Interfaces separated from the class implementation in separate projects? [closed]

I would distinguish between interfaces like this:

1. Standalone interfaces whose purpose you can describe without talking about the rest of your project. Put these in a single dedicated “interface assembly”, which is probably referenced by all other assemblies in your project. Typical examples: ILogger, IFileSystem, IServiceLocator.

2. Class coupled interfaces which really only make sense in the context of your project’s classes. Put these in the same assembly as the classes they are coupled to.

   An example: suppose your domain model has a Banana class. If you retrieve bananas through a IBananaRepository interface, then that interface is tightly coupled to bananas. It is impossible to implement or use the interface without knowing something about bananas. Therefore it is only logical that the interface resides in the same assembly as Banana.

   The previous example has a technical coupling, but the coupling might just be a logical one. For example, a IFecesThrowingTarget interface may only make sense as a collaborator of the Monkey class even if the interface declaration has no technical link to Monkey.

My answer does depend on the notion that it’s okay to have some coupling to classes. Hiding everything behind an interface would be a mistake. Sometimes it’s okay to just “new up” a class, instead of injecting it or creating it via a factory.