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

Interfaces make the most sense to me in the context of dependency injection and IoC frameworks. I like the idea that you can define a set of behaviors (methods) and "guarantee" those behaviors through the implementation of an interface. Now you can plug whole new functions into an existing system with a single assembly and a config file update.

Effective design of interfaces does require a good deal of forward planning, and I find they are most useful in the context of large systems and frameworks. When they're useful they're really useful. A few of my favorite:

• IComparable (YOU decide how your objects compare against each other)
• IQueryable (LINQ anyone?)
• IDisposable (keep your using statement handy)