Topic: Unit Testing - Limitations of unit testing

Very infomatic. Testing cannot be expected to catch every error in the program - it is impossible to evaluate all execution paths for all but the most trivial programs. The same is true for unit testing. Additionally, by definition unit testing only tests the functionality of the units themselves. Therefore it will not catch integration errors, or broader system level errors (such as functions performed across multiple units, or non-functional test areas such as performance). Unit testing is more effective if it is used in conjunction with other software testing activities. Like all forms of software testing, unit tests can only show the presence of errors; it cannot show the absence of errors.

Software testing is a combinatorial problem. For example, every boolean decision statement requires at least two tests: one with an outcome of "true" and one with an outcome of "false". As a result, for every line of code written, programmers often need 3 to 5 lines of test code. This obviously takes time and its investment may not be worth the effort. There are also many problems that cannot easily be tested at all— for example those that are nondeterministic. In addition, writing code for a unit test is as likely to be buggy as the code it is testing. Fred Brooks in The Mythical Man-Month quotes never take two chronometers to sea. Always take one or three. Meaning, if two chronometers contradict, how do you know which one is correct?

To obtain the intended benefits from unit testing, a rigorous sense of discipline is needed throughout the software development process. It is essential to keep careful records not only of the tests that have been performed, but also of all changes that have been made to the source code of this or any other unit in the software. Use of a version control system is essential. If a later version of the unit fails a particular test that it had previously passed, the version-control software can provide a list of the source code changes (if any) that have been applied to the unit since that time.

It is also essential to implement a sustainable process for ensuring that test case failures are reviewed daily and addressed immediately. If such a process is not implemented and ingrained into the team's workflow, the application will evolve out of sync with the unit test suite ― increasing false positives and reducing the effectiveness of the test suite.

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