MVC RC2中关于HtmlHelper给DropDownList设置初始选中值的问题

/// <summary>
 /// 生成下拉列表
 /// </summary>
 /// <param name="helper"></param>
 /// <param name="SelectListName">下拉列表的Name值</param>
 /// <param name="SelectItems">数据源</param>
 /// <param name="SelectedValue">选中值</param>
 /// <param name="Attributes">附加属性值，比如onchange="...."之类</param>
 /// <returns></returns>
 public static string DropDownList(this HtmlHelper helper, string SelectListName, IEnumerable<SelectListItem> SelectItems, string SelectedValue, string Attributes)
        {
            StringBuilder sb = new StringBuilder();
            sb.Append("<select");

 if (SelectListName.Trim() != "")
            {
                sb.Append(" name=\"" + SelectListName + "\"");
            }
 else
            {
 return "";
            }

 if (Attributes.Trim() != "")
            {
                sb.Append(" " + Attributes.Trim());
            }


            sb.Append(">");

 foreach (SelectListItem item in SelectItems)
            {
 if (item.Value == SelectedValue)
                {
                    sb.Append("<option value=\"" + item.Value + "\" selected=\"selected\">" + item.Text + "</option>");
                }
 else
                {
                    sb.Append("<option value=\"" + item.Value + "\">" + item.Text + "</option>");
                }
            }

            sb.Append("</select>");

 return sb.ToString();

        }

List<SelectListItem> _items = new List<SelectListItem>();
....
ViewData["Citys"] = _items;

<%=Html.DropDownList("SelName",  (IEnumerable<SelectListItem>)
ViewData["Citys"],item.CityId.ToString(),"onchange=\"alert('Test')\"") %>