c# Abstract Class implementing an Interface

Question

I\'ve seen the following code layout reading forums and other blog posts and adapted in order to ask a few questions.

public interface IService
{
    i         


        

Answer 1

Ad 1: The additional abstract base class allows you to evolve the interface without breaking the implementation. Supposed there was no abstract base class, and you'd extend the interface, let's say by adding a new method. Then your implementation was broken, because your class does not implement the interface any longer.

Using an additional abstract base class you can separate this: If you add a new method to the interface, you can provide a virtual implementation in the base class and all your sub-classes can stay the same, and can be adopted to match the new interface at a later point in time.

Moreover, this combination allows you to define a contract (using the interface) and provide some default mechanisms (using the abstract base class). Anyone who's fine with the defaults can inherit from the abstract base class. Anyone who wants super-duper fine control about any little detail can implement the interface manually.

Ad 2: From a technical point of view there is no NEED to implement the interface in the final class. But this, again, allows you to evolve things separately from each other. A CarService is for sure a Service<Car>, but maybe it's even more. Maybe only a CarService needs some additional stuff that should not go into the common interface nor into the service base class.

I guess that's why ;-)