Using IsNull or select COALESCE in Linq..? [duplicate]

Answer 1

It seems you have a typo near tmp1, you should do tree1.DefaultIfEmpty()

      var qry = from c in ctx.comment
      join p in ctx.professor
      on c.PID equals p.ID into tree
      join s in ctx.student
      on c.SID equals s.ID into tree1
      where c.VID == vID
      from tmp in tree.DefaultIfEmpty()
      from tmp1 in tree1.DefaultIfEmpty()
      select new
      {
          Firstnmae = tmp.firstname ?? tmp1.firstname,
          LastName = tmp.lastname ?? tmp1.lastname
      };