C# new not working as expected on derived class-inheritence

Question

Given the following code below. I am wondering why my output is not B A B, but instead B A A

class A
{
    public virtual void display()
    {
        Console.WriteLine("A");
    }
}
class B : A
{
    public new void display()
    {
        Console.WriteLine(" B ");
    }
}
class Program
{
    static void Main(string[] args)
    {
        A obj1 = new B();
        B obj2 = new B();
        obj2.display();
        A r;
        r = obj1;
        r.display();
        r = obj2;
        r.display();
        Console.ReadLine();
    }
}
//Output:
B
A
A

Since r now is a reference to class B(obj2) it should output B, but it is outputting A.

It's working exactly as most people would expect, so if it's not matching *your* expectations, explain what you were expecting and **why** those were your expectations. — Damien_The_Unbeliever, Oct 26 '18 at 17:02
okay reasoning is as follows 1.obj2 will output B because of the new keyword. — Shadman Mahmood, Oct 26 '18 at 17:05
@ShadmanMahmood yes obj2 currently being typed as B will call the `Display` method in B. `r` being type as `A` doesn't see `Display` in B as it's only for type of B. If it would have been an `override` then it would have called it — Franck, Oct 26 '18 at 17:11
@ShadmanMahmood See my edit, method hiding is all about what type the *variable* is, not what the type its pointing to is — BradleyDotNET, Oct 26 '18 at 17:14
@BradleyDotNET could you kindly give an example to what you mean by "what type the variable is" ? thankyou. — Shadman Mahmood, Oct 26 '18 at 17:18

BradleyDotNET · Accepted Answer · 2018-10-26T17:14:23.563

2

That's not how new works. It is how override works.

new (Method hiding) only works if your variable type (note that the actual type is irrelevant) is the derived type. Your second test is just polymorphism, and you need override for that.

class B : A
{
    public override void display()
    {
        Console.WriteLine(" B ");
    }
}

Side-note, you almost never need method hiding.

edited Oct 26 '18 at 17:14

answered Oct 26 '18 at 17:00

BradleyDotNET

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I know that, but this is an mcq question, so I am confused about the output – Shadman Mahmood Oct 26 '18 at 17:02
1

@ShadmanMahmood No idea what mcq is, perhaps the question is just wrong? – BradleyDotNET Oct 26 '18 at 17:03
mcq is a multiple choice question. I want to know why the output is like that. Thats all – Shadman Mahmood Oct 26 '18 at 17:16
@ShadmanMahmood Errr…. ok. The second paragraph (and the linked duplicates) explain this, you are missing the difference between hiding a method and using polymorphsim – BradleyDotNET Oct 26 '18 at 17:17

C# new not working as expected on derived class-inheritence

1 Answers1