Spring.NET + NHibernate + request scoped objects not playing well together

Question

I'm using Spring.NET 1.3.2, NHibernate 3.1 and the OSIV pattern in a ASP.NET application.

I have a custom EventListener that needs to be request scoped because it uses HttpContext.Current.Items as a constructor dependency.

Since ISession is also request scoped, I should be able to use Spring.NET to manage these dependencies for me.

The problem is that EventListener, like IInterceptor, is a property of ISessionFactory which is not request scoped (it's a singleton). The mismatch between the web object scopes is problematic.

I tried the following XML snippet, but the conditional expression always yields null. I think this is due to Spring creating EventListener object at an application level scope, and before HttpContext.Current.Items has had a chance to be populated.

  <object id="EventListener" scope="request" type="MyEventListener,DAL">

        <constructor-arg index="0" expression="T(System.Web.HttpContext).Current.Items.Contains('Principal')?T(System.Web.HttpContext).Current.Items['Principal']:null"/>

  </object>

So my requirement is:

1. Configure custom EventListener object in Spring so that it is created on a per-request basis

2. EventListener instantiation must occur late enough in the request lifecycle so that HttpContext.Current.Items['Princpial'] has been populated by a custom IHttpModule

3. The EventListener instance is injected into the the current OSIV ISession

Answer 1

I think you're trying to do this the wrong way around. According to the NHibernate documentation EventListeners should basically be considered singletons for your application.

Even when you register your custom EventListener with the SessionFactory, say at the start of the request, you still have no guarantee whatsoever that this listener will only receive events raised in the current HttpContext.

Instead you should register your eventhandlers globally, when configuring the session factory. If you need context information, such as a principal, you can inject a dependency with a (http)context-aware implementation or implement it as an ambient context.

From the custom listener, you can get access to the session that raised the event. Take for instance this ILoadEventListener implementation:

public class CustomLoadEventListener : ILoadEventListener
{
    private IPrincipalProvider _principalProvider;

    public CustomLoadEventListener(IPrincipalProvider provider)
    {
        _principalProvider = provider;
    }

    public void OnLoad(LoadEvent @event, LoadType loadType)
    {
        var sessionThatRaisedTheEvent = @event.Session;
        var principalForTheCurrentContext = _principalProvider.GetCurrentPrincipal();
    }
}

public interface IPrincipalProvider
{
    IPrincipal GetCurrentPrincipal();
}

public class HttpContextPrincipalProvider : IPrincipalProvider
{
    public IPrincipal GetCurrentPrincipal()
    {
        return System.Web.HttpContext.Current.User;
    }
}

This might not be the answer you hoped to get, but IMO what your are asking for would be fighting the infrastructure, which I generally try to avoid.

If you post some more details on what you are trying to achieve, we might be able to propose a better solution. What should your EventListener do?