DataTable people = (DataTable)Session["people"];
 
// Create excel file.
ExcelFile ef = new ExcelFile();
ExcelWorksheet ws = ef.Worksheets.Add("DataSheet");
ws.InsertDataTable(people, "A1", true);
 
Response.Clear();
 
// Stream file to browser, in required type.
switch (this.RadioButtonList1.SelectedValue)
{
    case "XLS":
        Response.ContentType = "application/vnd.ms-excel";
        Response.AddHeader("Content-Disposition", "attachment; filename=" +
             "Report.xls");
        ef.SaveXls(Response.OutputStream);
        break;
 
    case "XLSX":
        Response.ContentType = "application/vnd.openxmlformats";
        Response.AddHeader("Content-Disposition", "attachment; filename=" +
             "Report.xlsx");
        // With XLSX it is a bit more complicated as MS Packaging API
        // can't write directly to Response.OutputStream.
        // Therefore we use temporary MemoryStream.
        MemoryStream ms = new MemoryStream();
        ef.SaveXlsx(ms)
        ms.WriteTo(Response.OutputStream);
        break;
}
Response.End();