LINQ查询处理出错

Question

实际上,我正在尝试按城市分组学生名单.当我执行此操作时,我在s2.City附近的LINQ语句中出现"对象引用"错误.

class Groupby
    {
        Student[] s = new Student[10];

        public Groupby()
        {
            s[0] = new Student("Manikandan", "Jayagopi", "Chennai");

            s[1] = new Student("Ganesh", "Bayyanna", "Bangalore");

            s[2] = new Student("Laxman", "Bharghav", "Hyderabad");

            s[3] = new Student("Dinesh","Vellaiyan","Pune");

            s[4] = new Student("Natarajan","Nallathambi","Chennai");
        }

        public void Group()
        {                
            var groupQuery = from s2 in s
                             group s2 by s2.City;

            foreach (Student s1 in groupQuery)
                Console.WriteLine(" {0}", s1.FirstName);

        }
    }

class Program
    {
static void Main()
        {            
            Groupby objGroupby = new Groupby();

            objGroupby.Group();

            Console.ReadLine();
        }
    }

谁能帮我吗？

提前致谢

Answer 1

你有一个10项的数组,只有初始化5.其他5个是null因为数组有固定的长度.这意味着s2.City会导致NullReferenceException.所以其中一个:

• 不要超大数组:

  Student[] s = new Student[5];
  

• 使用a List<T>而不是数组:

  List<Student> s = new List<Student>();
  ///
  s.Add(new Student { ... }); // etc
  

• 检查null:

  var groupQuery = from s2 in s
                   where s2 != null
                   group s2 by s2.City;