Flutter: Test that a specific exception is thrown

user_9893693 0
Viewed 24136

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));

flutter-test dart flutter
6 Answers

This should do what you want:

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

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

user_2174080

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.')));
user_89949980

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

expect(() => operations.lookupOrderDetails(), throwsA(isInstanceOf<MyCustErr>()));
Ber_115270

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

expect(() => yourOperation.yourMethod(),
      throwsA(const TypeMatcher<YourException>()));
user_123204850

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

expect(operations.lookupOrderDetails, throwsA(isA<MyCustErr>()));`
Admin1

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!

user_44239340
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