I am trying to export a dataset to excel from asp.net page method(webmethod) using the following code. the download file dialog does not open up to ask user whether 'Save'/'Open'.thanks

<System.Web.Services.WebMethod()> _
   Public Shared Function exportexcel_TKNC(ByVal tenBang As String, ByVal dkien As String) As String
       Dim strText As String = ""
       Dim strTextTenTruong As String = ""
       Dim strTextMoTaTruong As String = ""
       Dim strFieldFillOrGet As String = ""
       Dim arrGiatri As New List(Of String)
       Dim arrAlias As New List(Of String)
       Dim arrColumns As New List(Of String)
       Dim iCount As Integer = 0
       Dim strSQL As String = ""
       Dim sql As String = ""
       Dim dtexport As New DataSet
       Dim dad As New OracleDataAdapter
       'pubTenBang = tenBang
       If tenBang <> "" Then
           'sql = "SELECT column_name, data_type, data_length FROM all_tab_columns WHERE "
           'sql &= " table_name = '" & tenBang & "' Order By COLUMN_ID"
           sql = "Select * From HE_THONG_COMMENTS_OF_FIELDS"
           sql &= " Where TEN_BANG = '" & tenBang & "'"
           Dim dtReader As OracleDataReader
           dtReader = clsTable.SelectTable_ReaderORACLE(clsTable.cnnOracle, sql)
           If dtReader.HasRows Then
               While dtReader.Read
                   If (dtReader.Item(0).ToString <> "") Then
                       arrColumns.Add(dtReader.Item(1) & "")
                       arrAlias.Add(dtReader.Item(2) & "")
                   End If

               End While

           End If
           dtReader.Close()

           strSQL = "Select "
           For iCount = 0 To arrColumns.Count - 1
               If iCount <> 0 Then
                   strSQL &= ", "
               End If
               strSQL &= "" & arrColumns(iCount)
           Next
           strSQL &= " from " & tenBang & " where " & dkien

           dad = New OracleDataAdapter(strSQL, clsTable.cnnOracle)
           dad.Fill(dtexport)
           Dim Response As HttpResponse = System.Web.HttpContext.Current.Response
           Dim GridView1 As New GridView()
           GridView1.AllowPaging = False
           GridView1.DataSource = dtexport
           GridView1.DataBind()

           Response.Clear()
           Response.Buffer = True
           Response.AddHeader("content-disposition", "attachment;filename=GridViewExport.xls")
           Response.Charset = ""
           Response.ContentType = "application/vnd.ms-excel"
           Dim sw As New StringWriter()
           Dim hw As New HtmlTextWriter(sw)

           For i As Integer = 0 To GridView1.Rows.Count - 1
               'Apply text style to each Row
               GridView1.Rows(i).Attributes.Add("class", "textmode")
           Next
           GridView1.RenderControl(hw)

           'style to format numbers to string
           Dim style As String = "<style> .textmode { mso-number-format:\@; } </style>"
           Response.Write(style)
           Response.Output.Write(sw.ToString())
           Response.Flush()
           Response.End()

           'Dim filename As String = tenBang + ".xls"
           'Dim tw As New System.IO.StringWriter()
           'Dim hw As New System.Web.UI.HtmlTextWriter(tw)
           'Dim dgGrid As New System.Web.UI.WebControls.DataGrid()
           'dgGrid.DataSource = dtexport
           'dgGrid.DataBind()
           ''Get the HTML for the control.
           'dgGrid.RenderControl(hw)
           ''Write the HTML back to the browser.
           'Dim Response As HttpResponse = System.Web.HttpContext.Current.Response
           'Response.ContentType = "application/vnd.ms-excel"
           'Response.AppendHeader("Content-Disposition", (Convert.ToString("attachment; filename=") & filename) + "")
           ''Me.EnableViewState = False
           'strText = tw.ToString
           ''Response.Write(tw.ToString())
           ''HttpContext.Current.Response.Flush()
           ''HttpContext.Current.Response.SuppressContent = True
           ''HttpContext.Current.ApplicationInstance.CompleteRequest()
           ''HttpContext.Current.Response.End()
       End If

       Return strText

   End Function