別ウィンドウでダウンロード処理

'WebForm1.aspx.vb（呼ぶ側の画面その１）
Private Sub Button1_Click(ByVal sender As System.Object, _
                          ByVal e As System.EventArgs) Handles Button1.Click
    Dim strHeader As String = "a,a,a,a"
    Dim strData As String = "1,1,1,1"
    Session("strHeader") = strHeader
    Session("strData") = strData
    Response.Redirect("CsvDownload.aspx")
End Sub

'WebForm2.aspx.vb（呼ぶ側の画面その２）
Private Sub Button1_Click(ByVal sender As System.Object, _
                          ByVal e As System.EventArgs) Handles Button1.Click
    Dim strHeader As String = "b,b,b,b"
    Dim strData As String = "2,2,2,2"
    Session("strHeader") = strHeader
    Session("strData") = strData
    Response.Redirect("CsvDownload.aspx")
End Sub

'CsvDownload.aspx.vb（呼ばれる側）
Private Sub Page_Load(ByVal sender As System.Object, _
                      ByVal e As System.EventArgs) Handles MyBase.Load
    Dim strHeader As String = Session("strHeader")
    Dim strData As String = Session("strData")
    Response.Charset = "Shift_JIS"
    Response.ContentEncoding = System.Text.Encoding.GetEncoding("Shift_JIS")
    Response.ContentType = "application/octet-stream"
    Response.AppendHeader("Content-Disposition", "attachment;filename=test.csv")
    Response.Write(strHeader & vbCrLf)
    Response.Write(strData & vbCrLf)
    Response.End()
End Sub