How do I use the Decorator Pattern with Unity without explicitly specifying every parameter in the InjectionConstructor

Question

This helpful article from David Haydn (EDIT: scam link removed, it could have been this article) shows how you can use the InjectionConstructor class to help y

Answer 1

I knocked up a fairly crude extension method for this, which behaved as expected when I ran it:

public static class UnityExtensions
{
    public static IUnityContainer Decorate(this IUnityContainer container, params InjectionMember[] injectionMembers)
        where TDecorator : class, TInterface
    {
        return Decorate(container, null, injectionMembers);
    }

    public static IUnityContainer Decorate(this IUnityContainer container, LifetimeManager lifetimeManager, params InjectionMember[] injectionMembers)
        where TDecorator : class, TInterface
    {
        string uniqueId = Guid.NewGuid().ToString();
        var existingRegistration = container.Registrations.LastOrDefault(r => r.RegisteredType == typeof(TInterface));
        if(existingRegistration == null)
        {
            throw new ArgumentException("No existing registration found for the type " + typeof(TInterface));
        }
        var existing = existingRegistration.MappedToType;

        //1. Create a wrapper. This is the actual resolution that will be used
        if (lifetimeManager != null)
        {
            container.RegisterType(uniqueId, lifetimeManager, injectionMembers);
        }
        else
        {
            container.RegisterType(uniqueId, injectionMembers);
        }

        //2. Unity comes here to resolve TInterface
        container.RegisterType(new InjectionFactory((c, t, sName) =>
        {
            //3. We get the decorated class instance TBase
            var baseObj = container.Resolve(existing);

            //4. We reference the wrapper TDecorator injecting TBase as TInterface to prevent stack overflow
            return c.Resolve(uniqueId, new DependencyOverride(baseObj));
        }));

        return container;
    }
}

And in your setup:

container.RegisterType();

// Wrap ProductRepository with CachingProductRepository,
// injecting ProductRepository into CachingProductRepository for
// IProductRepository
container.Decorate();

// Wrap CachingProductRepository with LoggingProductRepository,
// injecting CachingProductRepository into LoggingProductRepository for
// IProductRepository
container.Decorate();