Question

I have this code but I just can’t understand it.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1 {
    interface IStoreable {
        void Read();
        void Write();
    }
    class Person : IStoreable {
        public virtual void Read() { Console.WriteLine("Person.Read()"); }
        public void Write() { Console.WriteLine("Person.Write()"); }
    }
    class Student : Person {
        public override void Read() { Console.WriteLine("Student.Read()"); }
        public new void Write() { Console.WriteLine("Student.Write()"); }
    }
    class Demo {
        static void Main(string[] args) {
            Person s1 = new Student();
            IStoreable isStudent1 = s1 as IStoreable;

            // 1
            Console.WriteLine("// 1");
            isStudent1.Read();
            isStudent1.Write();           

            Student s2 = new Student();
            IStoreable isStudent2 = s2 as IStoreable;

            // 2
            Console.WriteLine("// 2");
            isStudent2.Read();
            isStudent2.Write();

            Console.ReadKey();
        }
    }    
}

I thought Student.Write() would be called in both cases, so I was puzzled by what I got:

// 1
Student.Read()
Person.Write()
// 2
Student.Read()
Person.Write()

Why is Person.Write() called instead of ‘Student.Write()`?

Answer 1

The new keyword indicates that you do not intend to override the base class’s Write() method (you can’t anyway, since Person‘s Write() method isn’t marked virtual). Since you’re calling it via IStoreable, there’s nothing about the IStoreable interface that links it to the Student class. Since Write() is not marked virtual, polymorphism for this function doesn’t apply.