Improved Testability Leads to Better Design
"Economic Testability" is a simple concept but not often seen in practice. Essentially it boils down to recognising that since tested code is a requirement and that we have some nice, economical, standards for testing in place (such as xUnit, Fit, etc), then the products that we build should always satisfy the requirement of ease-of-test in addition to any other requirements. Happily this requirement does not contradict, but reinforces best practice. Perhaps a few examples will help?
It is often the case that tests are impeded because certain major functions are not yet implemented. For example,an object needs to be tested but it makes use of a yet-to-be-built object is a pretty common occurrence. A way around this is to allow the provision of the yet-to-be-built object via a parameterized constructor. Then, in the test-case situation we can provide a dummy object without changing the internals at all; in the production model we can provide the real (tested) object to deliver the required functionality. When implemented via the constructor, as recommended, in the set-up region of the tests, the actual method signatures are not changed at all.
We can go further of course .. a lot further. If we wanted to introduce some kind of logging-trail for complex bug diagnosis, then we can provide the logging logic via the technique outlined above, by providing the logging object in the constructor parameters. At production time, using the null-object pattern, we can replace the logger with a harmless alternative. On the other hand, why not make this dynamically selectable? Then we could have a production system which can gracefully slip into logging mode. It may be unfortunate that you should need to do this, but if you do, then you can look like a hero, i.e., a professional!
Finally, if you build your systems so that testing is made simple, either by following the suggestion here, or by just making the interfaces very simple to use in the xUnit world, and simultaneously of course preserving simplicity in the production model, then our experience has been that the interfaces that you finally end up with are superior in terms of encapsulation and portability than if you had built you system with just one target environment in mind. In other words, the discipline in building an interface which is equally at home in two environments causes you to do a better job in the design-essentials than otherwise. It really is a win-win situation.