class ProxyObject : MarshalByRefObject
    {
        Assembly assembly = null;
        public void LoadAssembly(string choice)
        {
            //获取dll所在路径
            string dllPath = Config.GetValueByKey(choice, "dllPath");

            //获取dll名称/exe名称
            string dllName = Config.GetValueByKey(choice, "dllName");

            assembly = Assembly.LoadFile(AppDomain.CurrentDomain.BaseDirectory + dllPath + dllName);
        }
    }

ad = AppDomain.CreateDomain("DLL Unload  " + choiceOpiton);

ProxyObject instance = (ProxyObject)ad.CreateInstanceFromAndUnwrap("dll全路径","Proxy所在类名");