asp.net dropdownlist不更新所选值

Question

I have a page with a gridview on it and a dropdown list which controls how many items per page the gridview will display.

The pageSize value of the gridview is controlled by this dropdown list and it gets saved to a cookie. When the user loads the site the cookie is read so that it remembers what page size the user picked.

I have one problem and that is, if I pick another value on the dropdown list it does not update either cookie or dropdown list. It reverts back to the saved value.

This is the dropdown list created in the gridview pager template:

<PagerTemplate>
             <asp:Table ID="Table3" runat="server" Width="100%">
                 <asp:TableRow>
                     <asp:TableCell HorizontalAlign="Left">
                         <asp:PlaceHolder ID="ph" runat="server"></asp:PlaceHolder>
                     </asp:TableCell>
                     <asp:TableCell HorizontalAlign="Right" Width="10%">
                         Page Size
                            <asp:DropDownList runat="server" ID="ddlPageSize" AutoPostBack="true"
                                 OnSelectedIndexChanged="ddlPageSize_SelectedIndexChanged" OnLoad="ddlPageSize_Load">
                                <asp:ListItem>5</asp:ListItem>
                                <asp:ListItem>10</asp:ListItem>
                                <asp:ListItem>20</asp:ListItem>
                                <asp:ListItem>50</asp:ListItem>
                                <asp:ListItem>100</asp:ListItem>
                            </asp:DropDownList>
                     </asp:TableCell>
                 </asp:TableRow>
             </asp:Table>
        </PagerTemplate>

and this is where I try to load the value of the dropdown list from cookie:

protected void Page_Load(object sender, EventArgs e)
        {
            string pageSize = "10";
            //Try and load the PageSize cookie from the user's machine and default to 10 records if none is found.
            if (Request.Cookies["PageSize"] != null)
            {                
                if (Request.Cookies["PageSize"]["Value"] != null)
                {
                    pageSize = Request.Cookies["PageSize"]["Value"];
                    int _pageSize;
                    int.TryParse(pageSize, out _pageSize);
                    gvRecordsList.PageSize = _pageSize;
                    DropDownList ddlPageSize = (gvRecordsList.BottomPagerRow).FindControl("ddlPageSize") as DropDownList;
                    ddlPageSize.SelectedIndex = ddlPageSize.Items.IndexOf(new ListItem(pageSize));
                }
            }
            else
                gvRecordsList.PageSize = 10;


            if (IsPostBack)
            {
                ApplyPaging();               
            }
            else
            {
                gvRecordsList.DataSourceID = "RecordsListSqlDataSource";
                gvRecordsList.DataBind();
            }
        }

The dropdown list index changed code:

protected void ddlPageSize_SelectedIndexChanged(object sender, EventArgs e)
        {
            DropDownList ddlPageSize = (gvRecordsList.BottomPagerRow).FindControl("ddlPageSize") as DropDownList;
            gvRecordsList.PageSize = int.Parse(ddlPageSize.SelectedValue);
            Response.Cookies["PageSize"]["Value"] = ddlPageSize.SelectedValue;
            Response.Cookies["PageSize"].Expires = DateTime.Now.AddDays(1d);
        }

When I step through the code of the SelectedIndexChanged method, I can see that ddlPageSize.SelectedValue always contains the value from the cookie, 50, even though I select another value.

I guess the question is, where do I set the index of the dropdown list?

DropDownList ddlPageSize = (gvRecordsList.BottomPagerRow).FindControl("ddlPageSize") as DropDownList;
                        ddlPageSize.SelectedIndex = ddlPageSize.Items.IndexOf(new ListItem(pageSize));

Answer 1

The Page_Load event executes before the DropDownList SelectedIndexChanged event. And you are loading the cookie's value to the DropDownList on the PageLoad event.

I suggest you to try loading the cookie afterwards, on the OnPreRender event for example.

Or add a condition to your Page_Load logic, verifying if the PostBack is caused by the DropDownList :

DropDownList ddlPageSize = (gvRecordsList.BottomPagerRow).FindControl("ddlPageSize") as DropDownList;      
bool isDDLPostingBack = Request["__EVENTTARGET"] == ddlPageSize.UniqueID;

if (Request.Cookies["PageSize"]["Value"] != null && !isDDLPostingBack)
{
...
}