未在AppDomain中激发AssemblyLoad事件

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Reflection;
namespace DefaultAppDomainApp
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("***** Fun with the default app domain *****'n");
            InitDAD();
            DisplayDADStats();
            Console.WriteLine();
            ListAllAssembliesInAppDomain();
            Console.ReadLine();
        }
        #region Init the default app domain
        private static void InitDAD()
        {
            // This logic will print out the name of any assembly
            // loaded into the applicaion domain, after it has been
            // created. 
            AppDomain defaultAD = AppDomain.CurrentDomain;
            defaultAD.AssemblyLoad += (o, s) =>
                {
                    Console.WriteLine("{0} has been loaded!", s.LoadedAssembly.GetName().Name);
                };
        }
        #endregion
        #region Display basic stats
        private static void DisplayDADStats()
        {
            // Get access to the app domain for the current thread.
            AppDomain defaultAD = AppDomain.CurrentDomain;
            Console.WriteLine("Name of this domain: {0}", defaultAD.FriendlyName);
            Console.WriteLine("ID of domain in this process: {0}", defaultAD.Id);
            Console.WriteLine("Is this the default domain?: {0}", defaultAD.IsDefaultAppDomain());
            Console.WriteLine("Base directory of this domain: {0}", defaultAD.BaseDirectory);
        } 
        #endregion
        #region List loaded assemblies 
        static void ListAllAssembliesInAppDomain()
        {
            // Get access to the app domain for the current thread.
            AppDomain defaultAD = AppDomain.CurrentDomain;
            // Now get all loaded assemblies in the default app domain. 
            var loadedAssemblies = from a in defaultAD.GetAssemblies() orderby a.GetName().Name select a;
            Console.WriteLine("***** Here are the assemblies loaded in {0} *****'n",
              defaultAD.FriendlyName);
            foreach (var a in loadedAssemblies)
            {
                Console.WriteLine("-> Name: {0}", a.GetName().Name);
                Console.WriteLine("-> Version: {0}'n", a.GetName().Version);
            }
        } 
        #endregion
    }
}

上面的代码是我从AndrewTroelsen的"ProC#2010和.NET4平台"一书中得到的。在这里，当我运行这个代码时，控制永远不会到达行

            defaultAD.AssemblyLoad += (o, s) =>
                {
                    Console.WriteLine("{0} has been loaded!", s.LoadedAssembly.GetName().Name);
                };

为什么在运行此代码时不触发此事件？控制何时到达此处？