Exceptions

Your Dart code can throw and catch exceptions. Exceptions are errors indicating that something unexpected happened.

throw

To manually throw or raise an exception use throw statement.

throw FormatException('Expected at least 1 section');

You can also throw arbitrary objects:

throw "Some Error";

When you throw an exception, the program is terminated unless it is handled by try and catch block.

try-catch

Catching, or capturing, an exception stops the exception from propagating. Catching an exception gives you a chance to handle it:

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  buyMoreLlamas();
}

The following code shows how to specify multiple catch statement when you want to handle different specific type of exceptions:

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  // A specific exception
  buyMoreLlamas();
} on Exception catch (e) {
  // Anything else that is an exception
  print('Unknown exception: $e');
} catch (e) {
  // No specified type, handles all
  print('Something really unknown: $e');
}

You can use either on or catch or both. Use on when you need to specify the exception type. Use catch when your exception handler needs the exception object.

You can specify one or two parameters to catch(). The first is the exception that was thrown, and the second is the stack trace.

try {
  // ···
} on Exception catch (e) {
  print('Exception details:\n $e');
} catch (e, s) {
  print('Exception details:\n $e');
  print('Stack trace:\n $s');
}

finally

To ensure that some code runs whether or not an exception is thrown, use a finally clause. If no catch clause matches the exception, the exception is propagated after the finally clause runs:

try {
  breedMoreLlamas();
} finally {
  // Always clean up, even if an exception is thrown.
  cleanLlamaStalls();
}

The finally clause runs after any matching catch clauses:

try {
  breedMoreLlamas();
} catch (e) {
  print('Error: $e'); // Handle the exception first.
} finally {
  cleanLlamaStalls(); // Then clean up.
}