页面刷新时下拉列表值更改

Question

`我有一个带有复选框的Gridview。网格是根据两个下拉列表的值加载的。下拉列表中的值基于Web服务加载。

现在我的问题是，当选中该复选框时，刷新页面并将dropdownlist中的值设置为default。也就是说，在刷新页面的任何情况下，dropdownlists值都设置为default。只有下拉列表值更改，其他字段值保持不变。我尝试使用updatepanel但它没有工作。

任何帮助都将非常感激。

                                <td>
    <asp:DropDownList ID="ddl_Prov" CssClass="txtboxAll" Width="260px" 
    runat="server"> </asp:DropDownList>
    <asp:CascadingDropDown ID="CascadingDropDown1" runat="server"
    Category="facil" TargetControlID="ddl_Prov" PromptText="[SelectGroup]"
    LoadingText="Loading  Group..." ServicePath="~/Services/eservice.asmx"
    ServiceMethod="GetDropDownProgroup"></asp:CascadingDropDown>
    </td>
    <td>

   <asp:DropDownList ID="ddl_Pro" CssClass="txtboxAll" Width="260px"
   runat="server" ></asp:DropDownList>
   <asp:CascadingDropDown ID="cdd_fa" runat="server" Category="facility"  
   ParentControlID="ddl_Prov"  TargetControlID="ddl_Pro" 
   PromptText="[Select Pr]" LoadingText="Loading Provider..."
  ServicePath="~/Services/Freservice.asmx" 
  ServiceMethod="GetDropDownFacilitybyGr">
  </asp:CascadingDropDown> 
       </td>

            <asp:UpdatePanel ID="UpdatePanel1" runat="server" 
            UpdateMode="Conditional">
            <ContentTemplate>
            <asp:GridView ID="grd_ViewPayments" DataKeyNames="IN077_01" 
              runat="server" Width="100%"  
             DataSourceID="SqlDataSource1" AllowPaging="true" 
              showfooter="true" PageSize="20" AutoGenerateColumns="False" 
              OnSelectedIndexChanged="grd_ViewPayments_SelectedIndexChanged" 
              OnRowDataBound="grd_ViewPayments_RowDataBound">
            <SelectedRowStyle BackColor="#D8E4F2" />
                            <HeaderStyle cssclass="Header" />
                 <FooterStyle cssclass="Header" HorizontalAlign="Center" />
                             <RowStyle cssclass="rowsgrd" />
                            <HeaderStyle cssclass="Header" />
                            <PagerSettings FirstPageText="First" LastPageText="Last" Mode="NumericFirstLast" PageButtonCount="3" position="Bottom" />  
                            <PagerStyle HorizontalAlign="Center"  VerticalAlign="Middle" BorderColor="#36b0b6" BorderStyle="Dotted" BorderWidth="1px" BackColor="#E1E1E1" />

                <Columns>
                     <asp:TemplateField HeaderStyle-HorizontalAlign="Center" HeaderText="SlNo." 
                                   ItemStyle-HorizontalAlign="Center" ItemStyle-Width="30px">
                        <ItemTemplate>
                            <%# Container.DataItemIndex + 1%>
                        </ItemTemplate>
                        <HeaderStyle HorizontalAlign="Center" Width="3%" />
                        <ItemStyle HorizontalAlign="Center" />
                    </asp:TemplateField>
                    <asp:BoundField DataField="pid" HeaderText="VID" 
                        SortExpression="pid" >
                        <ItemStyle HorizontalAlign="Center" Width="150px" />
                                                                    </asp:BoundField>

                   <asp:BoundField DataField="pname"  HeaderText="Pr Name" ItemStyle-HorizontalAlign="Left"
                       SortExpression="pname">
                    <ItemStyle HorizontalAlign="Center" Width="150px" />
                                                                    </asp:BoundField>
                    <asp:BoundField DataField="amount" HeaderText="Amount" ItemStyle-HorizontalAlign="Center"
                        SortExpression="amount" >
                        <ItemStyle HorizontalAlign="Center" Width="100px" />
                                                                    </asp:BoundField>
                    <asp:BoundField DataField="date" HeaderText="Date" DataFormatString="{0:dd-MMM-yyyy}" ItemStyle-HorizontalAlign="Center"
                        SortExpression="date" >
                        <ItemStyle HorizontalAlign="Center" Width="100px" />
                                                                    </asp:BoundField>
                     <asp:TemplateField ItemStyle-HorizontalAlign="Center" ItemStyle-Width="3%">
                                     <HeaderTemplate>
                                     <asp:CheckBox ID="chk_All" runat="server" AutoPostBack="True" oncheckedchanged="chk_All_CheckedChanged"  />
                                     </HeaderTemplate>
                                     <ItemTemplate>
                                     <asp:CheckBox ID="chk_select" OnCheckedChanged="chk_select_CheckedChanged" AutoPostBack="True" runat="server"  />
                                     </ItemTemplate>
                                     <ItemStyle HorizontalAlign="Center" Width="3%" />
                                     </asp:TemplateField>

                </Columns>
        </asp:GridView>
        <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
                       ConnectionString="<%$ ConnectionStrings:NCPConnectionString %>"                                                
                       SelectCommand="SPPS_1" SelectCommandType="StoredProcedure">
                       <SelectParameters>
                           <asp:ControlParameter ControlID="ddl_Prov" DefaultValue="All" 
                               Name="pro1" PropertyName="SelectedValue" Type="String" />
                           <asp:ControlParameter ControlID="ddl_Pro" DefaultValue="All" 
                               Name="pr" PropertyName="SelectedValue" Type="String" />
                       </SelectParameters>
                    </asp:SqlDataSource>
        </ContentTemplate>
        <Triggers>


            <asp:AsyncPostBackTrigger ControlID="but_search" EventName="Click" />
            <asp:PostBackTrigger ControlID="grd_ViewPayments" />

        </Triggers>
        </asp:UpdatePanel>