Exceptions and Errors

Failing clearly

Dart can throw any non-null object, but by convention you throw objects implementing Exception for conditions callers may reasonably handle (a FormatException from parsing, a custom PaymentDeclinedException) and subclasses of Error for programming bugs that should not be caught in normal code (ArgumentError, StateError, RangeError, TypeError, UnimplementedError). Handle exceptions with try followed by on Type clauses (match by type), catch (e, stackTrace) to access the object and stack trace, and finally for cleanup. rethrow rethrows the current exception while preserving the stack trace. Dart has no checked exceptions, so document what a function may throw. Validate arguments early with ArgumentError.checkNotNull or by throwing ArgumentError.value(...). In asynchronous code, errors travel through Futures and Streams and are caught with try/catch around await. Some codebases prefer returning result types (a sealed class with success and failure subclasses) for expected failures, combining nicely with Dart 3 pattern matching.

Custom exceptions, on clauses and rethrow

Specific handlers first; programming errors are not swallowed.

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

String charge(double amount, String token) {
  if (amount <= 0) throw ArgumentError.value(amount, 'amount', 'must be positive');   // a bug in the caller
  if (token.startsWith('blocked')) throw PaymentDeclinedException('Card blocked by issuer');
  return 'pay_${DateTime.now().millisecondsSinceEpoch}';
}

void main() {
  try {
    final id = charge(499, 'blocked_tok');
    print('Paid $id');
  } on PaymentDeclinedException catch (e) {
    print('Declined: ${e.reason}');
  } on FormatException {
    print('Bad input');
  } catch (e, stackTrace) {
    print('Unexpected: $e');
    rethrow;                                   // keep the original stack trace
  } finally {
    print('Payment attempt finished');
  }
}

Do not catch Error subclasses routinely

Catching Error (such as RangeError or StateError) hides bugs that should be fixed. Catch specific Exception types you can handle and let programming errors reach a top-level handler and crash reports.

त्वरित जाँच: Which keyword rethrows the current exception while preserving its stack trace?

• throw
• finally
• rethrow
• on