Polymorphism and inheritance

[Goncalo Oliveira]

Let's see if I can explain my small doubt. I can go around this, but as I was coding something, I got curious if something wasn't possible to do in c#. In C++, you could have something like this: class A { public virtual void method() { // something } } class AA : public A { public void method() { // something else } } class AB : public B { public void method() { // even something else } } A *obj; if ( something ) { obj = (A)( new AA() ); } else { obj = (A)( new AB() ); } obj->method(); What happens here is that when you invoke the method function, you'll be executing the implementation from AA or from AB and not from A. Though, in C#, if I try to do something like this, you'll be executing the implementation from A. That fact to me, is a shame... I liked this behavior. And as pointers in C# only exist for values, I suppose this is not possible with C#. Best regards

Gonçalo A.

[originSH]

Why dont you try it and see what happens?

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

namespace VirtualMethods
{
class Program
{
static void Main(string[] args)
{
A a = new A();
A aa = new AA();
A ab = new AB();
A ac = new AC();

        Console.WriteLine(a.GetValue());
        Console.WriteLine(aa.GetValue());
        Console.WriteLine(ab.GetValue());
        Console.WriteLine(ac.GetValue());

        Console.ReadLine();
    }
}

}

class A
{
public virtual string GetValue()
{
return "A";
}
}

class AA : A
{
public override string GetValue()
{
return "AA";
}
}

class AB : A
{
public override string GetValue()
{
return "AB";
}
}

class AC : A
{
public override string GetValue()
{
return base.GetValue();
}
}

[Goncalo Oliveira]

override.... right. I feel like a newbie, but I guess these things happen when moving on to a new language :p Thank you for the quick reply, you helped a lot. :)

Gonçalo A.

[originSH]

no problem :) You can only override a virtual or abstract method, but you can "new" a method over any old one. i.e. class AA : A { public new string GetValue() { return "NEW"; } } But this will behave how you originally described ... if cast as 'A' it'll call the original method on the A class. It'll only call the 'new' method if cast as an AA.

[Goncalo Oliveira]

Yes, that was how I was doing it. The original was a virtual method, but I expected him to override automatically. Though, it does make more sense this way, and offers more possibilities. Thank you, once again, for the explanation.

Gonçalo A.

[Luc Pattyn]

Hi, you should pay attention to the messages the compiler emits. If you try and redefine a virtual method without either "new" or "override", a warning is issued. Don't lower the warning level (a project setting, 4 is a good value) to get rid of (some of) the messages, study the messages and adapt your code instead. :)

Luc Pattyn [Forum Guidelines] [My Articles]

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Happy 2008!

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[Goncalo Oliveira]

Yes, I noticed that already. Thanks.

Gonçalo A.