asp.net中根据数据库中数据自动设置dropdownlist中选中的值

//前台

 <asp:DropDownList ID="ddlmodel" runat="server" AutoPostBack="True"
                  onselectedindexchanged="ddlmodel_SelectedIndexChanged">
               <asp:ListItem Value="2">两</asp:ListItem>
                    <asp:ListItem Value="3" Selected="True">三</asp:ListItem>
              </asp:DropDownList>

//后台

SqlSen ss = new SqlSen();//调用APP_CODE中函数

            string text1, text2, text3, size1, size2, size3, width1, width2, width3;
            ss.SELIDData(id, out text1, out text2, out text3, out size1, out size2, out size3, out width1, out width2, out width3);

            this.t1.Text = text1;
            this.t2.Text = text2;
            this.t3.Text = text3;
            if (this.t3.Text.Equals(""))
            {
                ddlmodel.SelectedValue = "2";
                this.t3.Enabled = false;
            }
            if (this.t3.Text != "")
            {
                ddlmodel.SelectedValue = "3";
                this.t3.Enabled = true;
            }
            ddlSize1.SelectedValue = ddlSize1.Items.FindByValue(size1).ToString();//将数据库中的数据调给dropdownlist中,使其有自动选择效果
            ddlSize2.SelectedValue = ddlSize2.Items.FindByValue(size2).ToString();
            ddlSize3.SelectedValue = ddlSize3.Items.FindByValue(size3).ToString();