listbox failed to pick up value when reloading the page

Question

I'm trying to put a searchbox, a dropdownlist and lostbox control on my asp master page. Once a value is selected from the dropdownlist, the value passed to listbox;once some word is typed into the searchbox and enter is hit, the closest value is passed to the listbox.

My asp code for the control is below:

searchbox:

   <asp:TextBox ID="TextBox1" runat="server" ToolTip="Enter Company" Width="120px">

dropdownlist:

    <asp:DropDownList ID="DropDownListCI"  runat="server" DataSourceID="SqlDataSource3" DataTextField="company" DataValueField="id"  AppendDataBoundItems="true" AutoPostBack="True" width="160px"> 
                                <asp:ListItem Text="--Select One--" Value="" Selected="True" /> </asp:DropDownList>

     <asp:SqlDataSource ID="SqlDataSource3" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>" SelectCommand="select company, id from table1 order by company">

                            </asp:SqlDataSource>

listbox:

    <asp:ListBox ID="ListBox1" runat="server" DataTextField="company" DataValueField="id" Rows="1" Width="160px">
                              <asp:ListItem Text="--Null--" Value="" selected="true" />
                         </asp:ListBox>

data source from the searchbox is:

    <asp:SqlDataSource ID="SqlDataSourceSE" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>" SelectCommand="select company, id from table1 where company like '%'+ @Company + '%'">
                                <SelectParameters>
                                    <asp:ControlParameter ControlID="TextBox1" Name="Company" PropertyName="Text" />
                                </SelectParameters>
                            </asp:SqlDataSource>

datasource from dropdownlist is

      <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ConnectionString %>" SelectCommand="select company, id from table1 where id=@id">
                             <SelectParameters>
                                 <asp:ControlParameter ControlID="DropDownListCI" Name="ID" PropertyName="selectedvalue" />
                             </SelectParameters>
                         </asp:SqlDataSource>

and my vb code is below:

--Store session variable

          Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click


    Dim SelectionCI As String = Nothing
    Dim SelectionSE As String = Nothing


    If Not DropDownListCI.Text Is Nothing Then SelectionCI = DropDownListCI.Text
    Session("SelectedCI") = SelectionCI
    If Not TextBox1.Text Is Nothing Then SelectionSE = TextBox1.Text
    Session("SelectedSE") = SelectionSE

End Sub

--page load

     Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Not IsPostBack Then
        If Not Session("SelectedCI") Is Nothing Then DropDownListCI.Text = Session("SelectedCI").ToString
        If Not Session("SelectedSE") Is Nothing Then TextBox1.Text = Session("SelectedSE").ToString

    End If

    If Not DropDownListCI.Text = "" Then ListBox1.DataSourceID = SqlDataSource1.ID
    If Not TextBox1.Text = "" Then ListBox1.DataSourceID = SqlDataSourceSE.ID
End Sub

-----on change

 Protected Sub DropDownListCI_SelectedIndexChanged(sender As Object, e As EventArgs) Handles DropDownListCI.SelectedIndexChanged
    If Not DropDownListCI.SelectedIndex = 0 Then
        TextBox1.Text = ""
        ListBox1.DataSourceID = SqlDataSource1.ID
    Else : ListBox1.SelectedIndex = 0
    End If
End Sub


Protected Sub TextBox1_TextChanged(sender As Object, e As EventArgs) Handles TextBox1.TextChanged
    If Not TextBox1.Text = "" Then
        DropDownListCI.SelectedIndex = 0
        ListBox1.DataSourceID = SqlDataSourceSE.ID
    End If

End Sub

What I want to achieve is after I make a selection (say company A), when I refresh page, company A is still selected. Now it works well if company A is got through search box. But if I select company A from dropdownlist, then store the session variable, then refresh the page, it comes out company A is still selected in dropdownlist, but "--Null--" appears in listbox. I wonder what's the problem?

Thanks for any advice!

Answer 1

I finally realized that my page_load has a problem. The following is the correct page_load code

   Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

    If Not IsPostBack Then
        If Not Session("SelectedCI") Is Nothing And Session("SelectedCI") <> "" Then
            DropDownListCI.Text = Session("SelectedCI").ToString
            ListBox1.DataSourceID = SqlDataSource1.ID
        ElseIf Not Session("SelectedSE") Is Nothing And Session("SelectedSE") <> "" Then
            TextBox1.Text = Session("SelectedSE").ToString
            ListBox1.DataSourceID = SqlDataSourceSE.ID

        End If

    End Sub