ExportExcelDotnet.cs

using ClosedXML.Excel;

public async Task<FileResult> ExportPeopleInExcel()
{
    var people = await context.People.ToListAsync();
    var fileName = "people.xlsx";
    return GenerateExcel(fileName, people);
}

private FileResult GenerateExcel(string fileName, IEnumerable<Person> people)
{
    DataTable dataTable = new DataTable("People");
    dataTable.Columns.AddRange(new DataColumn[]
    {
        new DataColumn("Id"),
        new DataColumn("Name")
    });

    foreach (var person in people)
    {
        dataTable.Rows.Add(person.Id, person.Name);
    }

    using (XLWorkbook wb = new XLWorkbook())
    {
        wb.Worksheets.Add(dataTable);
        using (MemoryStream stream = new MemoryStream())
        {
            wb.SaveAs(stream);

            return File(stream.ToArray(),
                "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
                fileName);
        }
    }

}

reference: https://github.com/gavilanch/ExportExcelDemoMVC/blob/master/ExportExcelDemo/Controllers/HomeController.cs