = B(_a);
}

Best Practices

1. Prefer late variables for simple deferred initialization scenarios
2. Use constructors for complex initialization logic
3. Utilize getters for values that need to be computed on demand
4. Initialize in initState() for StatefulWidget properties
5. Avoid static variables unless you specifically need class-level sharing

Underlying Principle

Dart's initialization process follows a specific order:

1. Initialize superclass fields
2. Initialize superclass constructor
3. Initialize subclass fields
4. Initialize subclass constructor

This order means that instance fields cannot reference each other during initialization because there's no guarantee about which field will be initialized first. The solutions above work around this limitation by deferring initialization until the object is fully constructed.

By understanding these patterns, you can write cleaner, more maintainable Dart code that avoids initialization errors.