Today I encountered a pretty basic scenario that Unity can’t support out of the box: one NHibernate ISession per HttpContext.

The problem is that ISessions are provided by a factory method NHibernateHelper.OpenSession(), and I need to store them in a custom HttpContextLifetimeManager. Unity won’t let me use the two together because no RegisterFactory() overload allows you to specify a LifetimeManager.

Rupert Benbrook has posted a solution to this problem that overrides part of the Unity build-up pipeline (and even goes as far as emitting op codes directly). For simple applications, however, a plain extension method might suffice:

public static IStaticFactoryConfiguration RegisterFactory<T>(
    this StaticFactoryExtension extension,
    FactoryDelegate factoryMethod, LifetimeManager lifetimeManager)
{
    FactoryDelegate wrappedFactoryMethod = delegate
    {
        LifetimeManager manager = lifetimeManager;
        FactoryDelegate method = factoryMethod;
        IUnityContainer container = extension.Container;

        object o = manager.GetValue();
        if (o == null)
        {
            o = factoryMethod(container);
            manager.SetValue(o);
        }
        return o;
    };

    return extension.RegisterFactory<T>(wrappedFactoryMethod);
}

This simply wraps the real factory method in a closure that checks a LifetimeFactory first. I can then do:

container.Configure<StaticFactoryExtension>()
    .RegisterFactory<ISession>((c) => NHibernateHelper.OpenSession(),
    new HttpContextLifetimeManager<ISession>());

This seems to work pretty well.

May 26, 2009

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