Dependency Injection and Circular reference

Question

I am just starting out with DI & unit testing and have hit a snag which I am sure is a no brainer for those more experienced devs :

I have a class called MessageManager which receives data and saves it to a db. Within the same assembly (project in Visual Studio) I have created a repository interface with all the methods needed to access the db. The concrete implementation of this interface is in a separate assembly called DataAccess.

So DataAccess needs a project reference to MessageManager to know about the repository interface. And MessageManager needs a project reference to DataAccess so that the client of MessageManager can inject a concrete implementation of the repository interface. This is of courser not allowed

I could move the interface into the data access assembly but I believe the repository interface is meant to reside in the same assembly as the client that uses it

So what have I done wrong?

Answer 1

You should separate your interface out of either assembly. Putting the interface along with the consumer or the implementor defeats the purpose of having the interface.

The purpose of the interface is to allow you to inject any object that implements that interface, whether or not it's the same assembly that your DataAccess object belongs to. On the other hand you need to allow MessageManager to consume that interface without the need to consume any concrete implementation.

Put your interface in another project, and problem is solved.