Monday, April 22, 2013

Unit Tests vs Parametrized Unit Tests

Unit Tests

Using the conventions of NUnit unit tests as test methods contained in test classes. A parameterless method decorated with a custom attribute like [TestMethod] is a test method. Usually, each unit test explores a particular aspect of the behavior of the class-under-test.

Here is a unit test written in C# that adds an element to a .NET ArrayList instance. The test first creates a new array list, where the parameter to the constructor is the initial capacity, then adds a new object to the array list, and finally checks that the addition was correctly performed by verifying that a subsequent index lookup operation returns the new object.

void TestAdd() 
 ArrayList a = new ArrayList(0); 
 object o = new object(); 
 Assert.IsTrue(a[0] == o); 

It is important to note that unit tests include a test oracle that compares observed behavior with expected results. By convention, the test oracle of a unit test is encoded using assertions. The test fails if any assertion fails or an exception is thrown. Unit test frame- works can also deal with expected exceptions. For a quick introduction about NUnit refer previous posts.

A test suite produced by unit testing with high code coverage gives confidence in the correctness of the tested code. However, writing unit tests to achieve high coverage can be time-consuming and tedious given that test execution frameworks only automate the test executions. You still have to write your test cases !

To address this problem, several automatic unit test generation tools such as Parasoft JTest or jCUTE can automatically generate conventional unit tests. These tools, nevertheless, cannot guarantee high code coverage, unless testers manually write some tests.


Parametrized Unit Tests

The unit test above specifies the behavior of the array list by example. Strictly speaking, this unit test only says that by adding a new object to an empty array list, this object becomes the first element of the list. What about other array lists and other objects?

void TestAdd(ArrayList a, object o) 
 int i = a.Count;
 Assert.IsTrue(a[i] == o);

By adding parameters we can turn a closed unit test into a universally quantified conditional axiom that must hold for all inputs under specified assumptions.

Adding parameters to a unit test improves its expressiveness as a specification of intended behavior, but we lose concrete test cases. We can no longer execute this test axiom by itself. We need actual parameters. But which values must be provided to ensure sufficient and comprehensive testing? Which values can be chosen at all?

In the ArrayList example, if we study the internal structure of the .NET Framework implementation, we observe that there are two cases of interest. One occurs when adding an element to an array list that already has enough room for the new element (i.e. the array list’s capacity is greater than the current number of elements in the array list). The other occurs when the internal capacity of the array list must be increased before adding the element.

If we assume that library methods invoked by the ArrayList implementation are themselves correctly implemented, we can deduce that running exactly two test cases is sufficient to guarantee that the parametrized TestAdd(...) succeeds for all array lists and all objects.

void TestAddNoOverflow() 
 TestAdd(new ArrayList(1), new object());

void TestAddWithOverflow() 
 TestAdd(new ArrayList(0), new object());

Splitting axioms and test cases in this way is a separation of concerns:

  • First, we describe expected behavior as parametrized unit tests.
  • Then we study the case distinctions made by the code paths of the program under test to determine which inputs make sense for testing.

So, parametrized unit tests are more general specifications than conventional unit tests because it specifies the behavior for the whole input classes other than for a single concrete value of the input. But it needs concrete parameter values to be executed.

The good news is, Given a parametrized unit test, a test-generation tool, such as Microsoft Pex, can automatically generate tests with concrete inputs for the parameters to achieve high coverage. Pex explores the behaviors of a parametrized unit test using a technique called dynamic symbolic execution. Dynamic Symbolic Execution (DSE) is a variation of symbolic execution, which systematically explores feasible paths of the program under test by running the program with different test inputs to achieve high structural coverage. It collects the symbolic constraints on inputs obtained from predicates (condition statements like if else, switch,…) in branch statements along the execution and relies on a constraint solver [like Zap or Simplify] to solve the constraints and generate new test input for exploring new path. For each set of concrete test input that leads to a new path that achieves new coverage, Pex generates a corresponding conventional unit test. So, all what you have to do is write your parametrized unit test.

In this post we gave a brief overview about parametrized unit tests and its related research areas. It’s a very important concept to understand before start learning Microsoft Pex, which we will start . In later posts, we will explore Microsoft Pex in detail.