C# using virtual-override

Question

In this example it uses polymorphysm via virtual, override keywords

 abstract class A
    {
        public virtual string Print() { return "A"; }
    }
    
    class B : A
    {
        public override string Print() { return "B"; }
    }
    
    class C : B
    {
        public virtual new string Print() { return "C"; }
    }
    
    class D : C
    {
        public override string Print() { return "D"; }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            A a = new D();
    
            Console.WriteLine(a.Print());
        }
    }

Console prints B. Why B, not D? Thank you for answers

Whoever think it's a good question consider it's reposted typical [quiz question](https://codegalaxy.io/courses/csharp/questions/c2cbcce9d7d444ed9eccb1952331ed74/public-abstract-class-a-public-virtual-string-print-return-a-public-class-b-a-pu), without any research attempt. — Sinatr, Dec 16 '20 at 08:13

score 1 · Accepted Answer · answered Dec 16 '20 at 07:59

Simple, because the last time you override the Print function of class A is during the declaration of class B

    class B : A
    {
        public override string Print() { return "B"; }
    }

By declaring public virtual new, you are hiding the underlying implementation of Print. So during the D declaration you are overriding the new implementation, declared at class C

Now A, has no knowledge whatsoever of what you've done, once you've hidden the function on B

EricSchaefer · Answer 2 · 2020-12-16T08:09:24.537

1

That's the problem with methods that hide methods of their parent classes. It is not polymorphic. Since the variable is of type A the resolution of method Print() only knows about A.Print() and B.Print(). C.Print() and D.Print() are not overrides of A.Print(), because of the declation of C.Print() with new.

edited Dec 16 '20 at 08:09

answered Dec 16 '20 at 08:03

EricSchaefer

25,272
21
67
103

C# using virtual-override

2 Answers2