Linq 左连接 left join

var APPQuery = aa.query();
var ValidQuery = bb.query();

var query = from o in APPQuery
                            join v in ValidQuery
                            on o.Id equals v.Id into result
                            from r in result.DefaultIfEmpty()
                            select new GetFlightDto{
                               Calsign = r.Calsign
                                ........
                              };