一个gridview的DropDownList绑定问题！！！

我在gridview的编辑模板中放了一个DropDownList控件,控件是绑定数据库中的数据，当点击编辑铵钮时显示dropdownlist。数据绑定是实现了，可是在点击gridview中编辑按钮时dropdownlist中默认选中的不是该列的值。在更新时也得不到dropdownlist的selectedvalue值。不知为何，请各位帮看看，谢谢了~
代码如下：
<asp:GridView ID="gridMember" runat="server" AllowSorting="True" AutoGenerateColumns="False" Width="75%" DataKeyNames="proID" AllowPaging="True" OnPageIndexChanging="gridMember_PageIndexChanging" OnRowDataBound="gridMember_RowDataBound" OnRowEditing="gridMember_RowEditing" OnRowUpdating="gridMember_RowUpdating" OnRowCancelingEdit="gridMember_RowCancelingEdit">
                    <Columns>
                     <asp:TemplateField HeaderText="产品品牌">
                     <ItemTemplate><%#Eval("brandN")%></ItemTemplate>
                     <EditItemTemplate>
                     <asp:HiddenField runat="server" ID="hidtxt" Value='<%#Eval("brandN")%>' />
                   <asp:DropDownList runat="server" id="drop"></asp:DropDownList>
                     </EditItemTemplate>
                     </asp:TemplateField>
</Columns></asp:GridView>

后台代码：
protected void gridMember_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        string str = "select * from brands";
        DataSet dss = new DataSet();
        dss = dataFactory.ds(str, dss, "tb");
        if (((DropDownList)e.Row.FindControl("drop")) != null)
        {
            DropDownList drp = (DropDownList)e.Row.FindControl("drop");

            //  生成 DropDownList 的值，也可以取得数据库中的数据绑定
            drp.DataSource = dss.Tables["tb"];
            drp.DataTextField = "brand";
            drp.DataValueField = "brandID";
            drp.DataBind();
            //  选中 DropDownList
            drp.SelectedValue = ((HiddenField)e.Row.FindControl("hidtxt")).Value.ToString();
            //
        }
    }
更新代码：
protected void gridMember_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        string drp = ((DropDownList)gridMember.Rows[e.RowIndex].FindControl("drop")).SelectedValue;
        string str = "update proType set brand=@a1 where proID="+gridMember.DataKeys[e.RowIndex].Value+"";
        OleDbCommand cmd = new OleDbCommand(str,conn.connData());
        cmd.Parameters.Add("a1", OleDbType.Integer).Value = drp;
        conn.Open();
        cmd.ExecuteNonQuery();
        conn.Close();
        gridMember.EditIndex = -1;
        Init();
    }