在C# MVC中的表单验证中使用外键对象

public class Order
{
    public int OrderId { get; set; }
    public string OrderName { get; set; }
    public int CustomerId { get; set; }
    public Customer Customer { get; set; }
}

public class Customer
{
    public int CustomerId { get; set; }
    public string CustomerName { get; set; }
}

@model Order

@using (Html.BeginForm())
{
    @Html.LabelFor(model => model.OrderName)
    @Html.TextBoxFor(model => model.OrderName)
    
    @Html.LabelFor(model => model.CustomerId)
    @Html.DropDownListFor(model => model.CustomerId, new SelectList(ViewBag.Customers, "CustomerId", "CustomerName"), "Select Customer")
    
    <input type="submit" value="Submit" />
}

public class OrderController : Controller
{
    private readonly ApplicationDbContext _context;

    public OrderController(ApplicationDbContext context)
    {
        _context = context;
    }

    public IActionResult Create()
    {
        ViewBag.Customers = _context.Customers.ToList();
        return View();
    }

    [HttpPost]
    public IActionResult Create(Order order)
    {
        if (ModelState.IsValid)
        {
            _context.Orders.Add(order);
            _context.SaveChanges();
            return RedirectToAction("Index", "Home");
        }

        ViewBag.Customers = _context.Customers.ToList();
        return View(order);
    }
}

public class Order
{
    public int OrderId { get; set; }
    
    [Required(ErrorMessage = "Order Name is required.")]
    public string OrderName { get; set; }
    
    [Required(ErrorMessage = "Customer is required.")]
    public int CustomerId { get; set; }
    
    public Customer Customer { get; set; }
}

public class Customer
{
    public int CustomerId { get; set; }
    
    [Required(ErrorMessage = "Customer Name is required.")]
    public string CustomerName { get; set; }
}