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Asked: 2026-05-10T14:38:46+00:00

I am just starting out with DI & unit testing and have hit a

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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?

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1 Answer

  1. Are you using an Inversion of Control Container? If so, the answer is simple.

    Assembly A contains:

    • MessageManager
    • IRepository
    • ContainerA (add MessageManager)

    Assembly B contains (and ref’s AssemblyA):

    • Repository implements IRepository
    • ContainerB extends ContainerA (add Repository)

    Assembly C (or B) would start the app/ask the container for MessageManager which would know how to resolve MessageManager and the IRepository.

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