npoi写入excel是出现异常

//往excel的某个单元格填充内容
        public void FillExcelCell(string excelPath,  int rowIndex, int columnIndex, string content)
        {
            try
            {
                //读取excel
                try
                {
                    FileStream stream = new FileStream(excelPath, FileMode.Open, FileAccess.ReadWrite);
                    HSSFWorkbook workBook = new HSSFWorkbook(stream);
                    ISheet sheet = workBook.GetSheetAt(0);
                    if (sheet.GetRow(rowIndex) == null)
                    {
                        sheet.CreateRow(rowIndex);
                    }
                    if (sheet.GetRow(rowIndex).GetCell(columnIndex) == null)
                    {
                        sheet.GetRow(rowIndex).CreateCell(columnIndex);
                    }
                    sheet.GetRow(rowIndex).GetCell(columnIndex).SetCellValue("ssss");
                    workBook.Write(stream);
                    stream.Close();
                }
                catch (Exception ex)
                {
                    var a = ex.Message;
                }
            }
            catch (Exception ex)
            {
                Console.Write(ex.Message);
            }
        }

不知道为什么总会有这两个异常的产生：
Handle = “stream.Handle”引发了“System.ObjectDisposedException”类型的异常
IsReadOnly = “workBook.IsReadOnly”引发了“System.NotImplementedException”类型的异常
问一下各路大牛，本人是哪里写错了