Question

I have been using StructureMap for more than a year now. And all this time I used to have a wrapper class called IoC which looked like this

class IoC {
    public static T GetInstance<T>()
    {
        return (T)GetInstance(typeof(T));
    }
    public static IEnumerable<T> GetAllInstances<T>()
    {
        return ObjectFactory.GetAllInstances<T>();
    }

    public static IEnumerable GetAllInstances(Type type)
    {
        return ObjectFactory.GetAllInstances(type);
    }

    public static object GetInstance(Type type)
    {
        return ObjectFactory.GetInstance(type);
    }

    public static void Inject<T>(T obj)
    {
        ObjectFactory.Inject(obj);
    }
}

I added the wrapper assuming that I might want to change the IoC container at some point down the line. At this point I am thinking this is bad. One reason is: I cannot use ObjectFactory in my code to do other interesting things, I have to use this wrapper. The other thing is: Our code shouldn’t really have to be independent of the DependencyInjection container.

What are the pros/cons of using this approach?

Answer 1

For this reason the Common Service Locator project has been developed. It is an abstraction over DI frameworks and it defines an interface much like your IoC class. I even developed the Simple Service Locator library; an DI library that is a direct implementation of the Common Service Locator interface.

So in that sense, it’s not weird to have an abstraction over DI frameworks. However, when doing Dependency Injection correctly (and completely), the idea is to adjust the design of your application accordingly, configure the container in the application root, and have preferably just a single place in the application were types are assembled (read: were GetInstance is called). For an ASP.NET MVC application, this would be the ControllerFactory. For ASP.NET WebForms application you would typically need to override a PageHandlerFactory.

When you play by these rules, there is no reason to use such an abstraction, because you just call the container at a single place in your application anyway. However, if that’s not feasible for you, using either the Common Service Locator or your own abstraction is an alternative.

But please take a step back before you decide to let your code depend on an abstraction over your IoC library, because this causes a lot of problems and is seen as an anti-pattern in general. Calling back into the container from within your code:

• Makes the code much harder to test.
• Hides dependencies instead making the code harder to read and maintain.
• Disables compile-time support.
• Disallows your dependency graphs to be verified by a tool.