Where to put a common interface for a solution to increase decoupling?
<p>I have a solution that has an MVC3 web front end.  I have a separate assembly for view models.  I have a Services assembly that contains all domain objects and business functionality and business validation.  My Service also provides all view models to my web site. So the website is aware of the Service and Model assemblies. The Service is aware of the Models assembly only, and the Model assembly isn't aware of either.  </p>

<p>So I have a case where I need the Service to validate and process a Cart model. The items in the cart implement an interface, that I would like defined in the Service assembly. My problem is that in order for my Cart Model to use this interface, it needs to know about the Service assembly.  If I define the interface in my Models assembly, I've now coupled these two assemblies together in a way I don't want.</p>

<p>So I decided I could decouple these by adding another assembly that contains the interface, and each would reference it.  So, now they would both be tightly coupled to this assembly, but they are not coupled directly too each other.  </p>

<p>Is this 'Interface only' assembly a viable solution or should I be doing things differently?</p>
