Effect of LoaderOptimizationAttribute

I have written a small piece of code regarding the dynamic loading of assemblies and creating class instances from those assemblies, including an executable, a test lib to be dynamically loaded and a loader library to load dynamic assembly into a new Appdomain. Loader library is referenced by both executable and the dynamic library.

//executable
[System.STAThreadAttribute()]
[System.LoaderOptimization(LoaderOptimization.MultiDomain)]
static void Main(string[] args)
{       
    AppDomainSetup domainSetup = new AppDomainSetup()
    {
        ApplicationBase = AppDomain.CurrentDomain.SetupInformation.ApplicationBase,
        ConfigurationFile = AppDomain.CurrentDomain.SetupInformation.ConfigurationFile,
        ApplicationName = AppDomain.CurrentDomain.SetupInformation.ApplicationName,
        LoaderOptimization = LoaderOptimization.MultiDomain
    };
    AppDomain childDomain = AppDomain.CreateDomain("MyDomain", null, domainSetup);
    Console.WriteLine(AppDomain.CurrentDomain.SetupInformation.LoaderOptimization.ToString());
    Console.WriteLine(childDomain.SetupInformation.LoaderOptimization.ToString());

    byte[] assembly = null;
    string assemblyName = "CSTestLib"; 

    using (FileStream fs = new FileStream(assemblyName+".dll",FileMode.Open))
    {
        byte[] byt = new byte[fs.Length];
        fs.Read(byt,0,(int)fs.Length);
        assembly = byt;          
    }

    object[] pararmeters = {assemblyName,assembly}; 
    string LoaderAssemblyName = typeof(AssemblyLoader).Assembly.FullName;
    string LoaderClassName = typeof(AssemblyLoader).FullName;
    AssemblyLoader assloader = (AssemblyLoader)childDomain.CreateInstanceAndUnwrap(LoaderAssemblyName,LoaderClassName , true, BindingFlags.CreateInstance, null, parameters, null, null);


    object obj = assloader.Load("CSTestLib.Class1");
    object obj2 = assloader.Load("CSTestLib.Class2");

    AppDomain.Unload(childDomain);

    Console.ReadKey();
}

//Dynamic Lib
using System;


namespace CSTestLib
{
    public class Class1 :MarshalByRefObject
    {
        public Class1() { }
    }



    public class Class2 : MarshalByRefObject
    {
        public Class2() { }
    }
}

//Loader Library


using System;

namespace LoaderLibrary
{
    public class AssemblyLoader : MarshalByRefObject
    {
        string assemblyName;
        public AssemblyLoader(string assName, byte[] ass)
        {
            assemblyName = assName;
            AppDomain.CurrentDomain.Load(ass);
            Console.WriteLine(AppDomain.CurrentDomain.FriendlyName + " " + AppDomain.CurrentDomain.SetupInformation.LoaderOptimization.ToString());
        }

        public object Load(string className)
        {
            object ret = null;
            try
            {
                ret = AppDomain.CurrentDomain.CreateInstanceAndUnwrap(assemblyName, className);
            }
            catch (System.Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            return ret;
        }
    }
}

Here I set LoaderOptimizationAttribute on main() method but AppDomain.CurrentDomain.SetupInformation.LoaderOptimization.ToString(); says it is NotSpecified Why?
The differences between MultiDomain and MultiDomainHost is not so clear to me. Is MultiDomainHost for only GAC assemblies? For my situation which is more suitable?
According to this

JIT-compiled code cannot be shared for assemblies loaded into the load-from context, using the LoadFrom method of the Assembly class, or loaded from images using overloads of the Load method that specify byte arrays.

So how can I detect if an assembly is loaded domain-neutral or not? How can assure I it is loaded domain-neutral?

This attribute has only an effect if you precompile your assemblies with NGen to speed up a warm start of your application. When you specify MultiDomain or MultiDomainHost you enable the usage of precompiled (ngenned) assemblies. You can verify this with Process Explorer where you can look at the list of loaded modules.

This is one of the biggest startup time savers if your application consists of several executable instances which share assemblies. This enables .NET to share the code pages between processes which in turn saves real memory (one assembly exists only once in the physical memory but it is shared between one or more processes) and prevents JITing the same code over and over again in each process which takes time at the cost that the generated code is a little less efficient as it could be when it would be compiled with the regular JIT which can use more dynamic data to generate the most efficient code.

In your example you load the assembly into a byte array which is located in the managed heap and increases your private byte count. This makes it impossible to share data between processes. Only read only pages which have a counterpart on your hard disc can be shared between processes. This is the reason why the attribute has no effect. If you are after a factor 2 of warm startup performance this is the attribute you were seeking for. For anything else it is not relevant.

Now back to your original question:

It is set but when you start your application under a debugger this MultiDomain attribute is ignored. When you start it outside of a debugger you will get the expected results.
Yes MultiDomainHost does enable AppDomain neutrality only for signed assemblies all others are not shared.
Code sharing can only happen when it is precompiled. The real question is: How to check if the assembly is precompiled? I do it with Process Explorer by looking at the list of loaded modules. When my loaded assembly shows up with a path to the Native Image cache and an .ni extension I am sure the precompiled image is beeing used. You can check this also with fuslogvw when you set the radio button to Native Images to check why a native images was not used by the runtime.

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