Update HSSFWorkbook.cs

 public NPOI.SS.UserModel.ISheet GetSheet(String name)
        {
            HSSFSheet retval = null;

            for (int k = 0; k < _sheets.Count; k++)
            {
                String sheetname = workbook.GetSheetName(k);

                if (sheetname.Equals(name, StringComparison.OrdinalIgnoreCase))
                {
                    retval = (HSSFSheet)_sheets[k];
                    break; //返回，不用再遍历了
                }
            }
            return retval;
        }