Repository don't return value from my Test Method

I'm new in Unit Test and i'm trying to write a unit test for the below method of my serive

     public InventoryViewModel GetInventory(DateTime startDate, DateTime endDate, long roomServiceId)
            {
                InventoryViewModel inv = new InventoryViewModel();
                var roomService = _unitOfWork.RoomServiceRepository.GetByID(roomServiceId);
                if (roomService == null)
                    throw new InvalidOperationException("there is not the roomService");


                ....

                }

this method works correctly, but when i call this method from my TestMethod, RoomServiceRepository don't return the roomService, my testMehtod is like this :

     [TestClass]
        public class InventoryTest
        {

            private UnityContainer _container;
            readonly MockObjectsSetup _mos = new MockObjectsSetup();

            private IInventoryService _inventoryService;


            [TestInitialize]
            public void SetupTest()
            {
                _container = new UnityContainer();
                _mos.Setup(_container);
                Config.UnityTestConfig.RegisterTypes(_container);
                _container.RegisterType<IInventoryService, InventoryService>();
                _inventoryService = (InventoryService)_container.Resolve(typeof(InventoryService));
            }

    [TestMethod]
            public void Dont_Change_NotSelected_PriceValues()
            {
                DateTime startDate = DateTime.Parse("6/21/2016");
                DateTime endDate = DateTime.Parse("6/22/2016");

                long roomServiceId = 17;
                InventoryViewModel invViewModel = new InventoryViewModel();

                invViewModel.isUpdatingBoardPrice = invViewModel.isUpdatingPrice = invViewModel.isUpdatingFloatAvailability = true;
                invViewModel.isUpdatingCertainAvailability = false;
                invViewModel.StartDate = startDate;
                invViewModel.EndDate = endDate;
                invViewModel.RoomServiceId = roomServiceId;
                invViewModel.CertainAvailability = 0;
                invViewModel.FloatAvailability = 8;

                var inv = _inventoryService.GetInventory(startDate, endDate, roomServiceId);

               ....
            }

}