private void btnExportToExcel_Click(object sender, RoutedEventArgs e)
        {
            SaveFileDialog sfd = new SaveFileDialog();
            sfd.Filter = "excel文件|*.xls";
            if (sfd.ShowDialog() != true)
            {
                return;
            }
            string fileName = sfd.FileName;
            HSSFWorkbook workbook = new HSSFWorkbook();
            ISheet sheet = workbook.CreateSheet("员工信息表");
            IRow rowHeader = sheet.CreateRow(0);
            rowHeader.CreateCell(0, CellType.STRING).SetCellValue("姓名");
            rowHeader.CreateCell(1, CellType.STRING).SetCellValue("工号");
            rowHeader.CreateCell(2, CellType.STRING).SetCellValue("入职时间");
            rowHeader.CreateCell(3, CellType.STRING).SetCellValue("学历");
            rowHeader.CreateCell(4, CellType.STRING).SetCellValue("毕业院校");
            rowHeader.CreateCell(5, CellType.STRING).SetCellValue("基本工资");
            rowHeader.CreateCell(6, CellType.STRING).SetCellValue("部门");
            rowHeader.CreateCell(7, CellType.STRING).SetCellValue("职位");
            rowHeader.CreateCell(8, CellType.STRING).SetCellValue("合同签订日期");
            rowHeader.CreateCell(9, CellType.STRING).SetCellValue("合同到期日期");

            Employee[] employees = (Employee[])datagrid.ItemsSource;

            ICellStyle cellStyle = workbook.CreateCellStyle();
            IDataFormat dataFormat = workbook.CreateDataFormat();

            cellStyle.DataFormat = dataFormat.GetFormat("yyyy\"年\"m\"月\"d\"日\"");

            for (int i = 0; i < employees.Length; i++)
            {
                Employee employee = employees[i];
                IRow row = sheet.CreateRow(i + 1);
                row.CreateCell(0, CellType.STRING).SetCellValue(employee.Name);
                row.CreateCell(1, CellType.STRING).SetCellValue(employee.Number);
                ICell dateCell = row.CreateCell(2, CellType.NUMERIC);
                dateCell.CellStyle = cellStyle;
                dateCell.SetCellValue(employee.InDate);

                row.CreateCell(3, CellType.STRING).SetCellValue(IdNameDAL.GetEducationNameById(employee.EducationId));
                row.CreateCell(4, CellType.STRING).SetCellValue(employee.School);
                row.CreateCell(5, CellType.STRING).SetCellValue(employee.BaseSalary);
                row.CreateCell(6, CellType.STRING).SetCellValue(DepartmentDAL.GetById(employee.DepartmentId).Name);
                row.CreateCell(7, CellType.STRING).SetCellValue(employee.Position);
                ICell contractBegindateCell = row.CreateCell(8, CellType.NUMERIC);
                contractBegindateCell.CellStyle = cellStyle;
                contractBegindateCell.SetCellValue(employee.ContractStartDay);

                ICell contractEnddateCell = row.CreateCell(9, CellType.NUMERIC);
                contractEnddateCell.CellStyle = cellStyle;
                contractEnddateCell.SetCellValue(employee.ContractEndDay);
                
            }

            using (Stream stream = File.OpenWrite(fileName))
            {
                workbook.Write(stream);
            }

        }