Unity and Object Creation

Question

I am using unity as my IoC container. I am trying to implement a type of IProviderRepository. The concrete implementation has a constructor that accepts a type of IRepository. When I remove the constructor parameter from the concrete implementation everything works fine. I am sure the container is wired correctly. When I try to create the concrete object with the constructor I receive the following error:

"The current build operation (build key Build Key[EMRGen.Infrastructure.Data.IRepository1[EMRGen.Model.Provider.Provider], null]) failed: The current type, EMRGen.Infrastructure.Data.IRepository1[EMRGen.Model.Provider.Provider], is an interface and cannot be constructed. Are you missing a type mapping? (Strategy type BuildPlanStrategy, index 3)".

Is it possible to achieve the above mention functionality with Unity? Namely have Unity infer a concrete type from the Interface and also inject the constructor of the concrete type with the appropriate concrete object based on constructor parameters. Below is sample of my types defined in Unity and a skeleton class listing for what I want to achieve. IProviderRepository is implemented by ProviderRepository which has a constructor that expects a type of IRepository.

 <typeAlias alias="ProviderRepositoryInterface" type="EMRGen.Model.Provider.IProviderRepository, EMRGen.Model" />
 <typeAlias alias="ProviderRepositoryConcrete" type="EMRGen.Infrastructure.Repositories.Providers.ProviderRepository, EMRGen.Infrastructure.Repositories" />
 <typeAlias alias="ProviderGenericRepositoryInterface" type="EMRGen.Infrastructure.Data.IRepository`1[[EMRGen.Model.Provider.IProvider, EMRGen.Model]], EMRGen.Infrastructure" />
 <typeAlias alias="ProviderGenericRepositoryConcrete" type="EMRGen.Infrastructure.Repositories.EntityFramework.ApplicationRepository`1[[EMRGen.Model.Provider.Provider, EMRGen.Model]], EMRGen.Infrastructure.Repositories" />
 <!-- Provider Mapping-->
 <typeAlias alias="ProviderInterface" type="EMRGen.Model.Provider.IProvider, EMRGen.Model" />
 <typeAlias alias="ProviderConcrete" type="EMRGen.Model.Provider.Doctor, EMRGen.Model" />

Illustrate the call being made inside my class:

public class PrescriptionService
{
     PrescriptionService()
     {
         IUnityContainer uc = UnitySingleton.Instance.Container;
         UnityServiceLocator unityServiceLocator = new UnityServiceLocator(uc);
         ServiceLocator.SetLocatorProvider(() => unityServiceLocator);
         IProviderRepository pRepository =
             ServiceLocator.Current.GetInstance<IProviderRepository>();
     }
}

public class GenericRepository<IProvider> : IRepository<IProvider>
{
}

public class ProviderRepository :  IProviderRepository
{
    private IRepository<IProvider> _genericProviderRepository;

    //Explict public default constructor
    public ProviderRepository(IRepository<IProvider> genericProviderRepository) 
    {
        _genericProviderRepository = genericProviderRepository;
    }
}

Answer 1

What you want to do is possible, but you need to tell Unity how to map from interfaces to concrete types. AFAICT, your current configuration registers a lot of types, but doesn't specify how they relate to each other.

That said, static Service Locator is an anti-pattern. Consider changing your code to use proper Constructor Injection instead. That would also simplify your code considerably:

public class PrescriptionService
{
    private readonly IProviderRepository pRepository;

    public PrescriptionService(IProviderRepository pRepository)
    {
        if (pRepository == null)
        {
            throw new ArgumentNullException("pRepository");
        }

        this.pRepository = pRepository;
    }
}

Using Unity, you would be able to wire it up like this:

var container = new UnityContainer();
container.RegisterType<PrescriptionService>();
container.RegisterType<IProviderRepository, ProviderRepository>();
container.RegisterType<IRepository<IProvider>, MyRepository<IProvider>>();

var ps = container.Resolve<PrescriptionService>();

Configure the container and resolve all components in the application's Composition Root.

You should only use XML configuration if you need to be able to change certain components without recompiling your application.