Question

Does anyone know if it’s possible to use Ninject to resolve any unresolved abstract dependencies outside of the instantiation process? I’ve just been looking into constructor injection vs property/method/field injection, but it looks to me as though Ninject is still expecting to be the creator of the type using the IKernel.Get<>() method.

Basically, we’re using MVC3 to build our product, and we’ve come up against a situation where we want the default ModelBinder to map form values to an instance of the object, and then be able to call a method on the submitted ViewModel that is dependent on an abstract interface e.g.

public class InviteFriend {
    [Required]
    public string EmailAddress { get; set; }

    public void Execute() {
        var user = IUserRepository.GetUser(this.EmailAddress);

        if (user == null) {
               IUserRepository.SaveInvite(this.EmailAddress);
        }

        MailMessage toSend = new MailMessage(); // Obviously some logic to prepare the body, subject and other mail properties
        SmtpClient.Send(toSend);
    }
}

where the controller action would receive InviteFriend as the method argument. We want Ninject to be able to resolve that IUserRepository dependency, but I can’t quite work out how to since the object itself is instantiated by the MVC ModelBinder rather than Ninject IKernel.Get<>().

Maybe the solution is a Ninject-based ModelBinder, or does that seem a really bad idea?

EDIT TO ADD: After the comments below, I realise that my hastily mocked-up code sample doesn’t really reflect what we’re facing. I’ve updated the code sample to reflect that the logic for InviteFriend.Execute() is more complex than just calling a method on one repository. Potentially, this is logic representing a discrete task that could co-ordinate interactions between multiple different domain objects and multiple repositories. The repositories are defined abstractly, and ideally would be resolved by Ninject.

Answer 1

Thank you for all your comments, but I’ve subsequently found the information I was looking for.

The answer is that it is possible to inject dependencies post-instantiation with Ninject. The solution is as follows:

public class InviteFriend {
    [Inject]
    public IUserRepository UserRepo { get; set; }

    [Required]
    public string EmailAddress { get; set; }

    public void Execute() {
        var user = UserRepo.GetUser(this.EmailAddress);

        if (user == null) {
               UserRepo.SaveInvite(this.EmailAddress);
        }

        MailMessage toSend = new MailMessage(); // Obviously some logic to prepare the body, subject and other mail properties
        SmtpClient.Send(toSend);
    }
}

With client code then using the Ninject kernel as follows:

IKernel container = new StandardKernel(new ModuleWithMyBindings());
container.Inject(instanceOfInviteFriend);

The code itself is a bit more sophisticated than that i.e. I’m not instantiating a new IKernel each time I need it.

I realise that this is architecturally less pure than some of the suggestions put forward in comments, but in the spirit of YAGNI, this is good enough for now and we can always refactor later on with some of the good suggestions in Daniel’s answer. However, this was a question about the capabilities of Ninject rather than an architectural review question, and this is what I consider the answer to my own question 🙂