public ActionResult List(string category, string keyWord, int pageIndex = 1)
        {
            Parallel.Invoke(SetDropDownList);
            Task<List<Book>> task=Task<List<Book>>.Factory.StartNew(()=>{
                List<Book> books = null;
                if (!string.IsNullOrEmpty(category))
                {
                BookQueryCategories cate = (BookQueryCategories)Enum.Parse(typeof(BookQueryCategories), category);
                books = new BookManager().GetBooks(cate, keyWord);
                }
                else
                {
                    books = new BookManager().GetBooks();
                }
                     return books;
            });
            List<Book> book = task.Result;
            PagedList<Book> pagedBooks = new PagedList<Book>(book, pageIndex, pageSize);
            //Task t = Task.Factory.StartNew(SetDropDownList);
            //SetDropDownList(category);
            
            ViewData["keyWord"] = keyWord;

            return View("List", pagedBooks);
        }