Exceptions in Dart provide a powerful mechanism for managing errors and unexpected situations in your code. They allow you to gracefully handle runtime errors, maintaining program stability and improving user experience.

Understanding Dart Exceptions

An exception is an object representing an error or unexpected event that occurs during program execution. When an exception is thrown, it disrupts the normal flow of the program. Dart's exception handling system allows you to catch and respond to these errors.

Common Built-in Exceptions

Dart provides several built-in exception types:

Exception: A generic exception class
FormatException: Thrown when a string or some other data does not have an expected format
IOException: Related to input/output operations
StateError: Indicates that an object is in an invalid state

Throwing Exceptions

You can throw exceptions using the throw keyword. Here's a simple example:


void checkAge(int age) {
  if (age < 0) {
    throw ArgumentError('Age cannot be negative');
  }
}

Handling Exceptions with Try-Catch

To handle exceptions, use a try-catch block. This allows you to attempt potentially error-prone code and handle any exceptions that occur:


try {
  checkAge(-5);
} catch (e) {
  print('An error occurred: $e');
}

The Finally Clause

The finally clause executes code regardless of whether an exception was thrown or caught. It's useful for cleanup operations:


try {
  // Some risky operation
} catch (e) {
  print('Error: $e');
} finally {
  print('This always runs');
}

Custom Exceptions

You can create custom exceptions by extending the Exception class:


class InsufficientFundsException implements Exception {
  String errMsg() => 'Insufficient funds';
}

void withdraw(double amount) {
  if (amount > balance) {
    throw InsufficientFundsException();
  }
  // Proceed with withdrawal
}

Best Practices for Exception Handling

Only catch exceptions you can handle meaningfully
Avoid catching Exception or Error directly; be specific
Use rethrow to propagate exceptions you can't fully handle
Implement custom exceptions for domain-specific errors
Log exceptions for debugging and monitoring purposes

Conclusion

Effective exception handling is crucial for building robust Dart applications. By understanding and properly implementing exception handling techniques, you can create more reliable and user-friendly software. Remember to use async and await when dealing with asynchronous operations, as they have their own set of exception handling considerations.