在System.Web.Mvc.SelectListItem类中找不到“Disabled”属性

我从CodePlex下载并检查了微软的MVC源代码。根据源代码,类定义如下:

namespace System.Web.Mvc
{
    public class SelectListItem
    {
        /// <summary>
        /// Gets or sets a value that indicates whether this <see cref="SelectListItem"/> is disabled.
        /// </summary>
        public bool Disabled { get; set; }

        /// <summary>
        /// Represents the optgroup HTML element this item is wrapped into.
        /// In a select list, multiple groups with the same name are supported.
        /// They are compared with reference equality.
        /// </summary>
        public SelectListGroup Group { get; set; }

        public bool Selected { get; set; }

        public string Text { get; set; }

        public string Value { get; set; }
    }
}

但当我编写以下代码时

List<System.Web.Mvc.SelectListItem> statuses = (from s in DataContext.OfferStatuses
                                                        select new System.Web.Mvc.SelectListItem
                                                                  {
                                                                      Value = s.Id.ToString(),
                                                                      Text = s.Code,
                                                                  }).ToList();

在类属性定义中,我看不到“Disabled”属性。此外,当我遍历状态集合时,仍然无法看到该属性。

为什么我看不到一些定义为公共的财产?