ASP.NET MVC3 Dependency Injection misunderstanding

Question

public object GetService(Type serviceType)
{
    object resolvedObject = null;
    try
    {
        resolvedObject = _unityContainer.Resolve(serviceType);
    }
    catch(ResolutionFailedException e) { }

    return resolvedObject;
}

Is this best practice to catch exception and return null? By default mvc try to resolve IControllerFactory and IViewPageActivator, i.e. i get two catched exceptions. I don't prefer this solution.

My friend suggest to check serviceType on IControllerFactory, IViewPageActivator and if an exception is raised - catch and logged error. but as for me - it is not best solution, it like kind of hardcode.

This looks like you're using a service locator to resolve a dependency from an IoC container at run time. Just so you know, that's a really, really bad idea, in so many ways. — Jay Sullivan
– Jay Sullivan, Commented May 5, 2011 at 15:14
I should use DefaultControllerFactory like in mvc 2? Could you explain why it is realy bad idea, please. — Antony Blazer
– Antony Blazer, Commented May 5, 2011 at 15:16

Matthew Abbott · Accepted Answer · 2011-05-05 15:29:04Z

MVC3's DependencyResolver is a service locator, first and foremost. It is essentially the lovechild of MVC and the Common Service Locator (commonservicelocator.codeplex.com). The differences between a CSL service locator and MVC3's DependencyResolver is that the MVC architects decided that they needed a safer way of handling cases where services cannot be resolved through it, and the answer they chose was that the implementation of IDependencyResolver should return null in these circumstances.

With CSL implementations of IServiceLocator, the standard practice is to throw an ActivationException which allows for a standardised exception for handling these cases.

With MVC, the framework will call the DependencyResolver to try and resolve a configured service. If it cannot find a suitable service, it uses a default.

E.g., when it wants to find an IControllerFactory, it will check the DependencyResolver first. If it fails there, it will use the DefaultControllerFactory configured through ControllerBuilder.SetControllerFactory(...). Because of this check-first approach, it's better to return null then let a container specific exception bubble up.

This means that you must catch/swallow any exception at this point, as it is not MVC framework's responsibility to handle it for you (the onus should be on the container/IDependencyResolver itself).

You could log this at this stage, but the practice with MVC3 is to return null in these instances.

Kazi Manzur Rashid · Accepted Answer · 2011-05-05 18:36:14Z

0

This obviously not a best practice, But ASP.NET MVC is implemented in way that when the IoC is not able to resolve a service it should return null, so that the mvc framework can fallback to its default. Pls note there are number of services other than controller/view factory depends on it, so throwing exception will break those from functioning.

I argued with the design decision in the internal asp.net mailing list, but this how it currently stands :-(.

edited May 5, 2011 at 18:36

answered May 5, 2011 at 18:04

Kazi Manzur Rashid

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Comments

Stan Bashtavenko · Accepted Answer · 2012-04-03 15:41:07Z

0

For IControllerFactory you can

.RegisterType<IControllerFactory, DefaultControllerFactory>()

I am not sure which classes implement IViewPageActivator and ModelMetadataProvider. Event though an exception can be swallowed, supressing exceptions it is not a good practice simply because in general exceptions affect performance. Once an exception is caought the framework populates stack trace which take time.

answered Apr 3, 2012 at 15:41

Stan Bashtavenko

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