c# - Polymorphism - exercise -

alright, exercise:

define class named student, containing 3 grades of students. class have function calculates grades average. now, define class named student1 derived student , add function calculate sum of grades.in main program, define student variable , object of type student1. perform placement of object variable , run function of student1.

note: not homework, i'm learning own. code:

class student {     protected int grade1, grade2, grade3;     public student(int grade1, int grade2, int grade3)     {         this.grade1 = grade1;         this.grade2 = grade2;         this.grade3 = grade3;     }     public double average()     {         return (grade1 + grade2 + grade3) / 3;     } } class student1 : student {     public student1(int grade1, int grade2, int grade3)         : base(grade1, grade2, grade3)     {     }     public double sum()     {         return grade1 + grade2 + grade3;     } } class program {     static void main(string[] args)     {     } } 

i don't know in main class, how perform placement , also, wanted know what's benefit of doing it, let me know if have mistakes far, alot.

ok: guess they're looking for, although english little ropey:

1) declare student variable

 student s; 

2) declare student1 object

 student1 s1 = new student1(1,2,3); 

3) perform placement of object variable:

s = s1; 

4) run function (note you'll have cast s student1 type access type specific function sum)

console.writeline(((student1)s).sum());