PostExport.cs

public async Task<IActionResult> OnPostExport()
{
    string sWebRootFolder = _hostingEnvironment.WebRootPath;
    string sFileName = @"demo.xlsx";
    string URL = string.Format("{0}://{1}/{2}", Request.Scheme, Request.Host, sFileName);
    FileInfo file = new FileInfo(Path.Combine(sWebRootFolder, sFileName));
    var memory = new MemoryStream();
    using (var fs = new FileStream(Path.Combine(sWebRootFolder, sFileName), FileMode.Create, FileAccess.Write))
    {
        IWorkbook workbook;
        workbook = new XSSFWorkbook();
        ISheet excelSheet = workbook.CreateSheet("Demo");
        IRow row = excelSheet.CreateRow(0);

        row.CreateCell(0).SetCellValue("ID");
        row.CreateCell(1).SetCellValue("Name");
        row.CreateCell(2).SetCellValue("Age");

        row = excelSheet.CreateRow(1);
        row.CreateCell(0).SetCellValue(1);
        row.CreateCell(1).SetCellValue("Kane Williamson");
        row.CreateCell(2).SetCellValue(29);

        row = excelSheet.CreateRow(2);
        row.CreateCell(0).SetCellValue(2);
        row.CreateCell(1).SetCellValue("Martin Guptil");
        row.CreateCell(2).SetCellValue(33);

        row = excelSheet.CreateRow(3);
        row.CreateCell(0).SetCellValue(3);
        row.CreateCell(1).SetCellValue("Colin Munro");
        row.CreateCell(2).SetCellValue(23);

        workbook.Write(fs);
    }
    using (var stream = new FileStream(Path.Combine(sWebRootFolder, sFileName), FileMode.Open))
    {
        await stream.CopyToAsync(memory);
    }
    memory.Position = 0;
    return File(memory, "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet", sFileName);
}