Dependency Injection in a 3 layer asp.net mvc application

Question

I have a 3 layer application and the layers are:

• Web: Presentation Layer (ASP.NET MVC) --> only sees BLL
• BLL: Business Logic Layer --> only sees DAL
• DAL: Data Access Layer

So the Web layer doesn't know anything about my DAL layer. I have repository interfaces and concrete classes in my DAL, which are used in BLL layer in business logic classes. The question is, in order to decouple DAL and BLL, how do I setup Ninject to inject my repository implementations to the BLL layer?

The same question is for Web layer and BLL layer, I have interfaces and implementations on BLL which I use them in Web layer, how should I setup Niject for this?

Answer 1

The idea is that you define interfaces for your DAL and BLL. You then take an instance of such an interface as a constructor parameter. Example

interface IDatabase
{
    // Methods here
}

Your BLL class:

public class Bll
{
    IDatabase _db;
    public Bll(IDatabase db)
    {
        _db = db;
    }

    public void SomeMethod()
    {
        // Use db here
    }
}

Then in your composition root (in the web application) you use the kernel to configure these dependencies:

 kernel.Bind<IDatabase>().To<ConcreteDatabase();

You need the same thing from your controllers to your BLL, but it works the same way.

Apart from that, I think your dependencies are not correctly set up. In general you don't want these vertical dependencies. You should aim for a flatter hierarchy. I wrote a blog post about this: http://www.kenneth-truyers.net/2013/05/12/the-n-layer-myth-and-basic-dependency-injection/

In my blog post I explain what the problem is with such a hierarchy and how you can avoid it. Apart from that, it describes exactly your problem: ASP.NET MVC, BLL, DLL and Ninject to tie it together.