如何通过cookie asp.net更改下拉列表选择的值

Question

在Default.aspx中，我有一个下拉列表，用于从“对象数据”源中提取索引类型。下面的代码

<h4>Index to search</h4>
<asp:DropDownList ID="dlSearchIndex" runat="server" CssClass="input-group " 
DataTextField="INDEX" DataValueField="INDEX" Width="150px" 
DataSourceID="ObjectDataSource1">
</asp:DropDownList>
<asp:ObjectDataSource ID="ObjectDataSource1" runat="server" 
SelectMethod="GetIndexType" TypeName="the.dll"> 
</asp:ObjectDataSource>

我正在尝试使用cookie来设置此下拉列表的selectedvalue，如下所示：

If IsPostBack = False Then
        If Request.Cookies("IndexToSearch") IsNot Nothing Then
            dlSearchIndex.SelectedValue = 
            Request.Cookies("IndexToSearch").Value.ToString()
            Response.Cookies("IndexToSearch").Value = 
             dlSearchIndex.SelectedValue
            Response.Cookies("IndexToSearch").Expires = 
            DateTime.Now.AddSeconds(10)
        Else
            Response.Write("<p> No Cookie Found.</p>")
        End If
    End If

我正在这样创建cookie：

If Request.Cookies("IndexToSearch") Is Nothing Then
            Dim cookie As New HttpCookie("IndexToSearch")
            cookie.Value = Trim(dlSearchIndex.SelectedValue)
            cookie.Expires = DateTime.Now.AddSeconds(350)
            Response.SetCookie(cookie)
            Response.Write("<p>" & cookie.Name & ":" & cookie.Value & ":")
        End If

我第一次运行此代码时，它可以工作，但此后出现此错误：

"dlSearchIndex" has a SelectedValue which is invalid because it does not 
exist in the list of items.
Parameter name: value 

请，我在做什么错？预先谢谢你！