c# when to program to an interface?

Question

Ok the great thing about programming to an interface is that it allows you to interchange specific classes as long as the new classes implement everything in that interface.

e.g. i program my dataSource object to an interface so i can change it between an xml reader and a sql database reader.

does this mean ideally every class should be programmed to an interface? when is it not a good idea to use an interface?

Answer 1

When the YAGNI principle applies.

Interfaces are great but it's up to you to decide when the extra time it takes developing one is going to pay off. I've used interfaces plenty of times but there are far more situations where they are completely unnecessary.

Answer 2

Not every class needs to be flexibly interchanged with some other class. Your system design should identify the points where modules might be interchangeable, and use interfaces accordingly. It would be silly to pair every class with an additional interface file if there's no chance of that class ever being part of some functional group.

Every interface you add to your project adds complexity to the codebase. When you deal with interfaces, discoverability of how the program works is harder, because it's not always clear which IComponent is filling in for the job when consumer code is dealing with the interface explicitly.