XSSFWorkbook 多表头导出

  IWorkbook workbook = new XSSFWorkbook();
  IFont bodyCellFont = workbook.CreateFont();
  bodyCellFont.FontName = "宋体";
  bodyCellFont.FontHeightInPoints = 12; //设置字体大小

  IFont columnHeadFont = workbook.CreateFont();
  columnHeadFont.FontHeightInPoints = 13; //设置字体大小
  columnHeadFont.FontName = "黑体"; //设置字体

  ICellStyle headCellStyle = workbook.CreateCellStyle();
  headCellStyle.SetFont(columnHeadFont);
  headCellStyle.BorderTop = BorderStyle.Thin;
  headCellStyle.BorderLeft = BorderStyle.Thin;
  headCellStyle.BorderRight = BorderStyle.Thin;
  headCellStyle.BorderBottom = BorderStyle.Thin;
  headCellStyle.Alignment = HorizontalAlignment.Center;
  headCellStyle.VerticalAlignment = VerticalAlignment.Center;

  ICellStyle cellStyle = workbook.CreateCellStyle();
  cellStyle.BorderTop = BorderStyle.Thin;
  cellStyle.BorderLeft = BorderStyle.Thin;
  cellStyle.BorderRight = BorderStyle.Thin;
  cellStyle.BorderBottom = BorderStyle.Thin;
  cellStyle.SetFont(bodyCellFont);

  ISheet sheet1 = workbook.CreateSheet(dtProcessDateStart.Value.ToString("yy-MM") + "工资合计");
  //设置列标题
  List<string> titleList = new List<string> { "工段", "员工编号", "员工姓名", "月工资" };
  List<string> titleList2 = new List<string> { "计件工资", "补差工资", "计时工资", "最终工资" };
  foreach (string monthDay in monthDayList)
  {
      titleList.Add(monthDay);
  }

  var rowTitle = sheet1.CreateRow(0);
  var rowTitle2 = sheet1.CreateRow(1);
  int cellIndex = 0;
  foreach (string title in titleList)
  {
      var j = 0;
      if (cellIndex < 4)
      {
          var cellT = rowTitle.CreateCell(cellIndex);
          cellT.SetCellValue(title);
          cellT.CellStyle = headCellStyle;
　　　　　　//合并列
          CellRangeAddress region = new CellRangeAddress(0, 1, cellIndex, cellIndex);
          sheet1.AddMergedRegion(region);
      }
      else
      {
　　　　  //title 为天数，01,02,03,04...
          var cellT = rowTitle.CreateCell(4 * int.Parse(title));
          cellT.SetCellValue(title);
          cellT.CellStyle = headCellStyle;
　　　　　　//天数下面插入4列
          foreach (var title2 in titleList2)
          {
              var cellT2 = rowTitle2.CreateCell(4 * int.Parse(title) + j);
              cellT2.SetCellValue(title2);
              cellT2.CellStyle = headCellStyle;
              j++;
          }
　　　　　　//合并行,天数行                                        开始行   StartRow              结束行  EndRow
          CellRangeAddress region = new CellRangeAddress(0, 0, 4 * int.Parse(title), 4 * int.Parse(title) + 3);
          sheet1.AddMergedRegion(region);

      }

      if (cellIndex < 3)
      {
          sheet1.SetColumnWidth(cellIndex, 4000);
      }
      else
      {
          sheet1.SetColumnWidth(cellIndex, 2000);
      }
      cellIndex++;
  }

  //设置内容
  int nextInsertIndex = 2;
  foreach (dynamic employee in employeeList)
  {
      List<string> rowData = new List<string>();
      rowData.Add(employee.department_name.ToString());
      rowData.Add(employee.account.ToString());
      rowData.Add(employee.employee_name.ToString());

      var statList = employeeWageStatList.Where(s => s.process_employee_id == employee.id);
      rowData.Add(statList.Sum(s => (decimal)s.day_final_wage).ToString());

      foreach (string monthDay in monthDayList)
      {
          if (statList.FirstOrDefault(s => s.process_day == monthDay) != null)
          {
              rowData.Add(statList.FirstOrDefault(s => s.process_day == monthDay).day_process_wage.ToString());
              rowData.Add(statList.FirstOrDefault(s => s.process_day == monthDay).day_additional_wage.ToString());
              rowData.Add(statList.FirstOrDefault(s => s.process_day == monthDay).day_timing_wage.ToString());
              rowData.Add(statList.FirstOrDefault(s => s.process_day == monthDay).day_final_wage.ToString());
          }
          else
          {
              rowData.Add("");
              rowData.Add("");
              rowData.Add("");
              rowData.Add("");
          }
      }
      IRow row = sheet1.CreateRow(nextInsertIndex);
      for (int k = 0; k < rowData.Count; k++)
      {
          ICell cell = row.CreateCell(k);
          cell.CellStyle = cellStyle;
          if (!string.IsNullOrWhiteSpace(rowData[k]) && Regex.IsMatch(rowData[k], @"^[+-]?\d*[.]?\d*$")) //数字
          {
              if (k > 2)
              {
                  cell.SetCellValue(ConvertUtils.ToDouble(rowData[k], 0));
              }
              else
              {
                  cell.SetCellValue(rowData[k]);
              }
          }
          else
          {
              cell.SetCellValue(rowData[k]);
          }
      }
      nextInsertIndex++;
  }