Question

I user Autofac and wonder if it is possible to come around the DependencyResolutionException that is thrown if the instance that is returned by a registered type is null?
Consider the following example:

builder.Register( c => c.Resolve<HttpContextBase>().CurrentHandler as ITemplatePage
                ).InstancePerHttpRequest();

The example will return null if the current handler (for example the aspx-page that I am visiting) isn’t of type ITemplatePage, and Autofac will throw the DependencyResolutionException even if I use the ResolveOptional alternative like in this way:

container.ResolveOptional<ITemplatePage>(); 

Is there any way to come arond this and get an behaviour where Autofac returns null instead of thowing an exeption?

I have worked around this by adding a IsNull-proerty to ITemplatePage and always instanciate it, like this:

builder.Register( c => 
                  c.Resolve<HttpContextBase>().CurrentHandler as ITemplatePage
                  ?? new TemplatePage(true)
                 ).As<ITemplatePage>().InstancePerHttpRequest();

and then I can work with it in this way:

var templatePage = container.Resolve<ITemplatePage>();  
if(!templatePage.IsNull){
   // Do stuff
}

But I do not think that is an optimal solution to my problem.

Edit:
To catch the error is not an option since I use ITemplatePage in constructors of other types I resolve, and since I am using constructor injection to resolve these types I want them to return the instance created with the constructor without the ITemplatePage parameter rather than to crash as well.

Answer 1

The best way to do this is to register the service conditionally at the start of a web request, e.g. (pseudocode):

void Application_BeginRequest()
{
    var tp = HttpContext.CurrentHandler as ITemplatePage;
    if (tp != null)
    {
        var updater = new ContainerBuilder();
        updater.RegisterInstance(tp);
        updater.Update(ContainerProvider.RequestLifetime.ComponentRegistry);
    }
}

This way, you can simply make the ITemplatePage an optional constructor parameter of components that can use it (ITemplatePage tp = null) or use property injection/ResolveOptional().

Note there have been some reliability improvements around these kinds of delayed registrations in Autofac 2.5 so I recommend updating before using this approach.