下拉列表不分配所选值

我看过类似的问题，找不到一个能帮助我的答案。我在我的模型方法中分配SelectListItem Selected属性。我观察模型，因为它进入视图，它有选择的值。但是当它在视图中输出时，我只选择了第一个值。

模型:

public class ProductModel
{
    public int ID { get; set; }
    [Required(ErrorMessage = "Required")]
    [Index("ItemNumber", 1, IsUnique = true)]
    [Display(Name = "Item #")]
    public int itemNumber { get; set; }
    [Required(ErrorMessage = "Required")]
    [Display(Name = "Product")]
    [MaxLength(50)]
    public String product { get; set; }
    [Display(Name = "Description")]
    [MaxLength(500)]
    public String description { get; set; }
    [DefaultValue(true)]
    [Display(Name = "Active?")]
    public bool active { get; set; }
    [Display(Name = "Image Name")]
    public String imageName { get; set; }
    [Display(Name = "PDF Name")]
    public String PDFName { get; set; }
    [ForeignKey("Category")]
    public int CategoryID { get; set; }
    public virtual CategoryModel Category { get; set; }
    public IEnumerable<SelectListItem> CategoryList { get; set; }
    public static IEnumerable<SelectListItem> getCategories(int id = 0)
    {
        using (var db = new ProductContext())
        {
            List<SelectListItem> list = new List<SelectListItem>();
            var x = db.Categories.ToList();
            foreach (var y in x)
            {
                //SelectListItem sli = new SelectListItem { Value = y.ID.ToString(), Text = y.categoryName };
                SelectListItem sli = new SelectListItem { Value = y.ID.ToString(), Text = y.categoryName };
                if (id > 0 && y.ID == id)
                {
                    sli.Selected = true;
                }
                list.Add(sli);
            }
            return list;
        }
    }
    public ProductModel()
    {
        active = true;
    }
}

控制器方法赋值返回值:

public ActionResult EditProduct(int? id)
    {
        ProductModel model = new ProductModel();
        if (id != null)
        {
            using (var db = new ProductContext())
            {
                if (id != 0)
                {
                    model = db.Products.Find(id);
                }
                if (Session["dropdownID"] != null && Session["dropdownID"].ToString().Trim() != "")
                {
                    int dropdownID = Int32.Parse(Session["dropdownID"].ToString());
                    model.CategoryList = ProductModel.getCategories(dropdownID);
                }
                else
                {
                    model.CategoryList = ProductModel.getCategories();
                }
            }
        }
        return PartialView(model);
    }

视图代码:

<div class="form-group">
                @Html.LabelFor(model => model.Category, new { @class = "control-label col-md-8" })
                <div class="col-md-16">
                    @Html.DropDownListFor(model => model.CategoryID, Model.CategoryList, new { @class = "form-control"})
                    @Html.ValidationMessageFor(model => model.CategoryID)
                </div>
            </div>