程序代码：

    //People类
    public class People
    {
        public int Num;
        public int Age;
        public string Sex; 
        public void MyPed()
        {
            Console.WriteLine("ped");         
        }
    }
    //Student类，继承People类
    public class Student:People
    {
        public int StudentNum;
        public int Grade;
        public void MyStudent()
        {
            Console.WriteLine("student");
          
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            //建立一个People类型的数组
            People[] pedlist = new People[5];
            for (int i = 0; i < 5; i++)
            {
                Student stu = new Student();    //创建Student类的对象
                stu.Grade = i;                  //赋值
                pedlist[i] = stu;               //将对象存入pedlist中
            }

            for (int i = 0; i < pedlist.Length; i++)
               
            {              
                //////////////////////////////////////////
                //通过遍历pedlist，输出每个stu的Grade值，调用MyStudent()函数
                //
                ////////////////////////////////////////
                //这里不会了，用“pedlist[i].__”的方式不行呀
            }
            Console.ReadLine();
        }
    }