How can i use unioan and nested from linq?

Answer 1

will that suit your needs?

var id = 
  (from task in stock.CTasks select new { task.id, task.workorder, jobseqno= task.jobseqno } )
  .Union(from card in stock.NonRoutineCards select new { card.id, card.workorderno, jobseqno = card.cardno })
  .Union(from card in stock.AdditionalWorkCards select new { card.id, card.workorderno, jobseqno = card.cardno })
  .Where(x => x.workorderno.TrimEnd() == ToNo && x.jobseqno == ToSeq)
  .FirstOrDefault();