var v =from u in db.TableA
       let z=
       (from n in db.TableB
        select n.ID)
       where !z.Contains(u.ID)
       select u;

------解决方案--------------------

探讨

引用:
from a in A
join b in B on a.ID equals b.ID into AB
from ab in AB.DefaultIfEmpty()
where a.ID != b.ID
select new
{
ID=a.ID,
Name=a.Name
};

我试了，但到"!=b.ID"这里就出错，说上下文不存在……