直接走一個範例

using (var ms = new MemoryStream())
{
	var mapper = new Npoi.Mapper.Mapper();
	mapper.Put(你自己的東西, "Sheet名稱");
	var sheet = mapper.Workbook.GetSheetAt(0);
	var rowEnum = sheet.GetRowEnumerator();
	while (rowEnum.MoveNext())
	{
		IRow row = (IRow)rowEnum.Current;
		if (row.RowNum == 0) //略過標題欄位
		{
			continue;
		}
		XSSFFont boldFont = (XSSFFont)mapper.Workbook.CreateFont();
		boldFont.Boldweight = (short)FontBoldWeight.Bold;
		boldFont.Color = NPOI.HSSF.Util.HSSFColor.Red.Index;
		boldFont.FontHeightInPoints = 11;
		boldFont.FontName = "新細明體";

		XSSFCellStyle boldStyle = (XSSFCellStyle)mapper.Workbook.CreateCellStyle();
		boldStyle.SetFont(boldFont);
		for (int i = 0; i < row.Cells.Count; i++)
		{
			ICell cell = row.Cells[i];
			cell.CellStyle = boldStyle;
		}
	}

	mapper.Save(ms);
	 string fileName = $"{你的檔名}.xlsx";
	return File(ms.ToArray(), "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", fileName);
}

 NPOI中表單(Sheet)的列(Row)列舉器(Enumerator)使用

var rowEnum = sheet.GetRowEnumerator();
while (rowEnum.MoveNext())
{
	IRow row = (IRow)rowEnum.Current;
	...
}

 參考資料

• NPOI.Mapper Github
• ez2o
• mis2000lab
• Hot examples