Dart - Unit test for exception in constructor

Question

I write some simple project in Dart (1.9.3) with unit tests using unittest library. I have a problem with checking if constructor throws an error. Here's the sample code I write for this issue purposes:

class MyAwesomeClass {
    String theKey;

    MyAwesomeClass();

    MyAwesomeClass.fromMap(Map someMap) {
        if (!someMap.containsKey('the_key')) {
            throw new Exception('Invalid object format');
        }

        theKey = someMap['the key'];
    }
}

and here are the unit tests:

test('when the object is in wrong format', () {
    Map objectMap = {};

    expect(new MyAwesomeClass.fromMap(objectMap), throws);
});

The problem is the test fails with following message:

Test failed: Caught Exception: Invalid object format

What do I do wrong? Is it a bug in unittest or should I test exceptions with try..catch and check if exception has been thrown?
Thanks for all!

Recommend reopening because it isn't immediately obvious from the "duplicate" topic how to apply it to constructors. — Suragch, Oct 09 '19 at 13:56

score 9 · Accepted Answer · answered Apr 21 '15 at 21:40

You can test if the Exception has been thrown using:

    test('when the object is in wrong format', () {
       Map objectMap = {};

       expect(() => new MyAwesomeClass.fromMap(objectMap), throws);
    });

passing as first argument an anonymous function raising the exception.

Dart - Unit test for exception in constructor

1 Answers1