asp.net mvc SelectList 的selected 失效及解決方案

在asp.net mvc的強型別視圖（strongly typed view）下拉式清單，在Controller中設定的選擇項在View上選中失效了。
下面是My Code

AllUnSendMail
        [AcceptVerbs(HttpVerbs.Post)]
        public ActionResult AllUnSendMail(FormCollection collection)
        {
            int sysId = Convert.ToInt32(collection["ddlSysid"]);
            int selectedVal =-1;
            List<SystemInfoEntity> allSys = SystemInfoClient.GetAll();
            allSys.Insert(0, new SystemInfoEntity
            {
                SysID = -1,
                Name = "請選擇"
            });
            if (sysId > 0)
            {
                selectedVal = sysId;
            }
            SelectList serviceList = new SelectList(allSys, "SysID", "Name", selectedVal);
            ViewData["ddlDataSysid"] = serviceList;
            ViewData["Subject"] = string.Empty;
            var viewData = MessageQueries.GetAllUnSendMails(sysId, 1, 15);
            return View(viewData);
        }

相應的視圖的下列列表 

<%=Html.DropDownList("ddlDataSysid", (SelectList)ViewData["ddlDataSysid"])%>

問題就出在DropDownList的Name和ViewData相同了，只要修改一下Name如下就可以了

  <%=Html.DropDownList("ddlSysid", (SelectList)ViewData["ddlDataSysid"])%>