我正在填充这样的下拉列表:
SPList categories = SPContext.Current.Web.Lists["Categories"];
dropdownlist.DataSource = categories.Items;
dropdownlist.DataValueField = "ID";
dropdownlist.DataTextField = "Title";
dropdownlist.DataBind();
假设用户不想从下拉列表中选择任何值,但想选择一个空选项。我如何添加一个空的替代下拉列表?