C# – NUnit 3.0 and Assert.Throws

c++nunitnunit-3.0unit-testing

I am writing some unit tests with NUnit 3.0 and, unlike v2.x, ExpectedException() has been removed from the library.

Based on this answer, I can definitely see the logic in trying to catch specifically where in the test one expects their system to throw an exception (rather than just saying 'anywhere in the test').

However, I tend to be very explicit about my Arrange, Act, and Assert steps and this makes it a challenge.

I used to do something like:

[Test, ExpectedException(typeof(FormatException))]
public void Should_not_convert_from_prinergy_date_time_sample1()
{
    //Arrange
    string testDate = "20121123120122";

    //Act
    testDate.FromPrinergyDateTime();

    //Assert
    Assert.Fail("FromPrinergyDateTime should throw an exception parsing invalid input.");
}

Now I need to do something like:

[Test]
public void Should_not_convert_from_prinergy_date_time_sample2()
{
    //Arrange
    string testDate = "20121123120122";

    //Act/Assert
    Assert.Throws<FormatException>(() => testDate.FromPrinergyDateTime());
}

This isn't terrible, but muddies the Act and Assert, in my opinion. (Obviously, for this simple test, it's not hard to follow, but might be more challenging in larger tests).

I've had a colleague suggest I get rid of Assert.Throws altogether and just do something like:

[Test]
public void Should_not_convert_from_prinergy_date_time_sample3()
{
    //Arrange
    int exceptions = 0;
    string testDate = "20121123120122";

    //Act
    try
    {
        testDate.FromPrinergyDateTime();
    }
    catch (FormatException) { exceptions++;}

    //Assert
    Assert.AreEqual(1, exceptions);
}

Here, I stick with the strict AAA format, but at the expense of even more bloat.

So my question goes out to AAA-style testers: How would you do some sort of exception validation testing like I am trying to do here?

Best Solution

I see where you're coming from, even though I don't mind combining Act/Assert steps in this case.

The only thing I can think of is to store the actual delegate (here to FromPrinergyDateTime) into a variable as the "act" step and then assert it:

[Test]
public void Should_not_convert_from_prinergy_date_time_sample2()
{
    //Arrange
    string testDate = "20121123120122";

    //Act
    ActualValueDelegate<object> testDelegate = () => testDate.FromPrinergyDateTime();

    //Assert
    Assert.That(testDelegate, Throws.TypeOf<FormatException>());
}

I get that the "act" step isn't really acting, but rather defining what the action is. However, it does clearly delineate what action is being tested.