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I intend to use session variable to capture searching keywords and catagory input by user. I created datatable dTable to store "catagory", "keyword" and "ID" and save it into session variable session("MyCat") when page is firstly loaded.
If it is still within same session time frame, any new input will be automatically added into this datatable and session variable updated as well.
Anyway, my program can not get through as it is captured error in
dim dr as DataRow=dTable.NewRow() in my else statement.
May I know what is wrong with my coding in Sub Submit portion and how this can be implemented efficiently in ASP.NET?

<%@ Page Language="VB" EnableSessionState=true %>
<%@ Register TagPrefix="CityLibrary" TagName="Header_search" Src="..\include/header_search.ascx" %>
<%@ Register TagPrefix="CityLibrary" TagName="bar_empty" Src="..\include/bar_empty.ascx" %>
<%@ Import Namespace="System.Data" %>
<%@ Import Namespace="System.Data.OleDb" %>

<script runat="server">

Sub Submit(Sender As Object, e As EventArgs)
if NOT Page.IsPostBack then
dim dTable as New DataTable("mySearch")
dTable.Columns.Add("catagory", System.Type.getType("System.String"))
dTable.Columns.Add("keyword", System.Type.getType("System.String"))
dTable.Columns.Add("ID", System.Type.getType("System.Int32"))
dTable.Columns("ID").AutoIncrement=True
Dim PrimaryKeyColumns(0) As DataColumn
PrimaryKeyColumns(0)= dTable.Columns("ID")
dTable.PrimaryKey = PrimaryKeyColumns
dim dr as DataRow=dTable.NewRow()
dr(0)=Listbox1.SelectedItem.value
dr(1)=tbtext.Text
dTable.Rows.Add(dr)
session("myCat")= dTable

else
dim dTable as New DataTable
dTable= session("myCat")
dim dr as DataRow=dTable.NewRow()
dr(0)=Listbox1.SelectedItem.value
dr(1)=tbtext.Text
dTable.Rows.Add(dr)
session("myCat")= dTable
end if

session.Timeout=2

dim keyword as String
dim catagory as String
dim objitem as new CityLibrary.libraryItem

keyword = tbtext.Text
catagory = Listbox1.SelectedItem.value

dim objReader as OleDbDataReader
objReader = objitem.GetLibraryItemGeneral(catagory,keyword)
session("mydata")=objreader
objReader.Close()
objitem.Dispose
Response.Redirect("searchresult.aspx")
End Sub

</script>

<html><body>

<table height="100%" cellspacing=0 cellpadding=0 width="100%">
<CityLibrary:Header_Search runat="server"/>
<tr height="100%">
<CityLibrary:bar_empty runat="server"/>

<td align=left valign="top" width=100% nowrap>
<table height="100%" valign="top" align="left" cellspacing=0 cellpadding=0 width="100%">
<tr height="100%" valign="top">
<td nowrap>
<table cellspacing=0 cellpadding=0 width="100%">
<tr>
<td valign="top">
 <br>

<center>

<form action="Basicsearchmain.aspx" runat="server">

<h3> Name: <asp:Textbox id="tbText" type=text runat="Server"/><br><br>

UserGroup: <asp:Listbox runat="server" id="listbox1" width="200" rows="1"
SelectionMode="Single">
<asp:Listitem value="title">Title Keywords</asp:Listitem>
<asp:Listitem value="name">Author Last Name or FIrst Name</asp:Listitem>
<asp:Listitem value="subject">Subject Keywords</asp:Listitem>
<asp:Listitem value="kidstitle"> Kids' Reading Title</asp:Listitem>
<asp:Listitem value="kidssubject">Kids' reading Subject</asp:Listitem>
<asp:Listitem value="periodical">Periodical title</asp:Listitem>
<asp:Listitem value="keywordall">All Keywords</asp:Listitem>
</asp:Listbox><p>

<ASP:Button id="btSubmit" onclick="Submit" text="Lookup" runat="server"/>
</h3>
</form>

</center>
<hr>

</td>
</tr>
</table>
</td>
</tr>
</table>
</td>
</tr>
</table>
</body></html>