背景：需求需要读取数据导出成Excel文件，并且其中有一列需要赋值为公式，用于用户自己修改数据自动计算
导出Excel，我用到开源包MiniExcel
Gitee地址MiniExcel源码介绍，功能说明
Nuget安装 搜索MiniExcel

导出代码如下：
 

//多个Sheet
var sheets = new Dictionary<string, object>();
//保存文件位置
string filePath = Path.Combine(Directory.GetCurrentDirectory(), $"ExportTemplate\\{DateTime.Now.ToString("yyyyMMddHHmmss")}_StoreInventoryDt.xlsx");
{    
List<dynamic> value = new List<dynamic>();
    int index = 2;
    foreach (var itemDetail in storeInventoryExcelDto.ExportList)
    {
        itemDetail.saleInventory = itemDetail.startInventory + itemDetail.barInventory + itemDetail.todayPickUp - itemDetail.saleCount;
        // 创建计算公式，例如：B2 + C2
        //string formula = $"={MiniExcel.GetColumns(columnIndex)}2 + {MiniExcel.GetColumns(columnIndex + 1)}2";
        value.Add(new
        {
            品名 = itemDetail.goodsName,
            品类 = itemDetail.goodsType,
            期初数 = itemDetail.startInventory,
            吧台入库 = itemDetail.barInventory,
            本日拾遗 = itemDetail.todayPickUp,
            售卖 = itemDetail.saleCount,
            置换 = itemDetail.changeCount,
            销售库存 = itemDetail.saleInventory,
            寄存 = itemDetail.inStorageCount,
            取出 = itemDetail.outStorageCount,
            总库存 = itemDetail.saleInventory + itemDetail.inStorageCount - itemDetail.outStorageCount,// itemDetail.totalInventory,
            本日实盘 = itemDetail.todayInventory,
            盘点差异 = itemDetail.inventoryDif,//$"=SUM(K{index},-L{index})", //
            备注 = "",
        });
        index++;
    }
    sheets.Add(storeInventoryExcelDto.InvDate.ToString("MM月dd日"), value.ToArray());
}
MiniExcel.SaveAs(filePath, sheets);

导出结果如下：

找了很久MiniExcel没有设置公式的方法，所以使用了ClosedXml
ClosedXml GitHub源码地址 ClosedXml源码和说明
说明文档-英文版

就是重新读取刚刚保存的excel文件，设置M列=K列-L列，M=K-L
代码如下：

//读取当前excel文件的sheet 
var sheetNames = MiniExcel.GetSheetNames(filePath);
 // 打开现有的Excel文件
 using (var workbook = new XLWorkbook(filePath))
 {
     foreach (var sheet in sheetNames)
     {
         // 获取工作表
         var worksheet = workbook.Worksheet(sheet);
         //获取数据行数
         int rowCount = worksheet.RowsUsed().Count();

         for (int i = 2; i <= rowCount; i++)
         {
             // 设置特定单元格的公式
             worksheet.Cell($"M{i}").FormulaA1 = $"=K{i}-L{i}"; // 使用K列引用样式设置公式
             // 或者使用行和列的索引
             //worksheet.Cell(1, 1).FormulaA1 = "=B1*C1"; // 第1行，第1列的单元格
            
         }
         // 保存更改
         workbook.Save();
     }
 }

就完成excel文件中指定某列的计算公式

整体代码：

    var sheets = new Dictionary<string, object>();
string filePath = Path.Combine(Directory.GetCurrentDirectory(), $"ExportTemplate\\{DateTime.Now.ToString("yyyyMMddHHmmss")}_StoreInventoryDt.xlsx");
    List<dynamic> value = new List<dynamic>();
    //storeInventoryExcelDto数据源
    foreach (var itemDetail in storeInventoryExcelDto.ExportList)
    {
        itemDetail.saleInventory = itemDetail.startInventory + itemDetail.barInventory + itemDetail.todayPickUp - itemDetail.saleCount;
        // 创建计算公式，例如：B2 + C2
        //string formula = $"={MiniExcel.GetColumns(columnIndex)}2 + {MiniExcel.GetColumns(columnIndex + 1)}2";
        value.Add(new
        {
            品名 = itemDetail.goodsName,
            品类 = itemDetail.goodsType,
            期初数 = itemDetail.startInventory,
            吧台入库 = itemDetail.barInventory,
            本日拾遗 = itemDetail.todayPickUp,
            售卖 = itemDetail.saleCount,
            置换 = itemDetail.changeCount,
            销售库存 = itemDetail.saleInventory,
            寄存 = itemDetail.inStorageCount,
            取出 = itemDetail.outStorageCount,
            总库存 = itemDetail.saleInventory + itemDetail.inStorageCount - itemDetail.outStorageCount,// itemDetail.totalInventory,
            本日实盘 = itemDetail.todayInventory,
            盘点差异 = itemDetail.inventoryDif,//$"=SUM(K{index},-L{index})", //
            备注 = "",
        });
    }
    sheets.Add(storeInventoryExcelDto.InvDate.ToString("MM月dd日"), value.ToArray());

MiniExcel.SaveAs(filePath, sheets);
var sheetNames = MiniExcel.GetSheetNames(filePath);
// 打开现有的Excel文件
using (var workbook = new XLWorkbook(filePath))
{
    foreach (var sheet in sheetNames)
    {
        // 获取工作表
        var worksheet = workbook.Worksheet(sheet);

        int rowCount = worksheet.RowsUsed().Count();

        for (int i = 2; i <= rowCount; i++)
        {
            // 设置特定单元格的公式
            worksheet.Cell($"M{i}").FormulaA1 = $"=K{i}-L{i}"; // 使用A1引用样式设置公式
            // 或者使用行和列的索引
            //worksheet.Cell(1, 1).FormulaA1 = "=B1*C1"; // 第1行，第1列的单元格
           
        }
        // 保存更改
        workbook.Save();
    }
}