Registering factory method doesn't prevent Autofac from calling ctor

Question

I have a service called ServiceConfiguration wich has a non-default ctor. I'm trying to register it through Autofac with a factory method:

builder.Register(c=>LoadConfiguration())
       .As<ServiceConfiguration>();

And here is the simple LoadConfiguration method:

private ServiceConfiguration LoadConfiguration() {
    const string fileName = "app.json";
    var json = File.ReadAllText(fileName, Encoding.UTF8);
    var model = JsonConvert.DeserializeObject<ServiceConfiguration>(json);
    return model;
}

I expect Autofac to call the LoadConfiguration when I asked to resolve ServiceConfiguration . But it seems it's trying to call a ctor on ServiceConfiguration . Eg calling this line:

var c = container.Resolve<ServiceConfiguration>();

causes this error:

Autofac.Core.DependencyResolutionException: 'An error occurred during the activation of a particular registration. See the inner exception for details. Registration: Activator = ServiceConfiguration (ReflectionActivator), Services = [ServiceConfiguration], Lifetime = Autofac.Core.Lifetime.CurrentScopeLifetime, Sharing = Shared, Ownership = OwnedByLifetimeScope'

and the InnerException is:

DependencyResolutionException: None of the constructors found with 'Autofac.Core.Activators.Reflection.DefaultConstructorFinder' on type 'ServiceConfiguration' can be invoked with the available services and parameters: Cannot resolve parameter 'JobsConfiguration jobs' of constructor 'Void .ctor(JobsConfiguration)'.

Can you find out what I'm missing?

UPDATE: I'm not missing a registration of JobsConfiguration . Actually it's not a registrable type and I don't want to register it. I don't want to ctor get called at all (the service is getting build from a json file), instead whenever somebody asked for ServiceConfiguration I want Autofac to call LoadConfiguration() and use it's returned value.

UPDATE2:

After a while, it seems extracting an interface from ServiceConfiguration - say IServiceConfiguration - and registering/resolving the interface works just fine. But i cannot figure out why! I mean this:

builder.Register(c=>LoadConfiguration())
       .As<IServiceConfiguration>();

var c = container.Resolve<IServiceConfiguration>();

works. But the concrete version not. So why? What's the difference?

Answer 1

You should strive separating the loading of configuration files from Object Composition. This means that it's best not to load the configuration when Resolve is called. Changing this is trivial:

ServiceConfiguration configuration = LoadConfiguration();
builder.RegisterInstance(configuration);

That said, your given registration should work:

builder.Register(c => LoadConfiguration())
    .As<ServiceConfiguration>();

Which is identical to:

builder.Register(c => LoadConfiguration());

With this registration, Autofac will not Auto-Wire the ServiceConfiguration constructor, but will leave that to the supplied delegate. There might be something going on that your post is not showing.

I used the following MCVE , using Autofav 4.8.1, to verify that this works:

class ServiceConfiguration
{
    public ServiceConfiguration(int a, double b) => Console.WriteLine("ctor called.");
}

static ServiceConfiguration LoadConfiguration() => new ServiceConfiguration(2, 6);

static void Main(string[] args)
{
    var builder = new ContainerBuilder();

    builder.Register(_ => LoadConfiguration()).As<ServiceConfiguration>();

    builder.Build().Resolve<ServiceConfiguration>();
}

Last note: Make sure that ServiceConfiguration is not an application-wide configuration class that is injected into many consumers. To understand what's wrong with application-wide configuration classes, please read this Q/A .