NPOI的EXCEL转DATATABLE

/// <summary>
         /// EXCEL转DataTable
         /// </summary>
         /// <param name="postedfile">上传的文件对象</param>
         /// <returns>返回一张table</returns>
         static FileStream stream;
         static DataTable table;
         public static object obj = new object();
         public static DataTable ImportExcelToDataTable(HttpPostedFile postedfile)
         {
             lock (obj)
             {
                 if (postedfile != null)
                 {
                     try
                     {
                         string filename = DateTime.Now.ToString("yyyyMMddhhssmmffffff") + Path.GetExtension(postedfile.FileName);
                         string fullpath = HttpContext.Current.Server.MapPath("~/AdminSystem/TempFiles/");
                         if (!Directory.Exists(fullpath))
                         {
                             Directory.CreateDirectory(fullpath);
                         }
                         fullpath += filename;
                         postedfile.SaveAs(fullpath);


                         table = new DataTable(); 
                         stream = File.Open(fullpath, FileMode.Open, FileAccess.Read);


                         XSSFWorkbook workbook = new XSSFWorkbook(stream);
                         ISheet sheet = workbook.GetSheetAt(0);
                         //获取sheet的首行
                         IRow headerRow = sheet.GetRow(0);
                         int cellCount = headerRow.LastCellNum;
                         for (int i = headerRow.FirstCellNum; i < cellCount; i++)
                         {
                             DataColumn column = new DataColumn(headerRow.GetCell(i).StringCellValue);
                             table.Columns.Add(column);
                         }
                         int rowCount = sheet.LastRowNum;
                         for (int i = (sheet.FirstRowNum + 1); i <= sheet.LastRowNum; i++)
                         {
                             IRow row = sheet.GetRow(i);
                             DataRow dataRow = table.NewRow();
                             for (int j = row.FirstCellNum; j < cellCount; j++)
                             {
                                 if (row.GetCell(j) != null)
                                     dataRow[j] = row.GetCell(j).ToString();
                             }
                             table.Rows.Add(dataRow);
                         } 
                         stream.Close();
                         File.Delete(fullpath); 
                         return table;
                     }
                     catch
                     {
                         table.Dispose();
                         return null;
                     }
                     finally
                     {
                         stream.Close();
                         table.Dispose();
                     }
                 }
                 else
                 {
                     return null;
                 }
             }
         }