2 Why We Need Model-Based Testing

26 Why We Need Model-Based Testing
methods must be public void methods with no parameters, and must be labeled
with the [Test] attribute. We have also labeled these with the optional [Category("Convert")]
attribute so we can command the test runner to run just these
tests.
Each test method tests a run, a sample of program behavior consisting of a
sequence of method calls and other statements. Convert32 tests a very brief run
comprising a single method call. Its body is a single line:
Assert.AreEqual(Temperature.ConvertToCelsius(32), 0);
We happen to know that 32 ◦ F (the freezing point of water) corresponds to 0 ◦ C;
this test method checks that the ConvertToCelsius method computes the conversion
correctly for this case. The assertion here acts as the oracle, the authority that
provides the correct result to determine whether the test passed or failed.
A test method can make several method calls and check several assertions, so
it can test a run that is longer than a single method call. Our ConvertMany method
checks a few other well-known conversions. Its body is several lines:
Assert.AreEqual(Temperature.ConvertToCelsius(32), 0);
Assert.AreEqual(Temperature.ConvertToCelsius(98.6), 37);
Assert.AreEqual(Temperature.ConvertToCelsius(212), 100);
Assert.AreEqual(Temperature.ConvertToCelsius(212), 101); // Fail!
If any assertion in a test method fails, the test runner considers that test a failure.
Here we have deliberately put an incorrect assertion at the end of the test, in order to
demonstrate how NUnit indicates test failures. This is the cause of the test failure we
observed in the previous section. It also makes the point that a test failure does not
necessarily indicate that the IUT is defective. It might be the test that is defective!
The ConvertToCelsius method is an ideal subject for unit testing because it
gets all the information it needs from its argument (the temperature in Fahrenheit)
and provides all of the information it computes in its return value (the temperature
in Celsius). It uses no stored information, and computes no new information that
must be stored from one method call to the next. The same input always produces
the same output, so each method call is independent of all the others; the history
of previous calls doesn’t matter. Stored information is called state, so a method
with these properties is said to be state-independent. It behaves like a mathematical
function, so it is also said to be functional.
State-independent methods are ideal subjects for unit testing. Their behavior
does not depend on context, so the tests can be very short. Moreover, each test is
conclusive in this sense: in any context, when invoked with the same arguments, the
method will always behave exactly as it did in the test. The only problem in testing a
state-independent method is choosing arguments that achieve good coverage of the
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