在使用hssf可以直接按照行列插入。

 HSSFWorkbook book = new HSSFWorkbook();
              
                HSSFSheet sheet1 = (HSSFSheet)book.CreateSheet("Sheet1");
                HSSFPatriarch patriarch = (HSSFPatriarch)sheet1.CreateDrawingPatriarch();
                HSSFClientAnchor a1 = new HSSFClientAnchor(255, 125, 1023, 150, 0, 5, 2, 2);
                HSSFSimpleShape line1 = patriarch.CreateSimpleShape(a1);
                line1.ShapeType = HSSFSimpleShape.OBJECT_TYPE_LINE;
                line1.LineStyle = HSSFShape.LINESTYLE_SOLID;
                //在NPOI中线的宽度12700表示1pt,所以这里是0.5pt粗的线条。
                line1.LineWidth = 6350;
                using (FileStream fileStream = new FileStream("sss.xlsx", FileMode.Create))
                {
                    book.Write(fileStream);
                }

 但是呢，在使用XSSF得时候，使用同样方式得写法就不生效了，根据chatgpt给出的答案。

 我也不知道这个答案是否准确，但是我接纳他的建议，画一条斜向下得线，然后垂直翻转过来。

IWorkbook workbook = new XSSFWorkbook();
                ISheet sheet = workbook.CreateSheet("Sheet1");

                XSSFDrawing drawing = (XSSFDrawing)sheet.CreateDrawingPatriarch();
                
                XSSFClientAnchor anchor = (XSSFClientAnchor)drawing.CreateAnchor(0, 0, 0, 0, 0, 3, 4, 7);

                XSSFSimpleShape line1 = drawing.CreateSimpleShape(anchor);



                line1.ShapeType = 1;
                line1.SetFillColor(255, 255, 0);
                line1.LineStyle = NPOI.SS.UserModel.LineStyle.None;
                line1.LineWidth = 1;
                line1.SetLineStyleColor(255, 255, 0);
               // line1.GetCTShape().spPr.xfrm.flipH = true;
                line1.GetCTShape().spPr.xfrm.flipV = true;

                // line1.GetCTShape().spPr.xfrm.rot = 45;
                // XSSFRichTextString richText = new XSSFRichTextString("Rotated Shape");
                // richText.ApplyFont(0, richText.Length, new XSSFFont() { IsItalic = true });
                XSSFClientAnchor anchor2 = (XSSFClientAnchor)drawing.CreateAnchor(0, 0, 0, 0, 0, 3, 4, 7);

                XSSFSimpleShape line12 = drawing.CreateSimpleShape(anchor2);



                line12.ShapeType = 1;
                line12.SetFillColor(255, 255, 0);
                line12.LineStyle = NPOI.SS.UserModel.LineStyle.None;
                line12.LineWidth = 1;
                line12.SetLineStyleColor(255, 255, 0);
                // line1.GetCTShape().spPr.xfrm.flipH = true;
               // line12.GetCTShape().spPr.xfrm.flipV = true;


                // 将富文本字符串应用到形状
                //line1.Text = richText;
                using (FileStream fileStream = new FileStream("dcf.xlsx", FileMode.Create))
                {
                    workbook.Write(fileStream);
                }

                Console.WriteLine("Excel file created.");