public IActionResult OnPostBuy(double?ServicePrice, double?ProductPrice) { if (ModelState.IsValid) { var customer = customerData.GetCustomerById(Visit.CustomerId.Value); Visit.Customer = customer; if (ProductPrice.HasValue && ProductPrice.Value > 0) { Visit.BuyProduct(ProductPrice.Value); ProductPrice = 0; } if (ServicePrice.HasValue && ServicePrice.Value > 0) { Visit.BuyService(ServicePrice.Value); ServicePrice = 0; } } Message = Visit.CustomerId == null ? "No customer selected." : $"Total expences: {Visit.TotalExpences()}"; var customers = customerData.GetCustomers().ToList().Select(p => new { Id = p.Id, Display = $"{p.FirstName} {p.LastName}" }); Customers = new SelectList(customers, "Id", "Display"); return(Page()); }