Cookbooks
Dart
Chapter 5. Good Coding Practices
Handling errors

Dart:
Handling errors

How to:

Dart supports two types of errors: compile-time errors and run-time errors. Compile-time errors are detected by the Dart analyzer before the code runs, whereas run-time errors, or exceptions, occur during execution. Here’s how you handle exceptions in Dart:

Try-Catch

Use try-catch to capture exceptions and prevent them from crashing your application:

try {
  var result = 100 ~/ 0; // Attempting division by zero, throws an exception
} catch (e) {
  print('Caught an exception: $e'); // Handles the exception
}

Sample output: Caught an exception: IntegerDivisionByZeroException

Specific Exception

To handle specific exceptions, mention the exception after catch:

try {
  var result = 100 ~/ 0;
} on IntegerDivisionByZeroException {
  print('Cannot divide by zero.'); // Specifically handles dividing by zero exceptions
}

Sample output: Cannot divide by zero.

Stack Trace

To get a stack trace for debugging, use a second parameter in the catch block:

try {
  var result = 100 ~/ 0;
} catch (e, s) {
  print('Exception: $e');
  print('Stack trace: $s'); // Prints stack trace for debugging
}

Finally

Use finally to execute code after try/catch, regardless of whether an exception was thrown:

try {
  var result = 100 ~/ 0;
} catch (e) {
  print('Caught an exception: $e');
} finally {
  print('This is always executed.'); // Cleanup code or final steps
}

Sample output:

Caught an exception: IntegerDivisionByZeroException
This is always executed.

Third-Party Libraries

Although Dart’s core library is robust for error handling, you can also use third-party packages like dartz for functional programming which introduces concepts like Either and Option that can be used for error handling. Here’s an example using dartz for error handling:

  1. Add dartz to your pubspec.yaml file under dependencies:
dependencies:
  dartz: ^0.10.0
  1. Use Either for handling errors gracefully in your Dart code:
import 'package:dartz/dartz.dart';

Either<String, int> divide(int dividend, int divisor) {
  if (divisor == 0) {
    return Left('Cannot divide by zero.');
  } else {
    return Right(dividend ~/ divisor);
  }
}

void main() {
  final result = divide(100, 0);
  result.fold(
    (left) => print('Error: $left'), 
    (right) => print('Result: $right')
  );
}

Sample output: Error: Cannot divide by zero.

The Left part usually represents the error, and the Right part represents success. This pattern allows handling errors in a more functional way, offering clarity and control over error management.

Logging