Про Garbage Collector, Unity и слабые ссылки

interface IT {}
public IT CreateT(IA a, IB b, IC c, Type concreteType)
{
    // куча говнокода, который создает объект типа concreteType, определяемого в месте вызова в рантайме.
}

private static IT CreateITInternal(IA a, Type targetType)
{
    using (UnityContainer cont = new UnityContainer())
    {
        cont.RegisterInstance<IA>(a, new ExternallyControlledLifetimeManager());
        cont.RegisterType(typeof(IT), targetType, new ExternallyControlledLifetimeManager());
        return cont.Resolve<IT>();
    }
}

private static IT CreateIT()
{
    var a = new A();
    Type concreteType = typeof(T);
    return CreateITInternal(a, concreteType);
}

static void Main(string[] args)
{
    for (int i = 0; i < 1000; ++i)
    {
        CreateIT();
    }
}

public interface IA { };
public interface IT { };

public class A : IA
{
}

public class T : IT
{
    public T(IA a)
    {
    }
}

Unhandled Exception: Microsoft.Practices.Unity.ResolutionFailedException: Resolution of the dependency failed, type = "WeakRefTest.IT", name = "(none)".
Exception occurred while: while resolving.
Exception is: InvalidOperationException - The current type, WeakRefTest.IA, is an interface and cannot be constructed. Are you missing a type mapping?
-----------------------------------------------
At the time of the exception, the container was:

  Resolving WeakRefTest.T,(none) (mapped from WeakRefTest.IT, (none))
  Resolving parameter "a" of constructor WeakRefTest.T(WeakRefTest.IA a)
    Resolving WeakRefTest.IA,(none)
 ---> System.InvalidOperationException: The current type, WeakRefTest.IA, is an interface and cannot be constructed. Are you missing a type mapping?

   at Microsoft.Practices.ObjectBuilder2.DynamicMethodConstructorStrategy.ThrowForAttemptingToConstructInterface(IBuilderContext context)
   at lambda_method(Closure , IBuilderContext )
   at Microsoft.Practices.ObjectBuilder2.DynamicBuildPlanGenerationContext.<>c__DisplayClass1.<GetBuildMethod>b__0(IBuilderContext context)
   at Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context)
   at Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context)
   at Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context)
   at Microsoft.Practices.ObjectBuilder2.BuilderContext.NewBuildUp(NamedTypeBuildKey newBuildKey)
   at Microsoft.Practices.Unity.ObjectBuilder.NamedTypeDependencyResolverPolicy.Resolve(IBuilderContext context)
   at lambda_method(Closure , IBuilderContext )
   at Microsoft.Practices.ObjectBuilder2.DynamicBuildPlanGenerationContext.<>c__DisplayClass1.<GetBuildMethod>b__0(IBuilderContext context)
   at Microsoft.Practices.ObjectBuilder2.DynamicMethodBuildPlan.BuildUp(IBuilderContext context)
   at Microsoft.Practices.ObjectBuilder2.BuildPlanStrategy.PreBuildUp(IBuilderContext context)
   at Microsoft.Practices.ObjectBuilder2.StrategyChain.ExecuteBuildUp(IBuilderContext context)
   at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable`1 resolverOverrides)
   --- End of inner exception stack trace ---
   at Microsoft.Practices.Unity.UnityContainer.DoBuildUp(Type t, Object existing, String name, IEnumerable`1 resolverOverrides)
   at Microsoft.Practices.Unity.UnityContainer.Resolve(Type t, String name, ResolverOverride[] resolverOverrides)
   at Microsoft.Practices.Unity.UnityContainerExtensions.Resolve[T](IUnityContainer container, ResolverOverride[] overrides)
   at WeakRefTest.Program.CreateITInternal(IA a, Type targetType)
   at WeakRefTest.Program.CreateIT()
   at WeakRefTest.Program.Main(String[] args)

        private static IT CreateITInternal(IA a, Type targetType)
        {
            using (UnityContainer cont = new UnityContainer())
            {
                cont.RegisterInstance<IA>(a, new ExternallyControlledLifetimeManager());
                cont.RegisterType(typeof(IT), targetType, new ExternallyControlledLifetimeManager());
                GC.Collect();
                return cont.Resolve<IT>();
            }
        }

        private static void TestWeakRef()
        {
            var sa = new A();
            var wa = new WeakReference<A>(sa);
            GC.Collect();
            A sa2;
            wa.TryGetTarget(out sa2);
            Console.WriteLine("{0}", sa2 == null ? "null" : "not null");
            Console.ReadLine();
        }

        private static IT CreateITInternal(IA a, Type targetType)
        {
            using (UnityContainer cont = new UnityContainer())
            {
                cont.RegisterInstance<IA>(a, new ExternallyControlledLifetimeManager());
                cont.RegisterType(typeof(IT), targetType, new ExternallyControlledLifetimeManager());
                IT ret = cont.Resolve<IT>();
                GC.KeepAlive(a);
                return ret;
            }
        }