hi i am trying to export gridview data in xlsx file using below code, but when i open file its showing error, 
Response.Clear()
                Response.AddHeader("content-disposition", "attachment;filename=Report.xlsx")
                Response.Charset = ""
                Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
                Dim stringWrite As System.IO.StringWriter = New System.IO.StringWriter()
                Dim htmlWrite As System.Web.UI.HtmlTextWriter = New HtmlTextWriter(stringWrite)
                For Each r As GridViewRow In GridView1.Rows
                    If r.RowType = DataControlRowType.DataRow Then
                        For columnIndex As Integer = 0 To r.Cells.Count - 1
                            r.Cells(columnIndex).Attributes.Add("class", "text")
                        Next
                    End If
                Next
                GridView1.RenderControl(htmlWrite)
                Dim style As String = "<style> .text { mso-number-format:\@; } </style> "
                Response.Write(style)
                Response.Write(stringWrite.ToString())
                Response.[End]()