在c#中使用NPOI结合Magicodes.IE.excel将xlsx文件内存中转换为xls文件

项目中使用Magicodes.IE作为导出excel的组件，但只支持新格式xlsx，有需求要导出旧格式xls文件，因此只能考虑转换的方案，经多种方案尝试和查找相关解决方案，在一份使用NPOI转换的xlsx到xls的文章到找到相关代码，但代码中只支持XSSFWorkbook转换以HSSFWorkbook，扩展后支持byte[]原始数据转换，可以在内存中直接处理。同时原来代码不能处理单元格格式，这里修复后加入单元格格式支持，暂时不支持样式。以下为xlsx转换xls工具类代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using NPOI.HSSF.UserModel;
using NPOI.HSSF.Util;
using NPOI.SS.UserModel;
using NPOI.SS.Util;
using NPOI.Util;
using NPOI.XSSF.UserModel;

namespace Application.Util;
public static class ConvertXLSXToXLS
{
    /// 

    /// 转换xlsx到xls
    /// 
    /// 
    /// 
    public static byte[] ConvertWorkbookXSSFToHSSF(byte[] source)
    {
        using (var stream = new MemoryStream(source))
        {
            XSSFWorkbook xwb = new XSSFWorkbook(stream);


            HSSFWorkbook hwb = ConvertWorkbookXSSFToHSSF(xwb);

            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            hwb.Write(bos);
            return bos.ToByteArray();
        }
    }

    /// 

    /// 转换xlsx到xls
    /// 
    /// 
    /// 
    public static HSSFWorkbook ConvertWorkbookXSSFToHSSF(XSSFWorkbook source)
    {
        //Install-Package NPOI -Version 2.0.6
        HSSFWorkbook retVal = new HSSFWorkbook();
        for (int i = 0; i < source.NumberOfSheets; i++)
        {
            HSSFSheet hssfSheet = (HSSFSheet)retVal.CreateSheet(source.GetSheetAt(i).SheetName);

            XSSFSheet xssfsheet = (XSSFSheet)source.GetSheetAt(i);
            CopySheets(xssfsheet, hssfSheet, retVal);
        }
        return retVal;
    }

    private static void CopySheets(XSSFSheet source, HSSFSheet destination, HSSFWorkbook retVal)
    {
        int maxColumnNum = 0;
        Dictionary<int, XSSFCellStyle> styleMap = new Dictionary<int, XSSFCellStyle>();
        for (int i = source.FirstRowNum; i <= source.LastRowNum; i++)
        {
            XSSFRow srcRow = (XSSFRow)source.GetRow(i);
            HSSFRow destRow = (HSSFRow)destination.CreateRow(i);
            if (srcRow != null)
            {
                CopyRow(source, destination, srcRow, destRow, styleMap, retVal);
                if (srcRow.LastCellNum > maxColumnNum)
                {
                    maxColumnNum = srcRow.LastCellNum;
                }
            }
        }
        for (int i = 0; i <= maxColumnNum; i++)
        {
            destination.SetColumnWidth(i, source.GetColumnWidth(i));
        }
    }

    private static void CopyRow(XSSFSheet srcSheet, HSSFSheet destSheet, XSSFRow srcRow, HSSFRow destRow,
            Dictionary<int, XSSFCellStyle> styleMap, HSSFWorkbook retVal)
    {
        // manage a list of merged zone in order to not insert two times a
        // merged zone
        List<CellRangeAddress> mergedRegions = new List<CellRangeAddress>();
        destRow.Height = srcRow.Height;
        // pour chaque row
        for (int j = srcRow.FirstCellNum; j <= srcRow.LastCellNum; j++)
        {
            XSSFCell oldCell = (XSSFCell)srcRow.GetCell(j); // ancienne cell
            HSSFCell newCell = (HSSFCell)destRow.GetCell(j); // new cell
            if (oldCell != null)
            {
                if (newCell == null)
                {
                    newCell = (HSSFCell)destRow.CreateCell(j);
                }
                // copy chaque cell
                CopyCell(oldCell, newCell, styleMap, retVal);
                // copy les informations de fusion entre les cellules
                CellRangeAddress mergedRegion = GetMergedRegion(srcSheet, srcRow.RowNum,
                        (short)oldCell.ColumnIndex);

                if (mergedRegion != null)
                {
                    CellRangeAddress newMergedRegion = new CellRangeAddress(mergedRegion.FirstRow,
                            mergedRegion.LastRow, mergedRegion.FirstColumn, mergedRegion.LastColumn);
                    if (IsNewMergedRegion(newMergedRegion, mergedRegions))
                    {
                        mergedRegions.Add(newMergedRegion);
                        destSheet.AddMergedRegion(newMergedRegion);
                    }

                    if (newMergedRegion.FirstColumn == 0 && newMergedRegion.LastColumn == 6 && newMergedRegion.FirstRow == newMergedRegion.LastRow)
                    {
                        HSSFCellStyle style2 = (HSSFCellStyle)retVal.CreateCellStyle();
                        style2.VerticalAlignment = VerticalAlignment.Center;
                        style2.Alignment = HorizontalAlignment.Left;
                        style2.FillForegroundColor = HSSFColor.Teal.Index;
                        style2.FillPattern = FillPattern.SolidForeground;

                        for (int i = destRow.FirstCellNum; i <= destRow.LastCellNum; i++)
                        {
                            if (destRow.GetCell(i) != null)
                                destRow.GetCell(i).CellStyle = style2;
                        }
                    }
                }
            }
        }



    }

    private static void CopyCell(XSSFCell oldCell, HSSFCell newCell, Dictionary<int, XSSFCellStyle> styleMap, HSSFWorkbook retVal)
    {
        if (styleMap != null)
        {
            int stHashCode = oldCell.CellStyle.Index;
            XSSFCellStyle sourceCellStyle = null;
            if (styleMap.TryGetValue(stHashCode, out sourceCellStyle)) { }

            HSSFCellStyle destnCellStyle = (HSSFCellStyle)newCell.CellStyle;
            if (sourceCellStyle == null)
            {
                //sourceCellStyle = (XSSFCellStyle)oldCell.Sheet.Workbook.CreateCellStyle();
                sourceCellStyle = (XSSFCellStyle)oldCell.CellStyle;
            }
            //destnCellStyle.CloneStyleFrom(oldCell.CellStyle);
            CloneCellStyle(sourceCellStyle,ref destnCellStyle, retVal);
            if (!styleMap.Any(p => p.Key == stHashCode))
            {
                styleMap.Add(stHashCode, sourceCellStyle);
            }

            destnCellStyle.VerticalAlignment = VerticalAlignment.Top;
            newCell.CellStyle = (HSSFCellStyle)destnCellStyle;
        }
        switch (oldCell.CellType)
        {
            case CellType.String:
                newCell.SetCellValue(oldCell.StringCellValue);
                break;
            case CellType.Numeric:
    			newCell.SetCellValue(oldCell.NumericCellValue);
                break;
            case CellType.Blank:
                newCell.SetCellType(CellType.Blank);
                break;
            case CellType.Boolean:
                newCell.SetCellValue(oldCell.BooleanCellValue);
                break;
            case CellType.Error:
                newCell.SetCellErrorValue(oldCell.ErrorCellValue);
                break;
            case CellType.Formula:
                newCell.SetCellFormula(oldCell.CellFormula);
                break;
            default:
                break;
        }

    }


    private static CellRangeAddress GetMergedRegion(XSSFSheet sheet, int rowNum, short cellNum)
    {
        for (int i = 0; i < sheet.NumMergedRegions; i++)
        {
            CellRangeAddress merged = sheet.GetMergedRegion(i);
            if (merged.IsInRange(rowNum, cellNum))
            {
                return merged;
            }
        }
        return null;
    }

    private static bool IsNewMergedRegion(CellRangeAddress newMergedRegion,
            List<CellRangeAddress> mergedRegions)
    {
        return !mergedRegions.Contains(newMergedRegion);
    }

    public static void CloneCellStyle(XSSFCellStyle sourceCellStyle, ref HSSFCellStyle destnCellStyle, HSSFWorkbook retVal)
    {

        IDataFormat dataformat = retVal.CreateDataFormat();
        string cellStyleString = sourceCellStyle.GetDataFormatString();
        ICellStyle dateStyle = retVal.CreateCellStyle();
        dateStyle.DataFormat = dataformat.GetFormat(cellStyleString);

        destnCellStyle = (HSSFCellStyle)dateStyle;
    }
}

使用方式，使用了Magicodes.IE.excel，具体组件自行下载：

    /// 

    /// 导出XLS
    /// 
    /// 
    [ApiDescriptionSettings(Name = "ExportXLS"), NonUnify]
    [DisplayName("导出XLS")]
    public async Task<IActionResult> ExportXLS(RecordInput input)
    {
        var query = QueryData(input);
        IExcelExporter excelExporter = new ExcelExporter();
        var res = await excelExporter.ExportAsByteArray(query.ToList());
        return new FileStreamResult(new MemoryStream(ConvertXLSXToXLS.ConvertWorkbookXSSFToHSSF(res)), "application/octet-stream") { FileDownloadName = "导出数据" + DateTime.Now.ToString("yyyyMMddHHmmss") + ".xls" };
    }

通过这种一行行转换的方案，理论上也可以将旧格式xls转换为新格式xlsx。

参考：

在c#中使用NPOI将xlsx文件转换为xls文件
https://www.itbaoku.cn/post/1946308.html?view=all

HSSFWorkbook对象转换成输入流
https://www.cnblogs.com/slzys/p/13590907.html

java实现修改excel中数据格式
https://blog.csdn.net/weixin_45706856/article/details/130328932

C# NPOI 导出Excel 日期格式
https://blog.51cto.com/u_15976398/6099632