private readonly IService _Service;
            public LoginViewModel(IService Service=null) //Also i want to make this innjection optional so i set it to NULL
            {
                _Service = Service;
            }

            private void MethodWhereIWantToUseService()
            {
//_service is coming  NULL because configuration or mapping of injection calls after this
               _Service.MYMethod();
            }

public class ServicesModule : IModule
    {
        public void Initialize()
        {
            var container = ServiceLocator.Current.GetInstance<IUnityContainer>();
            container.RegisterType<IService, Service>();

        }
    }

public class Bootstrapper : UnityBootstrapper
        {
             protected override void ConfigureModuleCatalog()
                {
                    base.ConfigureModuleCatalog();
                    ModuleCatalog catalog = (ModuleCatalog)this.ModuleCatalog;
                    catalog.AddModule(typeof(ServicesModule));

                }
        }