Question

I am using the Unity IoC container, and I need to intercept any calls to Resolve for a certain base interface, and run my own custom code to construct those types.

In other words, in the sample code below, when I call container.Resolve<IFooN>(), if it hasn’t got an instance of the concrete implementing type, it calls MyFactoryFunction to construct one, otherwise I want it to return the cached copy.

The standard Unity container is not able to construct these objects (update: because they are .NET remoting objects, so the concrete classes do not exist in any assembly on the local computer), and I don’t want to create them up front and store them with RegisterInstance.

interface IFoo : IBase { ... }
interface IFoo2 : IBase { ... }

...
container.Resolve<IFoo2>();

...
IBase MyFactoryFunction(Type t)
{
    ...
}

I’m assuming I can create a Unity extension to do this, but I was wondering if there is already a solution out there I can borrow.

Answer 1

For completeness, I should add another answer that works under Unity 2, since my other answer no longer works. It is slightly more involved since you need to make a custom builder policy. Thanks to ctavares from the Unity project who provided lots of help on this thread in implementing this:

public class FactoryUnityExtension : UnityContainerExtension
{
    private ICustomFactory factory;
    private CustomFactoryBuildStrategy strategy;

    public FactoryUnityExtension(ICustomFactory factory)
    {
        this.factory = factory;
    }

    protected override void Initialize()
    {
        this.strategy = new CustomFactoryBuildStrategy(factory, Context);
        Context.Strategies.Add(strategy, UnityBuildStage.PreCreation);
        Context.Policies.Set<ParentMarkerPolicy>(new ParentMarkerPolicy(Context.Lifetime), new NamedTypeBuildKey<ParentMarkerPolicy>());
    }
}

public class ParentMarkerPolicy : IBuilderPolicy
{
    private ILifetimeContainer lifetime;

    public ParentMarkerPolicy(ILifetimeContainer lifetime)
    {
        this.lifetime = lifetime;
    }

    public void AddToLifetime(object o)
    {
        lifetime.Add(o);
    }
}

public interface ICustomFactory
{
    object Create(Type t);
    bool CanCreate(Type t);
}

public class CustomFactoryBuildStrategy : BuilderStrategy
{
    private ExtensionContext baseContext;
    private ICustomFactory factory;


    public CustomFactoryBuildStrategy(ICustomFactory factory, ExtensionContext baseContext)
    {
        this.factory = factory;
        this.baseContext = baseContext;
    }

    public override void PreBuildUp(IBuilderContext context)
    {
        var key = (NamedTypeBuildKey)context.OriginalBuildKey;

        if (factory.CanCreate(key.Type) && context.Existing == null)
        {
            context.Existing = factory.Create(key.Type);
            var ltm = new ContainerControlledLifetimeManager();
            ltm.SetValue(context.Existing);

            // Find the container to add this to
            IPolicyList parentPolicies;
            var parentMarker = context.Policies.Get<ParentMarkerPolicy>(new NamedTypeBuildKey<ParentMarkerPolicy>(), out parentPolicies);

            // TODO: add error check - if policy is missing, extension is misconfigured

            // Add lifetime manager to container
            parentPolicies.Set<ILifetimePolicy>(ltm, new NamedTypeBuildKey(key.Type));
            // And add to LifetimeContainer so it gets disposed
            parentMarker.AddToLifetime(ltm);

            // Short circuit the rest of the chain, object's already created
            context.BuildComplete = true;
        }
    }
}