public async Task <IActionResult> OnPostAsync() { if (!ModelState.IsValid) { return(Page()); } var item = _context.Project.SingleOrDefault(t => t.Id == Project.Id); var tags = _service.GetTagId(Project.Id); //delete id _service.RemoveProjectTags(tags.Except(TagId)); //add id _service.AddProjectTags(TagId.Except(tags), item.Id); if (item == null) { return(NotFound()); } item.Name = Project.Name; item.Title = Project.Title; item.Address = Project.Address; item.Description = Project.Description; _context.Update(item); try { await _context.SaveChangesAsync(); } catch (DbUpdateConcurrencyException) { if (!ProjectExists(Project.Id)) { return(NotFound()); } else { throw; } } return(RedirectToPage("./Index")); }