Error & Exception Handling: try, catch, on, finally & rethrow

1Concept

Dart uses try, on <Type> catch (e, stackTrace) to catch specific exception types, and finally for guaranteed execution. rethrow propagates exceptions up the call chain.

2Architecture Diagram

try {
    throw FormatException('Invalid JSON');
} on FormatException catch (e) {
    // Specific catch handler
} catch (e, stack) {
    // Catch-all with stack trace
} finally {
    // Always executes
}

3Code Example

class AuthException implements Exception {
  final String message;
  AuthException(this.message);
  @override
  String toString() => 'AuthException: $message';
}

void login(String username) {
  if (username.isEmpty) {
    throw AuthException('Username cannot be empty.');
  }
  print('Logged in as $username');
}

void main() {
  try {
    login('');
  } on AuthException catch (e) {
    print('[Handled] $e');
  } catch (e, stack) {
    print('[Unexpected] $e');
  } finally {
    print('Auth sequence completed.');
  }
}

4Expected Output

[Handled] AuthException: Username cannot be empty.
Auth sequence completed.

5Key Takeaways

• ✓Use 'on SpecificType catch (e)' to handle specific exception types.
• ✓stackTrace captures full call stack history.
• ✓Finally blocks always execute even if exceptions are thrown.