div>
            <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:Panel ID="Panel1" runat="server" Width="161" Height="127" 
                    BorderColor="#C2C2C2" BorderStyle="Dashed" BorderWidth="1px">
                <br />
                <br />
                学习AJAX<br/>
                就去playaspx.com</asp:Panel>
                    <cc1:AlwaysVisibleControlExtender ID="AlwaysVisibleControlExtender1" 
                    runat="server" TargetControlID="Panel1" VerticalSide="Middle">
                    </cc1:AlwaysVisibleControlExtender><asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True" 
                    onselectedindexchanged="DropDownList1_SelectedIndexChanged">
                    <asp:ListItem Value="None">默认</asp:ListItem>
                    <asp:ListItem Value="00">左上</asp:ListItem>
                    <asp:ListItem Value="01">左中</asp:ListItem>
                    <asp:ListItem Value="02">左下</asp:ListItem>
                    <asp:ListItem Value="10">中上</asp:ListItem>
                    <asp:ListItem Value="11">中中</asp:ListItem>
                    <asp:ListItem Value="12">中下</asp:ListItem>
                    <asp:ListItem Value="20">右上</asp:ListItem>
                    <asp:ListItem Value="21">右中</asp:ListItem>
                    <asp:ListItem Value="22">右下</asp:ListItem>
                </asp:DropDownList>
            </ContentTemplate>
            </asp:UpdatePanel>

 protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        switch (DropDownList1.SelectedValue[0].ToString())
        { 
            case "0":
                AlwaysVisibleControlExtender1.HorizontalSide = HorizontalSide.Left;
                break;
            case "1":
                AlwaysVisibleControlExtender1.HorizontalSide = HorizontalSide.Center;
                break;
            case "2":
                AlwaysVisibleControlExtender1.HorizontalSide = HorizontalSide.Right;
                break;
            default:
                AlwaysVisibleControlExtender1.HorizontalSide = HorizontalSide.Center;
                break;
        }
        switch (DropDownList1.SelectedValue[1].ToString())
        { 
            case "0":
                AlwaysVisibleControlExtender1.VerticalSide = VerticalSide.Top;
                break;
            case "1":
                AlwaysVisibleControlExtender1.VerticalSide = VerticalSide.Middle;
                break;
            case "2":
                AlwaysVisibleControlExtender1.VerticalSide = VerticalSide.Bottom;
                break;
            default:
                AlwaysVisibleControlExtender1.VerticalSide = VerticalSide.Middle;
                break;
        }
    }