Step 1 - Simple custom type

class ValidationException implements Exception {
  final String field;
  final String message;

  ValidationException(this.field, this.message);

  @override
  String toString() => 'ValidationException($field): $message';
}

void main() {
  try {
    throw ValidationException('email', 'missing @');
  } on ValidationException catch (e) {
    print(e.field);
    print(e.message);
  }
}

Step 2 - Extending built-in errors

class OutOfStockError extends StateError {
  OutOfStockError(String sku) : super('out of stock: $sku');
}

• Error vs Exception in Dart is conventional: Exception for expected failures, Error for programming mistakes. Many apps blur the line; stay consistent in your project.

Step 3 - Carrying a cause

class SaveFailed implements Exception {
  final Object cause;
  SaveFailed(this.cause);
}

void main() {
  try {
    throw SaveFailed(ValidationException('id', 'empty'));
  } on SaveFailed catch (e) {
    print(e.cause);
  }
}

Practice tasks

• Add AuthException with reason enum { badPassword, locked, network }.
• Catch ValidationException specifically, then a generic catch as last resort in one try (document why the generic catch exists).