AppDomain.Unload_MarshalByRefObject

        internal string GetClassInfo(string assemblyName, string className, string strField)
        {
            string ret = strField;
            if (!String.IsNullOrEmpty(assemblyName) && !String.IsNullOrEmpty(className))
            {
                string callingDomainName = AppDomain.CurrentDomain.FriendlyName;//Thread.GetDomain().FriendlyName;
                AppDomain ad = AppDomain.CreateDomain("Test Domain");
                ProxyObject obj = (ProxyObject)ad.CreateInstanceFromAndUnwrap(callingDomainName, "XXNamespace.ProxyObject");
                obj.LoadAssembly(assemblyName);
                //obj.Invoke("TestDll.Class1", "Test", "It's a test");
                ret = obj.GetClassInfo(className, strField);
                AppDomain.Unload(ad);
            }
            return ret;

        }

    class ProxyObject : MarshalByRefObject
    {
        Assembly assembly = null;
        public void LoadAssembly(string assemblyName)
        {
            assembly = Assembly.LoadFile(assemblyName);
        }

        public string GetClassInfo(string className, string strField)
        {
            string ret = strField;
            Type type = assembly.GetType(className, true, true);
            if (type != null)
            {
                FieldInfo fieldinfo = type.GetField(strField, BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Public | BindingFlags.Static);
                //return (String)fieldinfo.GetValue(assembly);
                ret = (String)fieldinfo.GetValue(null);
            }
            return ret;
        }

        public bool Invoke(string fullClassName, string methodName, params Object[] args)
        {
            if (assembly == null)
                return false;
            Type tp = assembly.GetType(fullClassName);
            if (tp == null)
                return false;
            MethodInfo method = tp.GetMethod(methodName);
            if (method == null)
                return false;
            Object obj = Activator.CreateInstance(tp);
            method.Invoke(obj, args);
            return true;
        }
    }