How to Create Custom Exception Classes in Dart?

In Dart, you can create custom exception classes by implementing or extending the Exception or Error classes. Generally, for exceptions that developers expect to handle through programmatic control logic, use Exception; for internal program errors, use Error.

Here are the steps to create a custom exception class:

1. Define a class: Implement the Exception interface or directly inherit from it.
2. Add a constructor: Typically, include a constructor that takes an error message.
3. Override the toString method: This allows for more descriptive error messages.

Here is an example demonstrating how to define a custom exception class named CustomException:

dart
class CustomException implements Exception {
  final String message;

  CustomException(this.message);

  @override
  String toString() => "CustomException: $message";
}

You can use this custom exception as follows:

dart
void someFunction() {
  throw CustomException('This is a custom error');
}

void main() {
  try {
    someFunction();
  } catch (e) {
    print(e);
  }
}

When someFunction is called, it throws CustomException, which is then caught and printed in the main function.