Question

I have a feeling this should be a common requirement, but I can’t work the proper way of doing it. I currently have a standard MVC3 site which is using Ninject to inject service classes (in singleton scope) from Project A into the constructor of the controllers – this is all working fine.

I have another class library – Project B – which requires classes from Project A. What I am wanting to do is inject the same singleton instance I use between in my MVC project within the Project B classes. Is this possible?

Currently in the global.asax, I have this for setting up the bindings.

private void SetupDependencyInjection()
        {
            // Create Ninject DI kernel
            IKernel kernel = new StandardKernel();

            kernel.Bind<IRepositoryA>().As<RepositoryA>().InSingletonScope();
            // A load more binding go here...

            // Tell ASP.NET MVC 3 to use our Ninject DI Container
            DependencyResolver.SetResolver(new NinjectResolver(kernel));
        }

Within my controller, I have something like

public ExampleController(IRepositoryA iRepositoryA, more params....)
        {
            this.iRepositoryA= iRepositoryA;
            var ProjectB.Class1 = new ProjectB.Class1(this.iRepositoryA);
            // more setup of params here....
        }

I have two classes in ProjectB which look like this

public class Class1
{
 public Class1(IRepositoryA iRepositoryA, more params...)
 {
  var class2 = new Class2(iRepositoryA, more params...);
 }
}

public class Class2
{
  public Class2(IRepositoryA iRepositoryA, more params...)
  {
   // Something goes here....
  }
}

What I am looking to do is instantiate a new instance of ProjectB.Class1 without having to pass iClass (plus potentially a load more) as a parameter. I believe I can expose IKernel from the global asax and then do something like iKernel.Get(). Is this the best way of doing this? The other problem I see is that the parameters injected into the controller may go down 3 or more levels, e.g. like example above, but contining further that just to Class2. Is it best in this instance to just keep passing the parameters down the chain?

Answer 1

You controller needs a Class1 instance, not an IRepositoryA, so the solution is to require a Class1 instance in your controller’s constructor:

public ExampleController(Class1 class1) {
    this.class1 = class1;
}

// Let Ninject provide these dependencies!
public Class1(IRepositoryA repositoryA, Dependency2 dependency2) {
    this.repositoyA = classB;
    this.dependency2 = dependency2;
}

(And see Dependency Injection Myth: Reference Passing)