SelectedValue的问题，是否是因为缺少指令集？

页面的代码如下：
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
public partial class Products : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
            int CurrPage = 0;
            string sSql = "";            ////Response.Write("IsPostBack: " + IsPostBack.ToString());            if (Master.SelectedValue == "")
                sSql = "SELECT ProductID, Name, ProductNumber, Color , Price FROM Product";
            else
                sSql = "SELECT ProductID, Name, ProductNumber, Color , Price FROM Product" + " where ProductSubcategoryID=" + Master.SelectedValue;            //Response.Write("<br>"+sSql);            DataSet ds = DataAccessHelper.RunQuery(sSql);            PagedDataSource pds = new PagedDataSource();
            pds.DataSource = ds.Tables[0].DefaultView;
            pds.AllowPaging = true;
            pds.PageSize = 18;            if (IsPostBack)
            {
                CurrPage = 1;
            }
            else
            {
                if (Request.QueryString["Page"] != null)
                    CurrPage = Convert.ToInt32(Request.QueryString["Page"]);
                else
                    CurrPage = 1;            }
            pds.CurrentPageIndex = CurrPage - 1;            //lblCurrPage.Text = "当前页： " + CurrPage.ToString();            if (CurrPage < pds.PageCount)
            {
                int NextPage = CurrPage + 1;
                lnkNext.NavigateUrl = "Products.aspx?Page=" + NextPage.ToString();
            }
            if (CurrPage > 1 && CurrPage < pds.PageCount)
            {
                int PrevPage = CurrPage - 1;
                lnkPrev.NavigateUrl = "Products.aspx?Page=" + PrevPage.ToString();
            }
            DataList1.DataSource = pds;
           DataList1.DataBind();        }
    }
}SelectedValue下一直有下划线，显示System.Web.UI.MasterPage不包含SelectedValue的定义。