Should a dependency be injected many "levels" up than it is needed?

Question

I'm writing a C# ASP.NET MVC web application using SOLID principles.

I've written a ViewModelService, which depends on a AccountService and a RepositoryService, so I've injected those two services in the the ViewModelServer.

The PermissionService depends on the HttpContextBase in order to use GetOwinContext() to get an instance of the UserManager. The controller has an instance of HttpContextBase that needs to be used - so it seems like I have to inject the HttpContextBase instance into the ViewModelService which then injects it into the PermissionService.

So, in terms of code I have:

public ViewModelService

public CategoryRepository(ApplicationDbContext context, IPermissionService permissionservice)

public AccountService(HttpContextBase httpcontext, IPrincipal securityprincipal)

to instantiate the ViewModelService, I then do this:

new ViewModelService(
    new CategoryRepository(
            new ApplicationDbContext(), 
            new PermissionService(
                new AccountService(HttpContext, Thread.CurrentPrincipal),
                new UserPasswordRepository(new ApplicationDbContext()),
                new ApplicationSettingsService())),
    new PasswordRepository(
            new ApplicationDbContext(), 
            new PermissionService(
                new AccountService(HttpContext, Thread.CurrentPrincipal), 
                new UserPasswordRepository(new ApplicationDbContext()),
                new ApplicationSettingsService())),
    new ModelValidatorService());

Should a dependency be injected from that many "levels" up, or is there a better way?

Answer 1

There's a balance to be struck.

On the one hand, you have the school of thought which would insist that all dependencies must be exposed by the class to be "properly" injected. (This is the school of thought which considers something like a Service Locator to be an anti-pattern.) There's merit to this, but taken to an extreme you find yourself where you are now. Just the right kind of complexity in some composite models, which themselves have composite models, results in aggregate roots which need tons of dependencies injected solely to satisfy dependencies of deeper models.

Personally I find that this creates coupling in situations like this. Which is what DI is intended to resolve, not to create.

On the other hand, you have the school of thought which allows for a Service Locator approach, where models can internally invoke some common domain service to resolve a dependency for it. There's merit to this, but taken to an extreme you find that your dependencies are less known and there's a potential for runtime errors if any given dependency can't be resolved. (Basically, you can get errors at a higher level because consuming objects never knew that consumed objects needed something which wasn't provided.)

Personally I've used a service locator approach a lot (mostly because it's a very handy pattern for introducing DI to a legacy domain as part of a larger refactoring exercise, which is a lot of what I do professionally) and have never run into such issues.

There's yin and yang either way. And I think each solution space has its own balance. If you're finding that direct injection is making the system difficult to maintain, it may be worth investigating service location. Conversely, it may also be worth investigating if the overall domain model itself is inherently coupled and this DI issue is simply a symptom of that coupling and not the cause of it.

Answer 2

Yes, the entire intent of Dependency Injection is that you compose big object graphs up-front. You compose object graphs from the Composition Root, which is a place in your application that has the Single Responsibility of composing object graphs. That's not any particular Controller, but a separate class that composes Controllers with their dependencies.

The Composition Root must have access to all types it needs to compose, unless you want to get into late-binding strategies (which I'll generally advise against, unless there's a specific need).

Answer 3

I am firmly of the opinion that Service Locators are worse than Dependency Injection. They can be a useful legacy technique, and a useful stepping stone on to something better, but if you are designing something new, then steer clear.

The main reason for this is that Service Locators lead to code that has implicit dependencies, and this makes the code less clear and breaks encapsulation. It can also lead to run time errors instead of compile time errors, and Interacting Tests.

Your example uses Constructor Injection, which is usually the most appropriate form of Dependency Injection:

public ViewModelService(ICategoryRepository categoryRepository, IPasswordRepository passwordRepository, IModelValidatorService modelValidator) { ... }

This has explicit dependencies, which is good. It means that you cannot create the object without passing in its dependencies, and if you try to you will get a compile time error rather than a run time one. It also is good for encapsulation, as just by looking at the interface of the class you know what dependencies it needs.

You could do this using service locators as below:

public ViewModelService() 
{ 
    var categoryRepository = CategoryRepositoryServiceLocator.Instance;
    var passwordRepository = PasswordRepositoryServiceLocator.Instance;   
    var modelValidator  FModelValidatorServiceLocator.Instance;

    ...
}

This has implicit dependencies, that you cannot tell just by looking at the interface, you must also look at the implementation (this breaks encapsulation). You can also forget to set up one of the Service Locators, which will lead to a run time exception.

In your example I thinky your ViewModelService is good. It references abstractions (ICategoryRepository etc) and doesn't care about how these abstractions are created. The code you use to create the ViewModelService is a bit ugly, and I would recommend using an Inversion of Control container (such as Castle Windsor, StructureMap etc) to help here.

In Castle Windsor, you could do something like the following:

container.Register(Classes.FromAssemblyNamed("Repositories").Pick().WithServiceAllInterfaces());
container.Register(Component.For<IAccountService>().ImplementedBy<AccountService>()); 
container.Register(Component.For<IApplicationDBContext>().ImplementedBy<IApplicationDBContext>()); 
container.Register(Component.For<IApplicationSettingsService>().ImplementedBy<IApplicationSettingsService>()); 
var viewModelService = _container.Resolve<ViewModelService>();

Make sure to read and understand the "Register, Resolve, Release" and "Composition Root" patterns before you start.

Good luck!