When are interfaces needed?

Question

(In the context of .NET for what its worth)

I tend to not use inheritance and rarely use interfaces. I came across someone who thinks interfaces are the best thing

Answer 1

One of the most important reasons for using an interface is the fact that they allow us to write unit tests for our classes and pass in our own dependencies. By putting the dependency behind an interface we open up "Seams" in our application code, where unit tests can easily be written. I don't see this test angle mentioned in many of the answers, but it is very important to understand that without interfaces, these dependencies (such as a web service, or a file system reference) can become very hard to test or at best are quite conditional. I have written a post here: http://jeffronay.com/why-use-an-interface/ that goes into much more detail with code examples showing a Service class without an interface, and then the same class re-written using an interface to demonstrate the testing flexibility when using interfaces.