Using Autofac with Dynamic Proxy that output message automatic

Question

public interface ILog
{
    void Write(string msg);
}

public class MyLog : ILog
{
    public void Write(string msg)
    {
        Console.WriteLine(msg);
    }
}

public interface ICanLog
{
    ILog Log { get; set; }
}

public interface IMyClass
{
    void Test();
}

public class MyClass : IMyClass, ICanLog
{
    public ILog Log { get; set; }
    public void Test()
    {
        Log.Write("Test");
    }
}

I am using Autofac with Castle DynamicProxy, and try to let MyClass Test Method output "BEGIN"/"END" automatic.

public class MyLogInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
        Console.WriteLine("BEGIN");
        invocation.Proceed();
        Console.WriteLine("END");
    }
}

The following is test code:

ContainerBuilder builder = new ContainerBuilder();
builder.RegisterType<MyLog>().As<ILog>();
builder.Register(c =>
{
    ProxyGenerator g = new ProxyGenerator();
    object proxy = g.CreateClassProxy(typeof(MyClass), new MyLogInterceptor());
    ICanLog proxyICanLog = (ICanLog)proxy;
    proxyICanLog.Log = c.Resolve<ILog>();
    return proxy;
}).As<IMyClass>();

using (var container = builder.Build())
{
    objectContext.Container = container;
    IMyClass myclass = container.Resolve<IMyClass>();
    myclass.Test();
}

But result no output "BEGIN"/"END", why ?

and if I create AutoLogModule that try build Log Property Instance automatic

    public class AutoLogModule : Autofac.Module
{
    protected override void AttachToComponentRegistration(IComponentRegistry componentRegistry, IComponentRegistration registration)
    {
        var type = registration.Activator.LimitType;
        if (HasPropertyDependencyOnClass(type))
        {
            registration.Activated += InjectClassViaProperty;
        }
    }

    private bool HasPropertyDependencyOnClass(Type type)
    {
        return type.GetProperties().Any(property => property.CanWrite && property.PropertyType==typeof(ILog));
    }

    private void InjectClassViaProperty(object sender, ActivatedEventArgs<object> evt)
    {
        var type = evt.Instance.GetType();
        var propertyInfo = type.GetProperties().First(x => x.CanWrite && x.PropertyType==typeof(ILog));

        ILog log = new MyLog();
        propertyInfo.SetValue(evt.Instance, log, null);
    }
}

The following is test code:

ContainerBuilder builder = new ContainerBuilder();
builder.RegisterType<MyLog>().As<ILog>();
builder.RegisterModule(new AutoLogModule());
builder.Register(c =>
{
    ProxyGenerator g = new ProxyGenerator();
    object proxy = g.CreateClassProxy(typeof(MyClass), new MyLogInterceptor());
    //ICanLog proxyICanLog = (ICanLog)proxy;
    //proxyICanLog.Log = c.Resolve<ILog>();
    return proxy;
}).As<IMyClass>();

using (var container = builder.Build())
{
    objectContext.Container = container;
    IMyClass myclass = container.Resolve<IMyClass>();
    myclass.Test();
}

The result is Test Method throw "Object reference not set to an instance of an object." in Log.Write("Test")

How to write this feature?

Answer 1

I know this is a rather old post but as I was trying to accomplish the same thing with Autofac and I found the documentation that helped me to achieve it. I will answer just in case it helps someone else.

In my case I'm using Autofac 4.92 and and extra package for DynamicProxy called Autofac.Extras.DynamicProxy 4.5.0 as the documentations sates.

I see a difference where you register your Interceptors. Even though what you are doing is what I would have done initially; is not what Autofac Documentation currently says about how to Register Interceptors:

 builder.RegisterType<MyClass>().As<IMyClass>().EnableInterfaceInterceptors();

// Typed registration
builder.Register(c => new MyLogInterceptor ();

Lastly, you need to Associate Interceptors with Types to be Intercepted:

[Intercept(typeof(MyLogInterceptor))]
public class MyClass : IMyClass, ICanLog
{
    public ILog Log { get; set; }
    public void Test()
    {
       Log.Write("Test");
    }
}

I hope this answer may help. In any case, the Autofac documentation explains step by step how to do it just in case my code may mistakenly skip some relevant part.