Intercept Unity 2.0 HandlerAttribute without an interface.

Question

Hi there.

I'm a first-time user of the AOP features of Unity 2.0 and would like some advice. My goal is to be able to log method calls in an ASPX page, like so:

public partial class Page2 : Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
        }

        [Log]
        private void Testing()
        {

        }
    }

Here is the code for the LogAttribute:

public class LogAttribute : HandlerAttribute
{
    public override ICallHandler CreateHandler(IUnityContainer container)
    {
        return new LogHandler(Order);
    }
}

Now the LogHandler:

public class LogHandler : ICallHandler
{
    public LogHandler(int order)
    {
        Order = order;
    }

    public IMethodReturn Invoke(IMethodInvocation input, GetNextHandlerDelegate getNext)
    {
        string className = input.MethodBase.DeclaringType.Name;
        string methodName = input.MethodBase.Name;

        string preMethodMessage = string.Format("{0}.{1}", className, methodName);
        System.Diagnostics.Debug.WriteLine(preMethodMessage); 

        return getNext()(input, getNext);
    }

    public int Order { get; set; }
}

The problem I have is how to use the [Log] attribute. I've seen plenty of example of how to configure the interception settings, for example:

container.AddNewExtension<Interception>();
container.Configure<Interception>().SetDefaultInterceptorFor<ILogger>(new InterfaceInterceptor());

But this implies that I have an interface to intercept, which I don't. I have the ASPX page which uses the [Log] attribute.

so how can I configure Unity to make use of the [Log] attribute? I've done this before using PostSharp and would like to be able to use Unity to do the same.

Cheers. Jas.

Answer 1

You're unfortunately not going to get this to work in an ASP.NET page with Unity interception.

Unity interception uses a runtime interception model. Depending on the interceptor you choose, you'll either get a subclass with virtual method overrides to call the call handlers (VirtualMethodInterceptor) or a separate proxy object (Interface or TransparentProxyInterceptor) which execute the call handlers and then forward to the real object.

Here's the issue - ASP.NET controls creation and calls to your page, and there's no easy way to hook into them. Without controlling the creation of the page object, you can't use the VirtualMethodInterceptor, because that requires that you instantiate a subclass. And you can't use the proxy version either, because you need ASP.NET to make calls through the proxy.

PostSharp gets around this because it's actually rewriting your IL at compile time.

Assuming you could hook into the creation of the page object, you'd have to use the VirtualMethodInterceptor here. It's a private method, so you want logging on "self" calls (calls from one method of the object into another method on the same object). The proxy-based interceptors can't see those, since the proxy is a separate instance.

I expect there is a hook somewhere to customize how ASP.NET creates object - BuildManager maybe? But I don't know enough about the details, and I expect it'll require some pretty serious hacking to get work.

So, how do you get around this? My recommendation (actually, I'd recommend this anyway) is to use the Model-View-Presenter pattern for your ASP.NET pages. Make the page object itself dumb. All it does is forward calls to a separate object, the Presenter. The Presenter is where your real logic is, and is independent of the details of ASP.NET. You get a huge gain in testability, and you can intercept calls on the presenter without all the difficulty that ASP.NET gives you.