How do you unittest exceptions in Dart?

Question

Consider a function that does some exception handling based on the arguments passed:

List range(start, stop) {
    i         


        

Answer 1

In this case, there are various ways to test the exception. To simply test that an unspecific exception is raised:

expect(() => range(5, 5), throwsException);

to test that the right type of exception is raised:

there are several predefined matchers for general purposes like throwsArgumentError, throwsRangeError, throwsUnsupportedError, etc.. for types for which no predefined matcher exists, you can use TypeMatcher.

expect(() => range(5, 2), throwsA(TypeMatcher()));

to ensure that no exception is raised:

expect(() => range(5, 10), returnsNormally);

to test the exception type and exception message:

expect(() => range(5, 3), 
    throwsA(predicate((e) => e is ArgumentError && e.message == 'start must be less than stop')));

here is another way to do this:

expect(() => range(5, 3), 
  throwsA(allOf(isArgumentError, predicate((e) => e.message == 'start must be less than stop'))));

(Thanks to Graham Wheeler at Google for the last 2 solutions).