Where to Define Interfaces

Hi,

We are bombarded by advice always to program to an interface, rather than an implementation, to reduce dependencies. I can see the benefits of this, and within an individual project, I use this technique to decouple classes wherever appropriate. But, what happens when you want to decouple two separate projects?

Imagine you have two separate libraries, LibA and LibB. You don't want either library to 'know' about the other, but, when you use them in a client application, you want LibA to be able to call a function in LibB. When I came across this problem, I thought, 'Oh, I'll use an interface,' but then I came upon the problem of where to define this.

If I put it in LibA, then, for LibB to be able to implement it, it becomes dependent upon LibA. The client application could define it, but then I would have to do a copy-paste of the code into any other applications that needed the same functionality.

There is a third option - creating the interface in a separate project/assembly of its own. This does the job, but seems like using a hammer to crack a nut.

Can anyone offer any advice on this? Am I stuck in the wrong mode of thinking? Should I be looking at something completely different? All advice gratefully received.

Kind wishes ~ Patrick

Tags: C#, General