Null Reference exception when I use the navigation property in the model class

Question

I am trying to add a new object to a database in a controller action.
This is my model class.

    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; }
}

These are the steps to create a new product in the database.

    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");
    }

CreateView has an appropriate model type (product type in this case). Code below

    @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>
}                                   

I always get a null reference instead of the model in action [HttpPost], but if I use ViewModel instead of Model, everything is fine (ViewModel code below)

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

I think this causes the model class to have virtual properties, but in any case, I don’t understand why this is normal when I use ViewModel. Can anyone answer me? thanks in advance.

Answer 1

virtual . , , . ; .