Exception Templates

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When handling a program exception, the two main concerns are in what context did it occur and what type of exception occured.

The library exception_templates provides parameterized classes that allow throwing and catching exceptions characterized by their type argument.

Using parameterized exceptions eliminates the need to define library or class specific exceptions and enables filtering exceptions based on their type argument.

To highlight the exception context use:

To emphasise the exception type use:


To use this library include exception_templates as dependency in your pubspec.yaml file.

The program below demonstrates how to throw and catch objects of type ExceptionOf<T> where T is a generic type. Colour output can be globally enabled or disabled by setting the static field colorOutput to ColorOutput.ON or ColorOutput.OFF, respectively,

import 'package:exception_templates/exception_templates.dart';

// Defining error/exception types:
class FailedToSerializeObject extends ErrorType{}
class InvalidDataFound extends ExceptionType{}

// Test class A.
class A {
  Object data;
  // Throwing a parameterized exception.
  void setUp() {
    data = List<int>.filled(3, 37);
    throw ExceptionOf<A>(
        message: 'Set up failed.',
        expectedState: 'A complete set of adapters.',
        invalidState: 'Adapter x078 is missing.');

  // Throwing a parameterized error.
  void tearDown() {
    throw ErrorOfType<FailedToSerializeObject>(
        message: 'An error occured in tearDown().',
        expectedState: 'All objects are serialized and stored.',
        invalidState: 'Failed to serialize object $data.');

// Test class B.
class B {
  final int data;

final a = A();

main(List<String> args) {
  // Catching an ExceptionOf<A>.
  try {
  } on ExceptionOf<A> catch (e) {

  // Turning off colour output globally.
  ExceptionOf.colorOutput = ColorOutput.OFF;

  // Catching an ExceptionOfType<InvalidDataFound>.
  // Colour output is switched off.
  try {
    throw ExceptionOfType<InvalidDataFound>(
        message: 'Something went wrong with class B.',
        expectedState: 'data > 0.',
        invalidState: 'data == null');
  } on ExceptionOfType catch (e) {
    print('// Switching of colour output.');
  } on ExceptionOf<A> catch (e) {
    print('Should not reach here! $e');

  // Throwing and catching an ErrorOfType<FailedToSerializeObject>.
  try {
  } on ErrorOfType<FailedToSerializeObject> catch (e) {
    print('${CYAN}DEMO only, errors should never be caught!');

A typical output produced when running the program above is shown below: Console Output


A copy of the program show in the section above can be found in the folder example.

Features and bugs

Please file feature requests and bugs at the issue tracker.


Provides generic exception and error templates. [...]