byte[] byteInfo = workbook.SaveToMemory(FileFormat.OpenXMLWorkbookMacroEnabled);
workStream.Write(byteInfo, 0, byteInfo.Length);
workStream.Position = 0;
return workStream;

byte[] byteInfo = workbook.SaveToMemory(FileFormat.OpenXMLWorkbookMacroEnabled);

void StreamExcelFile(byte[] bytes)
{
    Response.Clear();
    Response.ContentType = "application/force-download";
    Response.AddHeader("content-disposition", "attachment; filename=name_you_file.xls");
    Response.BinaryWrite(bytes);
    Response.End();
}