How to configure Ninject so that it would inject right instance depending on previously injected instance

Question

I can't find right words for my question so i will let my code speak instead.

I have Repository:

class Repository
{
    public Repository(DbContext ctx)
    {

    }
}

then i have this bindings:

Bind<IRepository>().To<Repository>();
Bind<DbContext>().To<UserStoreContext>().When...
Bind<DbContext>().To<CentralStoreContext>().When...

and then i have class that needs to access both db's

class Foo
{
    public Repository(IRepository userRepo, [CentralStoreAttribute]IRepository centralRepo)
    {

    }
}

How should i configure two DbContext bindings so that repositories with right contexts (based on CentralStoreAttribute) would be injected into Foo constructor?

Gilleod

I realized that my approach wont work because when i request IRepository in request context, i get same one (first) no matter if I ask for one with different DbContext. So i revert all my code to use Paul Equis suggested way. – Gilleod

Answer 1

Rather than relying on attributes in the right places, I usually create several types that are effectively just aliases. This is useful since with Ninject (and presumably other IoC containers) we're asking for dependencies by their type-name.

So if you need to be able to "request" a user repository vs a central one, I would create types that alias it like this:

interface IRepository {  /* methods and properties */ }
interface IUserRepository : IRepository {}
interface ICentralRepository : IRepository {}

class Foo
{
   public Foo(IUserRepository userRepo, ICentralRepository centralRepo)
   {
      // assign to fields
   }
}

I prefer this because then Ninject doesn't bleed into my app at all, it's more declarative, and I think it's simpler to remember than any convention based attribute approach like the one you're trying.

Brittani

@Paul Equis thats good approach, but still if applied to my sample code, how to inject right DbContext ? – Gilleod

@Paul Equis this is what you sugest if i understood correctly Bind<IRepository>().To<Repository>(); Bind<ICentralRepository>().To<Repository>(); Bind<DbContext>().To<UserStoreContext>().When... Bind<DbContext>().To<CentralStoreContext>().When... that still dosent solve problem of injecting right DbContext – Gilleod

Apply the same approach to the DbContext. Make one called IUserStoreContext and one called ICentralStoreContext and the repositories request the one they need. – Brittani

@Paul Equis and how my repository class then should look? DbContext is not my type, it's framework's. – Gilleod

Is DbContext sealed? If not, forget the interfaces and just inherit from it. Your repository constructor would look like this: UserRepository(UserStoreContext context) { /* assign stuff */ } – Brittani

@Paul Equis i don't need 2 implementations of repositories for each context, i need only one, and if possible i don't want to couple it with any custom context derived from DbContext, i want "generic" repository implementation that coupled only with DbContext – Gilleod

@Paul Equis I was able to use you suggested concept. I created 2 interfaces as you suggested, also added conditions to DbContext bindings like .When(r => r.ParentRequest != null && r.ParentRequest.Service.Name == typeof(ICentralStoreRepository).Name) – Gilleod

Answer 2

I tried this in a proof of concept but eventually went in a different direction.

Bind<IRepository>().ToMethod(x =>
{
  var repositoryType = x.Kernel
                .Get<IConfigObject>()
                .SomeStringPropertyDenotingTheRepository;

  switch (repositoryType )
  {
    case "1": return (IRepository)new Repository1();
    default: return (IRepository)new Repository2();
  }
}).InRequestScope();

While it worked, I never figured out if it was using my singleton instance of IObjectB or instantiating a new instance - should be pretty easy to figure out though. I figured it was calling ToMethod every time I used DI on IRepository - again not verified.

Denims

Answer 3

Use the When(Func<IRequest, bool> condition) overload to check recursivly if r.Target.IsDefined(typeof(TAttribute), false) is true for the given request or one of its anchestors r.ParentRequest

Gage

this works for constructor injections. But I also have places where i'm getting instance directly from kernel, like kernel.Get<IRepository>(...) what should i provide to Get method to get Repository with CentralStoreContext ? – Gilleod

@qrow: The answer is pretty simple. Do not use Ninject as service locator and you do not have this problem. – Gage

you mean stop getting instances directly from kernel? If so then i can't, i'm using framework that creates my types instead of me, so it needs parameter-less constructor. I don't know any other way then to get instances from kernel in that parameter-less constructor. – Gilleod

Answer 4

Bind<IRepository>().To<Repository>();
Bind<DbContext>().To<CentralStoreContext>()
    .When( context => context.Target != null 
        && context.Target.GetCustomAttributes( typeof( CentralStoreAttribute ) ) != null );

// make the general binding after the more specific one
Bind<DbContext>().To<UserStoreContext>();

Haircloth

this seems like it wont work? first GetCustomAttributes takes 2 parameters, and returns object[], but event if i change it to context.Target.GetCustomAttributes(typeof(CentralStoreAttribute), false).Any() it still looks for attribute at target that is DbContext but I specified attribute on IRepository – Gilleod

your example probably doing same thing as .WhenTargetHas() what i need is something like .WhenParentTargetHas() – Gilleod

sorry, I have an extension method that does exactly that, and was typing from memory. mine looks like: context.Target.HasAttribute<CentralStoreAttribute>() – Haircloth