public IActionResult Index(string searchString, int pageNum = 1, int pageSize = 3)
        {
            ViewBag.searchString = searchString;
            List <UserReadViewModel> users = _context.appUsers.Where(x => !string.IsNullOrEmpty(searchString) ? x.UserName.Contains(searchString) : true)
                                             .OrderBy(x => x.Id).Skip((pageNum - 1) * pageSize).Take(pageSize)
                                             .Select(n => new UserReadViewModel
            {
                Id       = n.Id,
                UserName = n.UserName,
                Gender   = n.Gender
            }).ToList();
            int TotalPage = (int)Math.Ceiling(Decimal.Divide(_context.appUsers.Count(x => !string.IsNullOrEmpty(searchString) ? x.UserName.Contains(searchString) : true), pageSize));

            UserPaginationViewModel PageData = new UserPaginationViewModel()
            {
                pagination = _paginationService.GetPagination(pageNum, pageSize, TotalPage),
                users      = users
            };

            return(View(PageData));
        }