Question

These 2-3 last years, many projects I see, like Cuyahoga open source C# CMS, tends to define persistent and non persistent classes as Interface. Why? Is there a good reason? TDD? Mocking? A design pattern? …

Answer 1

The main reason is that this makes techniques like dependency injection easier. This in turn allows for more flexibility in the software and easier reuse and recombination of existing code. Examples for where this is useful include the various forms of unit testing (as you mentioned), but also most other forms of "regular" code reuse.

A simple example:

Say you have a method that calculates emplyoee salaries. As part of its signature, it accepts an object that calculates their benefits, say an instance of BenefitCalculator:

calculateSalary(... BenefitCalculator bc, ...)

Originally, your design has only one class BenefitCalculator. But later, it turns out that you need more than one class, e.g. because different parts of the software are supposed to use different algorithms (maybe to support different countries, or because the algorithm is supposed to be user-configurable…). In that case, rather than bloat the existing implementation of BenefitCalculator, it makes sense to create new class(es), e.g. BenefitCalculatorFrance, or BenefitCalculatorSimple etc.

Now if you use the signature

calculateSalary(... BenefitCalculator bc, ...)

, you are kind of screwed, because you cannot supply different implementations. If however you use

calculateSalary(… IBenefitCalculator
bc, …)

you can just have all classes implement the interface.

This is actually just a special case of "loose coupling": Demand as little as possible from other parts of the code. In this case, don’t demand a certain class; instead just demand that certain methods exist, which is just what an Interface does.