Should I have a separate assembly for interfaces?

Question

We currently have quite a few classes in a project, and each of those classes implement an interface, mostly for DI reasons.

Now, my personal feeling is that these interfaces should be put into a separate namespace within the same assembly (so we have a MyCompany.CoolApp.DataAccess assembly, and within that there's an Interfaces namespace giving MyCompany.CoolApp.DataAccess.Interfaces).

However, somebody has suggested that these interfaces should actually be in their own assembly. And my question is - are they right? I can see that there are some benefits (eg. other projects will only need to consume the interface assembly), but at the end of they day all of these assemblies are going to need to be loaded. It also seems to me that there could be a slightly more complex deployment issue, as Visual Studio will not automatically pull the implementing assembly into the target's bin folder.

Are there best practice guidelines for this?

EDIT:

To make my point a little clearer: We already separate UI, DataAccess, DataModel and other things into different assemblies. We can also currently swap out our implementation with a different implementation without any pain, as we map the implementing class to the interface using Unity (IOC framework). I should point out that we never write two implementations of the same interface, except for reasons of polymorphism and creating mocks for unit testing. So we don't currently "swap out" an implementation except in unit tests.

The only downside I see of having the interface in the same assembly as the implementation is that the whole assembly (including the unused implementation) will have been loaded.

I can, however, see the point that having them in a different assembly means that developers won't accidentally "new" the implementing class rather than have it created using the IOC wrapper.

One point I haven't understood from the answers is the deployment issue. If I am just depending on the interface assemblies, I'll have a something like the following structure:

MyCompany.MyApplication.WebUI
    References:
        MyCompany.MyApplication.Controllers.Interfaces
        MyCompany.MyApplication.Bindings.Interfaces
        etc...

When I build this, the assemblies that are automatically put into the bin folder are just those interface assemblies. However, my type mappings in unity map different interfaces to their actual implementations. How do the assemblies that contain my implementations end up in the bin folder?

Answer 1

The usual expected^? practice is to place them in their own assembly, because then a given project consuming those interfaces doesn't require a hard reference to the implementation of those interfaces. In theory it means you can swap out the implementation with little or no pain.

That said, I can't remember when I last did this, to @David_001's point this isn't necessarily "usual". We tend to have our interfaces in-line with an implementation, our most common use for the interfaces being testing.

I think there are different stances to take depending on what you are producing. I tend to produce LOB applications, which need to interoperate internally with other applications and teams, so there are some stakeholders to the public API of any given app. However, this is not as extreme as producing a library or framework for many unknown clients, where the public API suddenly becomes more important.

In a deployment scenario, if you changed the implementation you could in theory just deploy that single DLL - thus leaving, say, the UI and interface DLLs alone. If you compiled your interfaces and implementation together, you might then need to redeploy the UI DLL...

Another benefit is a clean segregation of your code - having an interfaces (or shared library) DLL explicitly states to any on the development team where to place new types etc. I'm no longer counting this as a benefit as we haven't had any issues not doing it this way, the public contract is still easily found regardless of where the interfaces are placed.

I don't know if there are best practices for or against, the important thing arguably is that in code, you are always consuming the interfaces and never letting any code leak into using the implementation.