Assertions

Assertions are central to unit testing in any of the xUnit frameworks, and NUnit is no exception. NUnit provides a rich set of assertions as static methods of the Assert class.

If an assertion fails, the method call does not return and an error is reported. If a test contains multiple assertions, any that follow the one that failed will not be executed. For this reason, it's usually best to try for one assertion per test.

Each method may be called without a message, with a simple text message or with a message and arguments. In the last case the message is formatted using the provided text and arguments.

Two Models

In NUnit 3.0, assertions are written primarily using the Assert.That method, which takes constraint objects as an argument. We call this the Constraint Model of assertions.

In earlier versions of NUnit, a separate method of the Assert class was used for each different assertion. This Classic Model is still supported but since no new features have been added to it for some time, the constraint-based model must be used in order to have full access to NUnit's capabilities.

For example, the following code must use the constraint model. There is no real classic equivalent.

int[] array = new int[] { 1, 2, 3 };
Assert.That(array, Has.Exactly(1).EqualTo(3));
Assert.That(array, Has.Exactly(2).GreaterThan(1));
Assert.That(array, Has.Exactly(3).LessThan(100));

Where equivalents do exist, the two approaches will always give the same result, because the methods of the classic approach have all been implemented internally using constraints. For example...

Assert.AreEqual(4, 2+2);
Assert.That(2+2, Is.EqualTo(4));