C# 在模型类中使用导航属性时出现空引用异常

我尝试在控制器操作中在数据库中添加新实体。
这是我的模型课

    public class Product
    {
    public int ProductID { get; set; }

    [Required(ErrorMessage = "Please enter product name")]
    public string Name { get; set; }

    [Required(ErrorMessage = "Please enter product model")]
    public string Model { get; set; }

    [Required(ErrorMessage = "Please enter product serial")]
    public string Serial { get; set; }

    [Required(ErrorMessage = "Please choose dealer")]      
    public int DealerID { get; set; }

    [Required]        
    public Guid ClientID { get; set; }

    [Required(ErrorMessage = "Please choose employee")]
    public Guid EmployeeID { get; set; }

    public virtual Dealer Dealer { get; set; }
    public virtual Client Client { get; set; }
    public virtual Employee Employee { get; set; }

    [DisplayName("Commercial use")]
    public bool UseType { get; set; }
}

这是在数据库中创建新产品的操作

    public ViewResult Create()
    {
        PopulateDropDownLists();
        var model = new Product();
        return View(model);
    }

    [HttpPost]
    public ActionResult Create(Product model)
    {
        try
        {
            if (ModelState.IsValid)
            {
                _repo.GetRepository<Product>().Add(model);
                _repo.Save();
                TempData["message"] = "Product was successfully created";
                return RedirectToAction("List");
            }
        }
        catch (DataException)
        {
            TempData["error"] =
                "Unable to save changes. Try again, and if the problem persists, see your system administrator.";
            return View("Error");
        }

        PopulateDropDownLists();
        return View("Create");
    }

我认为这是因为模型类具有虚拟属性，但无论如何，我不明白为什么在使用ViewModel时它是可以的。 有人能回答我吗？

---

提前Thx。

虚拟属性不会改变结果。问题在于，视图是为了绑定到视图模型而编写的，因此接受模型是行不通的。如果要使用该模型；然后将视图绑定到模型。

我认为您也应该将视图放在模型上！！那么，如果您从属性中删除“virtual”会发生什么呢？如果我删除virtual属性，我可以使用model类，并且不需要编辑ViewModelQuestion就可以了。当我使用模型类ViewCreate时，我将视图代码放在其中，ViewCreate具有适当的类型（产品）。使用ViewModel时，ViewCreate具有ProductViewModel类型。

    @using System.Web.Mvc.Html
    @model STIHL.WebUI.Models.Product

    @using (Html.BeginForm())
    {
       @Html.EditorFor(m => m.Name)
       @Html.EditorFor(m => m.Model)
       @Html.EditorFor(m => m.Serial)

    <div class="form-group">
        @Html.LabelFor(m => m.DealerID, "Dealer")
        @Html.DropDownListFor(m => m.DealerID, new SelectList((IEnumerable)TempData["Dealers"],"DealerID", "DealerNumber"), string.Empty, new {@class = "form-control"})
        @Html.ValidationMessageFor(m => m.DealerID, null, new {@class = "help-block"})
    </div>

    <div class="form-group">
        @Html.LabelFor(m => m.EmployeeID, "Employee",new {@class = "control-label"})
        @Html.DropDownListFor(m => m.EmployeeID, new SelectList((IEnumerable)TempData["Employees"],"EmployeeID", "FullName"),string.Empty, new {@class="form-control"})
        @Html.ValidationMessageFor(m => m.EmployeeID, null, new {@class = "help-block"})       
    </div>

    <div class ="ok-cancel-group">
        <input class="btn btn-primary" type="submit" value="Create" />
        @Html.ActionLink("Cancel", "List","Product",new {@class = "btn btn-primary"})
    </div>
}                                   

public class ProductViewModel
{
    public Product Product { get; set; }
}