Question

we want to use Unity for IOC.
All i’ve seen is the implementation that there is one global static service (let’s call it the the IOCService) which holds a reference to the Unity container, which registers all interface/class combinations and every class asks that object: give me an implementation for Ithis or IThat.

Frequently i see a response that this pattern is not good because it leads to a dependency from ALL classes to the IOCService (not to the Unity container because it is only known inside the IOCService).

But what i don’t see often, is: what is the alternative way?

Michel

EDIT: found out that the global static service is called the service locator, added that to the title.

Answer 1

The alternative is to have a single instance of your container at the highest application level only, then use that container to resolve every object instance you need to create in that layer.

For example, the main method of most executables just looks like this (minus exception handling):

private static void main(string[] args) {

     Container container = new Container();

     // Configure the container - by hand or via file

     IProgramLogic logic = container.Resolve<IProgramLogic>();

     logic.Run();
}

Your program (represented here by the IProgramLogic instance) doesn’t have to know anything about your container, because container.Resolve will create all its dependencies – and its dependencies’ dependencies, on down to leaf classes with no dependencies of their own.

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ASP.NET is a harder case, because web forms doesn’t support constructor injection. I typically use Model-View-Presenter in my web forms applications, so my Page classes really only have one dependency each – on their presenter. I don’t unit test them (everything interesting and testable is in my presenters, which I do test), and I don’t ever substitute presenters. So I don’t fight the framework – I just expose a container property on my HttpApplication class (in global.asax.cs) and use it directly from my Page files:

protected void Page_Load(object sender, EventArgs args) {
    ICustomerPresenter presenter = Global.Container.Resolve<ICustomerPresenter>();
    presenter.Load();
}

That’s service locator of course – though the Page classes are the only thing coupled to the locator: your presenter and all of its dependencies are still fully decoupled from your IoC container implementation.

If you have a lot of dependencies in your Page files (that is, if you do not use Model-View-Presenter), or if it’s important to you to decouple your Page classes from your Global application class, you should try to find a framework that integrates into the web forms request pipeline and use property injection (as suggested by Nicholas in the comments below) – or write your own IHttpModule and perform the property injection yourself.