Flutter: Test that a specific exception is thrown

Question

in short, throwsA(anything) does not suffice for me while unit testing in dart. How to I test for a specific error message or type?

Here is the error I would like to catch:

class MyCustErr implements Exception {
  String term;

  String errMsg() => 'You have already added a container with the id 
  $term. Duplicates are not allowed';

  MyCustErr({this.term});
}

here is the current assertion that passes, but would like to check for the error type above:

expect(() => operations.lookupOrderDetails(), throwsA(anything));

This is what I want to do:

expect(() => operations.lookupOrderDetails(), throwsA(MyCustErr));

You can check out this post for answers on how to match for specific messages as well: https://stackoverflow.com/questions/13298969/how-do-you-unittest-exceptions-in-dart — Almund, Aug 19 '19 at 00:25

score 76 · Accepted Answer · edited Jul 15 '21 at 07:55

76

This should do what you want:

expect(() => operations.lookupOrderDetails(), throwsA(isA<MyCustErr>()));

if you just want to check for exception check this answer:

edited Jul 15 '21 at 07:55

Ber

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answered Jan 17 '19 at 17:41

Günter Zöchbauer

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1

there's a `instanceOf ` matcher too. – Rémi Rousselet Jan 17 '19 at 17:41
Right, that's a shorthand – Günter Zöchbauer Jan 17 '19 at 17:42
2

That did not work for me. maybe due to my setup however: ```Expected: Actual: dynamic>``` removing the closure does not work either – xpeldev Jan 17 '19 at 19:06
throwsA method works fine. I tried the isInstanceOf first – xpeldev Jan 17 '19 at 19:08
2

nossir it did not for me but so long as I have an alternative. thank you! – xpeldev Jan 17 '19 at 19:59
3

As a note to future people, I wasn't able to get the first alternative to work. I discovered it was because VS Code Intellisense had me import `'package:flutter/{widgets-material-cupertino}.dart'` when you actually want to import `'package:matcher/matcher.dart'`. – Abion47 Sep 26 '19 at 22:51
@Abion47, removing import for wrong package `'package:flutter/{widgets-material-cupertino}.dart'` did the trick. Thanks! – Despotovic Oct 16 '19 at 15:31
2

The TypeMatcher<> is deprecated in Flutter 1.12.1 onwards. For me, this matcher works: `throwsA(isInstanceOf())` – Ber Jan 27 '20 at 08:52
4

Both `TypeMatcher` and `isInstanceOf` have been deprecated. Use `throwsA(isA())` instead. – rlat Jul 11 '20 at 21:27

Brett Sutton · Answer 2 · 2021-04-30T04:46:42.860

As of April 2021 this is the correct method.

CORRECT METHOD

import 'package:dcli/dcli.dart';
import 'package:test/test.dart';

 /// GOOD: works in all circumstances.
 expect(() => restoreFile(file), throwsA(isA<RestoreFileException>()));

Some examples show :

INCORRECT METHOD

import 'package:dcli/dcli.dart';
import 'package:test/test.dart';
 /// BAD: works but not in all circumstances
 expect(restoreFile(file), throwsA(isA<RestoreFileException>()));

Note the missing '() => ' after the expect.

The difference is that the first method will work for functions that return void whilst the second method won't.

So the first method should be the prefered technique.

To test for a specific error message:

CHECK CONTENTS OF EXCEPTION AS WELL

import 'package:dcli/dcli.dart';
import 'package:test/test.dart';

    expect(
        () => copy(from, to),
        throwsA(predicate((e) =>
            e is CopyException &&
            e.message == 'The from file ${truepath(from)} does not exists.')));

This certainly works but there's no logical reason why should one pass lambdas for it to work! This is straight-up unintuitive syntax comparing to something like JUnit. — Arrowsome, May 01 '21 at 18:41
@Arrow if the method being called returns null then you must use () otherwise you get a compile error: This expression has a type of 'void' so its value can't be used. ... I do have lints turned up to 11 which might be why you see a difference. — Brett Sutton, May 02 '21 at 23:29

score 17 · Answer 3 · answered Jan 27 '20 at 10:48

17

After `TypeMatcher<>' has been deprecated in Flutter 1.12.1 I found this to work:

expect(() => operations.lookupOrderDetails(), throwsA(isInstanceOf<MyCustErr>()));

answered Jan 27 '20 at 10:48

Ber

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`isInstanceOf` has been deprecated as well, in favor of `isA`. So the code should now be `expect(() => operations.lookupOrderDetails(), throwsA(isA()));` – rlat Jul 11 '20 at 21:24

score 2 · Answer 4 · answered Oct 09 '19 at 18:44

2

In case anyone wants to test with an async function like I had to do all you need to do is add async keyword in the expect, bearing in mind that the lookupOrderDetails is an async function:

expect(() **async** => **await** operations.lookupOrderDetails(), throwsA(const TypeMatcher<MyCustErr>()));

expect(() **async** => **await** operations.lookupOrderDetails(), isInstanceOf<MyCustErr>()));

It still uses Gunter's answer which is very good!

answered Oct 09 '19 at 18:44

Mihai Neagoe

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5

Not sure why, but neither of those variants worked for me, however `expect(() async => await resultFuture, throwsA(isInstanceOf()));` did – Matt R Oct 14 '19 at 10:50

score 1 · Answer 5 · answered Nov 14 '19 at 18:57

1

First import correct package 'package:matcher/matcher.dart';

expect(() => yourOperation.yourMethod(),
      throwsA(const TypeMatcher<YourException>()));

answered Nov 14 '19 at 18:57

Rajat Arora

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score 1 · Answer 6 · answered Feb 22 '21 at 08:32

1

The current proper way to expect that a function call throws an exception is:

expect(operations.lookupOrderDetails, throwsA(isA<MyCustErr>()));`

answered Feb 22 '21 at 08:32

Flutter: Test that a specific exception is thrown

6 Answers6

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