Summary - Advanced error handling

Combine hierarchy, Result and async errors to write a robust service.

Summary - Advanced error handling

In this group you learned advanced error handling: Exception hierarchy, Error vs Exception, Stack trace, Result pattern and async error handling.

Summary: custom exceptions by extending Exception; not catching Error; StackTrace with catch (e, s); type-safe errors with sealed Result; Future errors with async/await.

The right tool: Exception (or Result) for expected conditions, Error fixes for programmer mistakes.

A combined example

A function combining everything: AppException extends Exception, Future<int> fetchCount() async { throw AppException('timeout'); }.

The caller: try { await fetchCount(); } on AppException catch (e) { print(e.message); }. Here async error + custom exception + message access come together.

This pattern is common in real apps: a network call throws a custom exception, caught with async/await.

Exercise Combined error handling

Define an AppException class (extends Exception, final String message + constructor). Write Future<int> fetchCount() async { throw AppException('timeout'); }. Make main async: call it with try { await fetchCount(); } on AppException catch (e) { print(e.message); }.

Show example solution

class AppException extends Exception {
  final String message;
  AppException(this.message);
}

Future<int> fetchCount() async {
  throw AppException('timeout');
}

void main() async {
  try {
    await fetchCount();
  } on AppException catch (e) {
    print(e.message);
  }
}

Test yourself

Which is part of the Result pattern?
- Error and Exception
- try and catch
- throw and rethrow
- Success and Failure
Show answer

Success and Failure — Result consists of Success and Failure subclasses.
Error handling summary

extends Exception

Custom exception
catch (e, s)

StackTrace
Result<T>

Type-safe
await + catch

Async error

The rethrow keyword

try { risky(); } catch (e) { ____; }

Show answer

try { risky(); } catch (e) { rethrow; }

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