Self-testability in unit testing

An often-proposed approach to decrease costs of software development projects is to reuse existing software units. Reusing software units has indeed the potential to decrease costs, but this decrease of costs has to be put in relation to the risks inherent in third-party software. In particular, third-party software units can have a high inherent complexity complicating testing, even worse, source code and other information required for testing might not be available at all. In such cases, self-testability, if the software unit has been augmented appropriately, can be a method for ensuring that the software unit reused fits into the system to be developed with respect to its quality.