Question

I’ve developing a Silverlight application, and I’ve introduced Unity into it.

The problem I have is I don’t know how to get an instance of container.

I create this intance in ApplicationStartup method on the App

_container = new UnityContainer();
            _container.RegisterType<IMyAppServiceAgent, MyAppServiceAgent>(new InjectionConstructor(OriginalHandlerId, W2OGuid, ServiceEndpointAddr));

and I write a getter

public IUnityContainer Container
        {
            get { return _container; }
        }

Everything works fine, and this how I use my container:

    public static void CreateMemberSearch()
    {
        if (_memberSearch == null)
        {

            _memberSearch =
                new MemberSearchViewModel((App.Current as App).Container.Resolve<IMyAppServiceAgent>());

        }
    }

Above example is from ViewModelLocator (from MVVM Light Toolkit).

I need to know how to refactor my code to go along with the IOC principles.

Answer 1

The code shown above implements the ServiceLocator anti-pattern. You should never call the container directly.

The container is setup in the Composition Root. For a Silverlight application that would be your ApplicationStartup method or a dedicated bootstrapper like in Caliburn.Micro.

This container instance is called exactly once to resolve your main view oder viewmodel (depending on wether you use a view first or viewmodel first approach).

And that should be it. You should never call your container again. If a class depends on some other component: inject that component using constructor injection. If that class needs to create other objects: inject a factory. Unity has a nice feature called automatic factories

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Update

If the ViewModelLocator is part of your infrastructure and you never use it directly in your application code … maybe. Please see my comment on @MikePost’s question.