How to inject dependency name as a constructor parameter

Question

Using Autofac, I can register a class to resolve against an interface using property injection, using the following code:

builder.RegisterType

        

Answer 1

You only use logName to effectively resolve by name an ILog, so why not just inject an ILog?

public class Log4NetAdapter : ILogger
{
    private readonly ILog _logger;

    public Log4NetAdapter(ILog logger)
    {
        _logger = logger;
    }

    ...
}

OK, so now I've just moved the problem a bit, but I've also made this less coupled to other classes, namely the LogManager.

So if I was using unity, I would then do this to ensure I get the right logger:

var childContainer = container.CreateChildContainer();
childContainer.RegisterInstance<ILog>(LogManager.GetLogger(logName));
var adaptor = childContainer.Resolve<Log4NetAdapter>();

The child container prevents any other code getting access to that ILog. You can do this as high up as you like, I don't know any more about your code.

Answer 2

Update: Building on the LogInjectionModule sample and how Autofac does property injection, I have extended the module to do both constructor and property injection.

Note: I've fixed the type passed to LogManager in OnComponentPreparing to use the declaring type. This makes e.g. Resolve<Func<Service>> use the correct log type.

    using System.Linq;
    using log4net;

    public class LogInjectionModule : Module
    {
        protected override void AttachToComponentRegistration(IComponentRegistry registry, IComponentRegistration registration)
        {
            registration.Preparing += OnComponentPreparing;
            registration.Activating += OnComponentActivating;
        }

        private static void OnComponentActivating(object sender, ActivatingEventArgs<object> e)
        {
            InjectLogProperties(e.Context, e.Instance, false);
        }

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

        private static void InjectLogProperties(IComponentContext context, object instance, bool overrideSetValues)
        {
            if (context == null) throw new ArgumentNullException("context");
            if (instance == null) throw new ArgumentNullException("instance");

            var instanceType = instance.GetType();
            var properties = instanceType
                .GetProperties(BindingFlags.Public | BindingFlags.Instance)
                .Where(pi => pi.CanWrite && pi.PropertyType == typeof(ILog));

            foreach (var property in properties)
            {
                if (property.GetIndexParameters().Length != 0)
                    continue;

                var accessors = property.GetAccessors(false);
                if (accessors.Length == 1 && accessors[0].ReturnType != typeof(void))
                    continue;

                if (!overrideSetValues &&
                    accessors.Length == 2 &&
                    (property.GetValue(instance, null) != null))
                    continue;

                ILog propertyValue = LogManager.GetLogger(instanceType);
                property.SetValue(instance, propertyValue, null);
            }
        }
    }

On how to use the module, here's a sample:

public class Service
{
    public Service(ILog log) { ... }
}

var cb = new ContainerBuilder();
cb.RegisterModule<LogInjectionModule>();
cb.RegisterType<Service>();
var c = cb.Build();

var service = c.Resolve<Service>();