以流方式导出到EXCEL时，怎么保持原有格式

 Public Shared Sub ExportToExcel(ByVal dgv As DataGridView, Optional ByVal strfilename As String = "")
        Dim saveFileDialog As New SaveFileDialog()
        saveFileDialog.Filter = "Execl files (*.xls)|*.xls"
        saveFileDialog.FilterIndex = 0
        saveFileDialog.FileName = strfilename
        saveFileDialog.RestoreDirectory = True
        'saveFileDialog.CreatePrompt = True
        saveFileDialog.Title = "保存为Excel文件"
        saveFileDialog.ShowDialog()

        If saveFileDialog.FileName.IndexOf(":") < 0 Then
            Exit Sub
        End If
        '被点了"取消" 
        Dim myStream As IO.FileStream
        myStream = saveFileDialog.OpenFile()
        Dim sw As New IO.StreamWriter(myStream, System.Text.Encoding.GetEncoding(-0))
        Dim columnTitle As String = ""
        Try
            '写入列标题 
            For i As Integer = 0 To dgv.ColumnCount - 1
                If i > 0 Then
                    columnTitle += vbTab
                End If
                columnTitle += dgv.Columns(i).HeaderText
            Next

            sw.WriteLine(columnTitle)

            '写入列内容 
            For j As Integer = 0 To dgv.Rows.Count - 1
                Dim columnValue As String = ""
                For k As Integer = 0 To dgv.Columns.Count - 1
                    If k > 0 Then
                        columnValue += vbTab
                    End If
                    If dgv.Rows(j).Cells(k).Value Is Nothing Then
                        columnValue += ""
                    Else
                        columnValue += dgv.Rows(j).Cells(k).FormattedValue.ToString.Trim()
                    End If
                Next
                sw.WriteLine(columnValue)
            Next
            sw.Close()
            myStream.Close()
        Catch e As Exception
            MessageBox.Show(e.ToString())
        Finally
            sw.Close()
            myStream.Close()
        End Try
    End Sub

#Region "EXCEL用格式(边框、格式)"
     ''' <summary>
     ''' EXCEL用格式(边框、格式)
     ''' </summary>
     ''' <param name="iPosV">开始行号码</param>
     ''' <param name="iPosH">开始列号码</param>
     ''' <param name="iArrFmtInfo">格式情报</param>
     ''' <param name="iArrPosInfo">位置情报</param>
     ''' <param name="iRowCnt">数据行数</param>
     ''' <param name="iColCnt">数据列数</param>
     ''' <param name="bBorder">边框有无</param>
     ''' <param name="bTitle">标题行有无</param>
     ''' <returns>处理结果</returns>
     ''' <remarks></remarks>
     Private Function XlsEditTblFmt(ByVal iPosV As Integer, _
                                    ByVal iPosH As Integer, _
                                    ByVal iArrFmtInfo() As XLS_FMT, _
                                    ByVal iArrPosInfo() As XLS_POS, _
                                    ByVal iRowCnt As Integer, _
                                    ByVal iColCnt As Integer, _
                                    ByVal bBorder As Boolean, _
                                    ByVal bTitle As Boolean) As XLS_RESULT

         Dim iCol As Integer
         Dim sRowS As String     'Excel字符串开始行号码
         Dim sRowE As String     'Excel字符串结束行号码
         Dim sColS As String     'Excel字符串开始列号码
         Dim sColE As String     'Excel字符串结束列号码

         Try
             '标题行的开始行号码、开始列号码、结束列号码设定
             sRowS = iPosV.ToString
             sColS = GetXlsRange(iPosH)
             sColE = GetXlsRange(iPosH + iColCnt - 1)

             '标题行
             If bTitle = True Then
                 '标题行Range设定
                 oRange = oSheet.Range(sColS & sRowS, sColE & sRowS)
 
                 '标题行设定(背景色)
                 oInterior = oRange.Interior
                 oInterior.ColorIndex = Excel.XlPattern.xlPatternGrid
                 MRComObject(oInterior)
 
                 '标题行设定(居中)
                 oRange.HorizontalAlignment = Excel.Constants.xlCenter
                 MRComObject(oRange)
             Else
                 iPosV -= 1
             End If
 
             If iRowCnt > 0 Then
                 '数据行开始行号码、结束行号码设定
                 sRowS = (iPosV + 1).ToString
                 sRowE = (iPosV + iRowCnt).ToString

                 '数据行格式设定
                 For iCol = 0 To iColCnt - 1
 
                     '数据行的开始列号码设定
                     sColS = GetXlsRange(iPosH + iCol)

                     oRange = oSheet.Range(sColS & sRowS, sColS & sRowE)

                     Select Case iArrFmtInfo(iCol)
                         Case XLS_FMT.FMT_NORMAL        '通常设定
                         Case XLS_FMT.FMT_NUMBER        '数值设定
                             oRange.NumberFormatLocal = "#,##0"
                         Case XLS_FMT.FMT_STRING        '字符串设定
                             oRange.NumberFormatLocal = "@"
                         Case XLS_FMT.FMT_DATE          '日期设定
                             oRange.NumberFormatLocal = "yyyy/mm/dd"
                         Case XLS_FMT.FMT_DECIMAL
                             oRange.NumberFormatLocal = "0.000_ "
                         Case XLS_FMT.FMT_DIGITONE     '小数点后一位
                             oRange.NumberFormatLocal = "0.0_ "
                         Case XLS_FMT.FMT_DIGITTWO      '小数点后二位
                             oRange.NumberFormatLocal = "0.00_ "
                         Case XLS_FMT.FMT_DIGITTHR       '小数点后三位
                             oRange.NumberFormatLocal = "0.000_ "
                     End Select

                   MRComObject(oRange)

                 Next

             Return XLS_RESULT.XLS_OK

         Catch ex As Exception
             Me.XlsFileRelease()
             Return XLS_RESULT.XLS_NG
         End Try
     End Function
 #End Region