Passing column value from specific row index of a GridView - asp.net

Question

Suppose I have a gridview containing some bound fields and a template field. The template field contains a button (btnUpload).
btnUpload triggers a modalpopupextender to display a panel which contains some controls and a submit button.
What I need to be able to do is to get the value from cell 0 of the row btnUpload was clicked on and pass it to the panel so I can retrieve some data into the panel's controls in the panel_load event based on that value from cell 0 of the gridview.
I thought I could do this by storing the value from cell 0 in a session variable but not sure if this is the 'best' way of doing this?
UPDATE - Using hidden fields on the panel to store row index as suggested by Karl
I have added two hidden fields to the panel and I populate them as below:
If e.CommandName = "Upload" Then
Dim row As GridViewRow = CType(CType(e.CommandSource, Control).NamingContainer, GridViewRow)
Dim ref As String = CType(row.Cells(0), DataControlFieldCell).Text
Dim dt As String = CType(row.Cells(2), DataControlFieldCell).Text
hfRef.Value = ref
hfDate.Value = dt
End If
This is working and populating the hidden fields OK. However in the btnSubmit_Click event procedure on the panel I am doing the following the variables I am assigning are not getting a value from the hidden fields:
If fuCertificate.HasFile Then
Dim fileName As String = Path.GetFileName(fuCertificate.PostedFile.FileName)
fuCertificate.SaveAs(Server.MapPath("~/CalibrationCerts/" & fileName))
Dim ref As String = hfRef.Value
Dim dt As String = hfDate.Value
Dim dc As New ExternalCalibrationDataContext
Dim thisRecord = (From x In dc.ext_calibration_records
Where x.calibration_no = ref AndAlso
x.calibration_date = dt).ToList()(0)
thisRecord.certificate_no = txtCertNumber.Text
thisRecord.certificate = "~/CalibrationCerts/" & fileName
Else
lblError.Text = "Please select a file to upload"
End If

Answer 1

I would recommend putting a HiddenField control in the panel used by the ModalPopupExtender. It will be easier to work with the HiddenField control than Session, because you will not need to do the casting of the hidden field Value to a string, because it already is a string. All Session objects are stored as Object, so it is necessary to cast to a string when retrieving the value from Session.
Hidden field option:
<asp:Panel>
// Other stuff for your modal popup
<asp:HiddenField id="HiddenField1` runat="server" />
</asp:Panel>
// Set hidden field
HiddenField1.Value = cell0.Text
// Get hidden field
string theValue = HiddenField1.Value
Session option:
// Set the value in Session
Session["Cell0Text"] = cell0.Text
// Retrieve the value from Session
// First check to see if the Session value still exists
if(Session["Cell0Text"] != null)
{
// Now cast the Session object to a string
string theValue = Session["Cell0Text"].ToString()
}