How can I test void methods?

Question

I have some void methods and I need to test them, but I'm not sure about how to do it. I just know how to test methods that return something, using Assert. Someone knows how to do it? Do you guys know some links with exercices in this style?

Answer 1

You can test two things:

• State changes after void method call (state-based testing)
• Interaction with dependencies during void method call (interaction testing)

First approach is simple (NUnit sample):

var sut = new Sut();
sut.Excercise(foo);
Assert.That(sut.State, Is.EqualTo(expectedState)); // verify sut state

Second approach requires mocks (Moq sample):

var dependencyMock = new Mock<IDependency>();
dependencyMock.Setup(d => d.Something(bar)); // setup interaction
var sut = new Sut(dependencyMock.Object);
sut.Excercise(foo);
dependencyMock.VerifyAll(); // verify sut interacted with dependency

Well, you also can test if appropriate exceptions are thrown.