用NPOI导出百万级数据，导出报错，System.OutOfMemoryException

然后想到分段查询出数据，多个sheet导出，但还是导不出数据
代码如下：
//查询出数据
var exportData =getData();
byte[] export;
//2.使用NPOI构建Excel文件
var workbook = new XSSFWorkbook();
//数据大于5W分不同sheet导出
if (exportData.Count>50000)
{
decimal maxNum = 50000;
decimal sheetNum= Math.Ceiling(exportData.Count / maxNum);
int allNum = 0;
for (int i = 1; i <= sheetNum; i++)
{
var sheet = workbook.CreateSheet("Sheet"+i);
#region 构建表头
//定义CellStyle
var cellStyle = workbook.CreateCellStyle();
cellStyle.WrapText = false;
cellStyle.VerticalAlignment = VerticalAlignment.Center;
cellStyle.Alignment = HorizontalAlignment.Center;

                //2.1构建表头
                var headRow = sheet.CreateRow(0);

                //定义HeadStyle
                var headFont = workbook.CreateFont();
                headFont.FontHeightInPoints = 10;
                headFont.Boldweight = 700;
                var headStyle = workbook.CreateCellStyle();
                headStyle.WrapText = false;
                headStyle.VerticalAlignment = VerticalAlignment.Center;
                headStyle.Alignment = HorizontalAlignment.Center;
                headStyle.SetFont(headFont);

                headRow.CreateCell(0).SetCellValue("数据1");
                headRow.GetCell(0).CellStyle = headStyle;
                headRow.CreateCell(1).SetCellValue("数据2");
                headRow.GetCell(1).CellStyle = headStyle;
                headRow.CreateCell(2).SetCellValue("数据3");
                headRow.GetCell(2).CellStyle = headStyle;
           
                // 设置列宽度
                sheet.SetColumnWidth(0, 15 * 256);
                sheet.SetColumnWidth(1, 20 * 256);
                sheet.SetColumnWidth(2, 50 * 256);
                sheet.SetColumnWidth(3, 50 * 256);
                sheet.DefaultRowHeight = 200;
                #endregion 
                //2.2构建数据
                var currentRowIndex = 1;

                for (int j = 0; j < (exportData.Count/sheetNum); j++)
                {
                    if (allNum > exportData.Count)
                        continue;
                    IRow newRow = sheet.CreateRow(currentRowIndex);
                    newRow.CreateCell(0).SetCellValue(exportData[allNum].dataNum);
                   
                    //设置默认行高
                    newRow.HeightInPoints = 50;
                    newRow.CreateCell(1).SetCellValue(exportData[allNum].dataNum1);
                    newRow.CreateCell(2).SetCellValue(exportData[allNum].dataNum2);

                    currentRowIndex++;
                    allNum++;
                }

            }

 
        }
        using (var ms = new MemoryStream())
        {
            workbook.Write(ms);
            ms.Flush();
            ms.Position = 0;
            export = ms.ToArray();
        }