PagedDataSource not working with previous/next pages - asp.net

Question

I'm having a hard time with this scenario and hoping someone can help me clarify where I'm missing a step/logic. I tried searching online but I couldn't find an example that addressed this issue.
We are setting up a search page that when first loads shows a bunch of options (e.g., textboxes, checkboxes, etc). The user will fill out the form and submit the form back to itself. Once posted back, the page will build and run a SQL query (e.g., SELECT ID FROM Customers WHERE Company = 'Acme' AND AmtDue = 3) against the database with the user's options and then the results will show up. This part works ok.
The part that breaks down is when I am trying to add pagination. The result set is a DataTable bound to a Repeater. I am using a PagedDataSource to add pagination. Pagination works great for the first page but for subsequent pages, it does not work. Basically, instead of returning the next page of the result requested (e.g., SELECT ID FROM Customers WHERE Company = 'Acme' AND AmtDue = 3), the results returned is the SQL query before appending the user's search options (e.g., SELECT ID FROM Customers).
I think my basic problem is that I'm not sure how to distinguish a normal Page.IsPostBack and paginating through results. The reason why this is causing problems is because I don't want to be regathering form data, rebuilding query, and requerying the DB.
The examples that I found online involved pagination of a DataTable that is rebuilt every time the page loads (e.g., Not Page.IsPostBack).
Here is a rough outline of our code:
Public dtCustomers As DataTable = New DataTable()
Sub Page_Load(ByVal Sender As Object, ByVal E As EventArgs) Handles Me.Load
' When the page first loads, show the search form with search options.
If Not Page.IsPostBack Then
ShowSearchForm()
' Once the form is submitted, show the search results.
Else
' ----------------------------------------
' This is the part that I'm having trouble with. How do I skip the following two steps (GatherFormData() and BuildDT()) when the user requests a subsequent page?
GatherFormData()
dtCustomers = BuildDT()
' ----------------------------------------
BindData()
End If
End Sub
Sub BindData()
Dim objPDS As PagedDataSource = New PagedDataSource()
objPDS.DataSource = dtCustomers.DefaultView
objPDS.AllowPaging = True
objPDS.PageSize = intNumPerPage
objPDS.CurrentPageIndex = Me.ViewState("_CurrentPage")
rptSearchResults.DataSource = objPDS
End Sub
' Subroutine called when the previous page button is pressed.
Sub GoToPrevPage()
' Set viewstate variable to the previous page.
Me.ViewState("_CurrentPage") -= 1
BindData()
End Sub
' Subroutine called when the next page button is pressed.
Sub GoToNextPage()
' Set viewstate variable to the next page.
Me.ViewState("_CurrentPage") += 1
BindData()
End Sub
Note: I understand that the DataTable will have to be put into cache or a session variable, but haven't decided the best method.
Please excuse the code outline but the actual code is massive so the simplification is to make it easier to get to the heart of the issue.
Let me know if any of that was unclear. Thanks in advance!

Answer 1

I am assuming that you are going to store your data in session/cache (I would prefer the later but there can be use case for storing it in session) - you can store the key in the view-state and presence of key could be use to determine if post-back is for pagination or not. For example,
if (ViewState["dataKey"] == null)
{
// first form submittal, do the search
GatherFormData();
dtCustomers = BuildDT();
// store it in cache
ViewState["dataKey"] = Guid.NewGuid().ToString(); // create a key
Cache.[ViewState["dataKey"].ToString()] = dtCustomers; // TODO use Add method to control expiration etc
}
Its important that you clear the view-state when the search is reset (or similar condition)
Lastly, it is also possible to do the paging from database/data-store (for example, using ranking functions in SQL Server) so that you need not have to store the search results into the session/cache but rather do a database trip at each post-back. Such approach is useful when the complete result set size can be huge.