This question appears near the top of all google searches for information on response.end so for other searches like myself who wish to post CSV/XML/PDF etc in response to an event without rendering the entire ASPX page, this is how I do it. (overriding the render methods is overly complex for such a simple task IMO)
// Add headers for a csv file or whatever
Response.ContentType = "text/csv"
Response.AddHeader("Content-Disposition", "attachment;filename=report.csv")
Response.AddHeader("Pragma", "no-cache")
Response.AddHeader("Cache-Control", "no-cache")
// Write the data as binary from a unicode string
Dim buffer As Byte()
buffer = System.Text.Encoding.Unicode.GetBytes(csv)
Response.BinaryWrite(buffer)
// Sends the response buffer
Response.Flush()
// Prevents any other content from being sent to the browser
Response.SuppressContent = True
// Directs the thread to finish, bypassing additional processing
HttpContext.Current.ApplicationInstance.CompleteRequest()