linq查询或筛选的结果怎么使用啊，强转成要绑定的ObservableCollection类型的集合抛出"未将对象引用设置到对象的实例"

Question

ObservableCollection<Student> a = new ObservableCollection<Student>();
            Student ab = new Student();
            ab.Name = "1.jpg";
            Student cc = new Student();
            cc.Name = "3.jpg";
            Student bb = new Student();
            bb.Name = "2.jpg";
            Student ww = new Student();
            ww.Name = "aaa";
            Student ss = new Student();
            ss.Name = "bbb";
            a.Add(ab);
            a.Add(cc);
            a.Add(bb);
            a.Add(ww);
            a.Add(ss);
            var query = from Student in a
                        orderby Student.Name ascending /*descending*/
                        select Student;
            a = query as ObservableCollection<Student>;
            foreach (Student k in a)
            {
                Console.WriteLine(k.Name);
            }
            Console.ReadKey();

初次使用linq排序或筛选，但不知道怎么使用结果