entity framework实现left join功能

 1 var query=(from a in db.TableA
 2                   join b in db.TableB
 3                   on a.ID equals b.NodeID into c
 4                   from x in c.DefaultIfEmpty()
 5                   where a.ID==param
 6                   select new
 7                   {
 8                           a,
 9                           x.Key
10                   }).FirstOrDefault();