Testing for exceptions with [TestCase] attribute in NUnit 3?

Question

How do I test for exceptions in a TestCase with NUnit3?

Let's say I have a method Divide(a,b) defined as follows:

public double Divide(double a, double b) { if(Math.Abs(b) < double.Epsilon) throw new ArgumentException("Divider cannot be 0"); return a/b; }

I want to test this method using NUnit 3.0 test cases, so maybe I have:

[TestCase(-10, 2, -5)] [TestCase(-1, 2, -0.5)] public void TestDivide(double a, double b, double result) { Assert.That(_uut.Divide(a, b), Is.EqualTo(result)); }

Is there a way to specify a Test Case that will cause Divide() to throw an ArgumentException and somehow have this as the expected result, e.g. something along the lines of:

[TestCase(-10, 2, -5)] [TestCase(-1, 2, -0.5)] [TestCase(-1, 0, ExpectedResult = TypeOf(ArgumentException)] public void TestDivide(double a, double b, double result) { Assert.That(_uut.Divide(a, b), Is.EqualTo(result)); }

(Of course I could define a separate test method and use Assert.Throws() in this, so this is purely out of curiosity)

Chris · Accepted Answer · 2016-08-09 09:10:09Z

ExpectedException would have been the correct method for NUnit 2.X, but it was removed from NUnit 3.

There's a various snippets of discussion in the NUnit Google Group and the equivalent Dev group - but it looks like the decision was made that it's generally a better design pattern to test expected outcomes, and exceptions in separate methods. (link)

The only way to do this in NUnit 3, would be to break it down in to two separate tests. (Confirmed in a similar question answered by the NUnit core team, here.)

[TestCase(-10, 2, -5)] [TestCase(-1, 2, -0.5)] public void TestDivide(double a, double b, double result) { Assert.That(_uut.Divide(a, b), Is.EqualTo(result)); } [TestCase(-1, 0)] public void TestDivideThrows(double a, double b) { Assert.That(() => _uut.Divide(a, b), Throws.TypeOf<ArgumentException>()); }

If you really wanted to force the two into a single test case, you could add a boolean throwsException argument and test it. However, the presence of an if - then - else and two entirely different expectations would kind of prove that this really should be two different tests.
"generally a better design pattern to test expected outcomes, and exceptions in separate methods" - I can't update nunit framework on my code without refactoring 1000+ tests because of a "generally better design pattern". That's hardly a nice way of justifying breaking changes

Oliamster · Accepted Answer · 2020-12-01 13:48:47Z

I agree with the Corpus Gigantus's answer but why don't you simplify thing like this:

[TestCase(-10, 2, -5)] [TestCase(-1, 2, -0.5)] [TestCase(-1, 0, null, typeof(ArgumentException))] public void TestDivide(double a, double b, double result, Type exception = null) { if(exception != null) { Assert.Throws(exception, delegate { _uut.Divide(a, b); }); } else Assert.That(_uut.Divide(a, b), Is.EqualTo(result)); }

In this way, the expected exception is part of the test parameters as well, note that you can only have two outcome, either the division succeeds or throws.

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