ajaxtoolkit：CalendarExtender无法在asp.net网页上工作

Question

我想在单击图像按钮时显示日历，所选日期将显示在文本框中。我使用了ajaxtoolkit，但是有错误显示不是已知元素（ToolkitScriptManager，CalendarExtender）。

的web.config：

<system.web>
  <compilation debug="true" targetFramework="4.5" />
  <httpRuntime targetFramework="4.5" />
  <pages>
    <controls>
      <add tagPrefix="ajaxToolkit" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit"/>
    </controls>
  </pages>
</system.web>

我的页面：

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit" %>

<table class="auto-style18">
                    <tr>
                        <td class="auto-style19">   
                        <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"></asp:ToolkitScriptManager>   <- error occur there(show not known element)                         
                        <asp:TextBox ID="txtExpiredDate" runat="server" Width="260px"></asp:TextBox>
                        &nbsp; </td>
                        <td>
                        <asp:ImageButton ID="ImageButton1" runat="Server" AlternateText="Click here to display calendar" Height="21px" ImageUrl="~/Images/calender.png" Width="26px" />                                    
                        <asp:CalendarExtender  ID="CalendarExtender1"  TargetControlID="txtExpiredDate" runat="server" />  <- error occur there(show not known element)                         
                        </td>
                    </tr>
                </table>