npoi单元格样式问题

 ICellStyle cellStyle = workbook.CreateCellStyle();          cellStyle.DataFormat = HSSFDataFormat.GetBuiltinFormat("0.00%");          SumRow.CreateCell(17).CellStyle = cellStyle; 
SumRow.CreateCell(17).SetCellValue(Convert.ToDouble(sumExamineGrossRate)); 

我是这么加样式的为什么不起作用啊？
[解决办法]

/// <summary>
        /// 获取单元格样式
        /// </summary>
        /// <param name="hssfworkbook">Excel操作类</param>
        /// <param name="font">单元格字体</param>
        /// <param name="fillForegroundColor">图案的颜色</param>
        /// <param name="fillPattern">图案样式</param>
        /// <param name="fillBackgroundColor">单元格背景</param>
        /// <param name="ha">垂直对齐方式</param>
        /// <param name="va">垂直对齐方式</param>
        /// <returns></returns>
        public static ICellStyle GetCellStyle(HSSFWorkbook hssfworkbook, IFont font, HSSFColor fillForegroundColor, FillPatternType fillPattern, HSSFColor fillBackgroundColor, HorizontalAlignment ha, VerticalAlignment va)
        {
            ICellStyle cellstyle = hssfworkbook.CreateCellStyle();
            cellstyle.FillPattern = fillPattern;
            cellstyle.Alignment = ha;
            cellstyle.VerticalAlignment = va;
            if (fillForegroundColor != null)
            {
                cellstyle.FillForegroundColor = fillForegroundColor.GetIndex();
            }
            if (fillBackgroundColor != null)
            {
                cellstyle.FillBackgroundColor = fillBackgroundColor.GetIndex();
            }
            if (font != null)
            {
                cellstyle.SetFont(font);
            }
            //有边框
            cellstyle.BorderBottom = CellBorderType.THIN;
            cellstyle.BorderLeft = CellBorderType.THIN;
            cellstyle.BorderRight = CellBorderType.THIN;
            cellstyle.BorderTop = CellBorderType.THIN;
            return cellstyle;
        }

[解决办法]
没看到你对cellStyle?设置过样式属性咧！你要设置相关属性啊，如字体、边距、背景等，如：
//设置字体
Font font1 = hssfworkbook.CreateFont();
font1.Color = HSSFColor.RED.index;
font1.IsItalic = false;
font1.FontHeightInPoints = 10;

cellStyle.SetFont(font1);
//设置边框背景等
cellStyle.BorderLeft = CellBorderType.THIN;
cellStyle.LeftBorderColor = HSSFColor.GREY_40_PERCENT.index;
cellStyle.BorderRight = CellBorderType.THIN;
cellStyle.RightBorderColor = HSSFColor.GREY_40_PERCENT.index;
cellStyle.FillForegroundColor = NPOI.HSSF.Util.HSSFColor.GREY_40_PERCENT.index;
cellStyle.FillPattern = FillPatternType.SOLID_FOREGROUND;