VB .NET server generating Excel file on the fly - html

Question

I have a VB .NET web application running on a server with multiple requests and performance requirements.
I have a function that retrieves some data from a DB and has to generate an excel report to show to the system users.
I somehow did it, but my solution has not the performance I'd like it to have: basically what my code does is:
The server accepts the request of report generating
The server fills an excel file
The server saves locally the xls file
The server attaches the file to the html response and the user downloads it
The server deletes the file (when? I need to handle borderline cases too)
The code snippet is like:
Public Sub ExportaDataTableToExcel(ByVal dt As System.Data.DataTable, ByVal Page As System.Web.UI.Page, ByVal ReportName As String)
Dim oExcel As Excel.Application
Dim oBook As Excel.Workbook
Dim oSheet As Excel.Worksheet
Dim colIndex As Integer
Dim rowIndex As Integer
oExcel = New Excel.Application
oBook = oExcel.Workbooks.Add(Type.Missing)
oSheet = oBook.Worksheets(1)
'Export the Columns to excel file
For Each dc In dt.Columns
colIndex = colIndex + 1
oSheet.Cells(1, colIndex) = dc.ColumnName
Next
oBook.SaveAs("C:\file.xls")
oSheet = Nothing
oBook.Close()
Page.Response.AddHeader("content-disposition", "attachment;filename=" & ReportName & ".xls")
Page.Response.Charset = String.Empty
Page.Response.ContentType = "vnd.application/ms-excel"
Page.Response.TransmitFile("C:\file.xls")
Page.Response.Flush()
Page.Response.End()
oExcel.Quit()
End Sub
As you can see the server generates the xls file locally in C:\file.xls, then trasmittes the file in the response page.
What I'd like to do, and I really don't know if it's possible as I did not found any example on this, is generating the xls file on the fly without saving it locally (maybe returning as a Stream of bytes or something like this) and then assembling this as xls file in the response page, without saving the file locally.
I tried using both Page and System.IO.StringWriter with no luck, maybe I'm doing something wrong.
The following code acts abnormally, asking me to save modifies on the file (on the server side), and downloading an xls file that has parts of the html of the page, generating errors about missing css files and displaying part of the page. So, with this approach I'm almost at zero. Here it is the code:
Public Sub ExportaDataTableToExcel(ByVal dt As System.Data.DataTable, ByVal Page As System.Web.UI.Page, ByVal ReportName As String)
Dim oExcel As Excel.Application
Dim oBook As Excel.Workbook
Dim oSheet As Excel.Worksheet
Dim colIndex As Integer
Dim rowIndex As Integer
oExcel = New Excel.Application
oBook = oExcel.Workbooks.Add(Type.Missing)
oSheet = oBook.Worksheets(1)
'Export the Columns to excel file
For Each dc In dt.Columns
colIndex = colIndex + 1
oSheet.Cells(1, colIndex) = dc.ColumnName
Next
oSheet = Nothing
oBook.Close()
Dim stringWrite As System.IO.StringWriter = New System.IO.StringWriter
Dim htmlWrite As System.Web.UI.Html32TextWriter = New System.Web.UI.Html32TextWriter(stringWrite)
Page.Response.Clear()
Page.Response.AddHeader("content-disposition", "attachment;filename=" & ReportName & ".xls")
Page.Response.Charset = String.Empty
Page.Response.ContentType = "vnd.application/ms-excel"
Page.Response.WriteFile(stringWrite.ToString)
Page.Response.Flush()
Page.Response.End()
oExcel.Quit()
End Sub

Answer 1

I know I can help with at least one of those problems you are having, Lateralus. Ive taken a snippet from our live server here to show you.
Edit: This one transmits a CSV file and we had the problem initially with the HTML showing up in the file. We opted to not use excel in this particular instance but, the problems are likely one in the same.
I believe that you need to add an additional header to your page response in order to remove the HTML that you are getting in the file.
Here is an example. As a warning, I'm not entirely sure how secure this method is. We use this within our network so it didn't need to be as locked down.
Dim LiveFileStream As FileStream = New FileStream("C:\inetpub\wwwroot\" &_
"Website\CSVFile.csv", FileMode.Open, FileAccess.Read)
Dim fileBuffer(CInt(LiveFileStream.Length)) As Byte
LiveFileStream.Read(fileBuffer, 0, CInt(LiveFileStream.Length))
LiveFileStream.Close()
Response.Clear()
Response.Charset = "utf-8"
Response.ContentType = "text/plain"
'I believe if you are to add the following header
'it will fix the problem with HTML showing in document
Response.AddHeader("Content-Length", fileBuffer.Length.ToString)
Response.AddHeader("Content-Disposition", "attachment; filename=CSVFile.csv")
Response.BinaryWrite(fileBuffer)
Response.End()