Explicit resolving of ILog in Autofac when using with Log Injection Module

Question

I use the following code in order to register log4net for all the classes that need it.

public class LogInjectionModule : Module
{
    private readonly string _configPath;

    public LogInjectionModule(string configPath)
    {
        _configPath = configPath;
    }

    protected override void AttachToComponentRegistration(IComponentRegistry registry,
        IComponentRegistration registration)
    {
        XmlConfigurator.Configure(new FileInfo(_configPath));

        registration.Preparing += OnComponentPreparing;
    }

    private static void OnComponentPreparing(object sender, PreparingEventArgs e)
    {
        var t = e.Component.Activator.LimitType;
        e.Parameters = e.Parameters.Union(new[]
                                          {
                                              new ResolvedParameter((p, i) => p.ParameterType == typeof (ILog),
                                                  (p, i) => LogManager.GetLogger(t))
                                          });
    }
}

All the classes are registered using autofac's types scanning:

builder.RegisterAssemblyTypes(typeof (IResourceFinder).Assembly)
 .AsImplementedInterfaces();

And it works fine!

One class needs to be registered explicitly tries to resolve ILog and fails

builder.Register(x => new ClassThatNeedsILog(x.Resolve<ILog>())).AsImplementedInterfaces();

Here is that class

public class ClassThatNeedsILog
{
    public ClassThatNeedsILog(ILog log)
    {

    }
}

I am getting the following exception:

Autofac.Core.Registration.ComponentNotRegisteredException : The requested service 'log4net.ILog' has not been registered. To avoid this exception, either register a component to provide the service, check for service registration using IsRegistered(), or use the ResolveOptional() method to resolve an optional dependency.

Answer 1

Your LogInjectionModule never registers any ILog into the container only supplies paramters for the resolved instances on the preparing face, and it only works for instances created by Autofac.

So when you write builder.Register(x => new ClassThatNeedsILog(x.Resolve<ILog>())) you are creating the ClassThatNeedsILog manually. with new ClassThatNeedsILog(...)

Hence Autofac does not know about your instance creation (so your OnComponentPreparing won't run) and because you haven't really registered any ILog implementation you get the ComponentNotRegisteredException.

You have two options:

• register an ILog in the container directly
• let Autofac create your ClassThatNeedsILog type.

So you can just register an ILog in the container with:

builder.RegisterInstance(LogManager.GetLogger("Logger")).As<ILog>();

Then your code will work fine.

Or if you anyway creating the ClassThatNeedsILog by hand can just supply directly the ILog there:

 builder.Register(x => new 
     ClassThatNeedsILog(LogManager.GetLogger(typeof(ClassThatNeedsILog))))
     .AsImplemen‌​tedInterfaces();

The other options is to let Autofac create the instances for you, so change your registration to:

 builder.RegisterType<ClassThatNeedsILog>()
     .AsImplemen‌​tedInterfaces();

In this case Autofac will handle the instance creation for you and it will call the OnComponentPreparing method of your module.

If you want to supply additional constructor parameters you can use WithParameter and WithParameters methods:

 builder.RegisterType<ClassThatNeedsILog>()
     .AsImplemen‌​tedInterfaces()
     .WithParameters(new Parameter[] {
         ResolvedParameter.ForNamed<IAnotherInterface>("NAME"),
         ResolvedParameter.ForNamed<IYetAnotherInterface>("ANOTHERNAME")});