Question

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How will I know when to create an interface?

Hi guys,

This will sound a bit thick, I guess, but I am battling to understand the reason to use interfaces. People keep saying that they are ‘contracts’ for classes. But, why use them? If I was a single developer, on an application, that I knew no one would ever work on (I know – not a common example, but I am just trying to understand), would I use Interfaces? They seem to just duplicate work. It seem I define what a class must implement, and then go an implement it. I’m doing it twice – why?

Please note: I am not in anyway saying they’re useless… I’m just tying to find out why, in projects I work on, they define an IClass, and then based on that, define the class which they use…

Sorry if it’s very basic… Just hoping someone can help me out.

Answer 1

I use interfaces because it makes my code a lot more modular. Using interfaces in combination with an inversion of control container (http://code.google.com/p/autofac/) will allow you to swap in various implementations of an interface easily.

Also, interfaces are easier to unit test.

Those are just a couple good reasons; really, there are more. But those are strong enough to make me want to use interfaces.