网格视图排序箭头未显示

Question

我正在对网格进行排序view.The排序工作perfect.Now我正在尝试将排序箭头添加到每个标题列旁边。我尝试了几乎所有的方法，但箭头不显示在我的UI.PFB中我的代码：

CSS：

th .ascending a {
    background: url(images/ascArrow.gif) no-repeat;
    display: block;
    padding: 0 4px 0 15px;
}

th .descending a {
    background: url(images/descArrow.gif) no-repeat;
    display: block;
    padding: 0 4px 0 15px;
}

代码隐藏：

protected void RPMData_Sorting(object sender, GridViewSortEventArgs e)
    {
        if (TextData.Text == String.Empty)
        {
            SqlCommand cmd = new SqlCommand("select Customer_Name,Site_Type,Source,Destination,Latency,Jitter_Priority_Real_Time,Jitter_Real_Time,PacketLoss_Priority_Real_Time,PacketLoss_Real_Time,PacketLoss_Other_Classes from RPM", con);
            con.Open();
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            da.Fill(dt1);
            con.Close();
            if (dt1.Rows.Count > 0)
            {
                string sortingDirection = string.Empty;
                if (direction == SortDirection.Ascending)
                {
                    direction = SortDirection.Descending;
                    RPMData.HeaderStyle.CssClass = "descending";
                    sortingDirection = "Desc";
                }
                else
                {
                    direction = SortDirection.Ascending;
                    RPMData.HeaderStyle.CssClass = "ascending";
                    sortingDirection = "Asc";
                }

                DataView sortedView = new DataView(dt1);
                sortedView.Sort = e.SortExpression + " " + sortingDirection;
                Session["SortedView"] = sortedView;
                RPMData.DataSource = sortedView;
                RPMData.DataBind();
            }
        }

//根据应用的过滤器，排序逻辑有两个条件。我在这里只添加了第一个条件

GridView标签：

<asp:GridView CssClass="infoTable" ID="RPMData" runat="server" OnSelectedIndexChanged="Button1_Click" AllowPaging="True" PageSize="10" OnPageIndexChanging="RPMData_PageIndexChanging" AllowSorting="True" OnSorting="RPMData_Sorting" AutoGenerateColumns="False" SortedAscendingHeaderStyle-CssClass="ascending" SortedDescendingHeaderStyle-CssClass="descending">
    <HeaderStyle CssClass="ascending" />
        <Columns>

请帮帮我。