public IActionResult OnPost() { if (ModelState.IsValid) { var customer = customerData.GetCustomer(Visit.CustomerId.Value); Visit.Customer = customer; if (visitTP.TotalPrice(Visit) == 0) { TempData["Message2"] = "Please buy something next time !"; return(RedirectToPage("./List")); } Visit = visitData.Add(Visit); visitData.Commit(); TempData["Message2"] = "Thank you for your purchase !"; return(RedirectToPage("./List")); } Customers = customerData.GetCustomers().Select(x => new SelectListItem { Value = x.Id.ToString(), Text = x.Name }).ToList(); return(Page()); }