ajaxToolkit:SlideShowExtender怎么显示当前图片第几张？

HTML code

<!--这个两个literal用来显示当前图片数和图片总数-->
<asp:Literal ID="Literal_currNum" runat="server"></asp:Literal></span>/<asp:Literal ID="Literal_Count" runat="server"></asp:Literal>

<ajaxToolkit:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</ajaxToolkit:ToolkitScriptManager>
<div>
        <asp:Label ID="Label_picTitle" runat="server" Visible="false"></asp:Label>
        <asp:Image ID="Image1" runat="server" Height="228px" Width="284px"  ImageUrl="images/01.jpg" BorderWidth="1px" BorderStyle="solid" />
        <asp:Label ID="Label_picIntro" runat="server"></asp:Label>
        <ajaxToolkit:SlideShowExtender ID="SlideShowExtender1" runat="server" 
                    TargetControlID ="Image1" 
                    ImageDescriptionLabelID ="Label_picIntro" 
                    ImageTitleLabelID ="Label_picTitle"  
                    SlideShowServicePath="ImagesService.asmx" 
                    SlideShowServiceMethod="GetSlides" 
                    AutoPlay="false" 
                    Loop="false"      
                    PlayInterval="2000"  
                    NextButtonID="Next"
                    PreviousButtonID="Prev" UseContextKey="True" >
        </ajaxToolkit:SlideShowExtender>
        <asp:Button ID="Prev" runat="server" Text="上一张" OnClick="Prev_Click" />
        <asp:Button ID="Next" runat="server" Text="下一张" OnClick="Next_Click" />
        <asp:Button ID="Stop" runat="server" Text="停" />
</div>

图片正常显示，下一张，上一张均没问题，问题是我在下一张 写了Next_Click事件（事件里面去改写当前图片序号），
这个Next_Click事件不执行，已经单步跟踪过，Prev_Click也是一样。为何？

可有知晓的？