C# poly and method hiding issue

[harrylmh]

Hi,
I'm learning C# and I just don't quite understand the need for
polymorphism. why do we need to use it? how does a base class variable
holding a derived class instance do any good?

Also, what's the difference between method hiding and overriding when
they're both still overriding the base method.

Thanks

[Sami Vaaraniemi]

Re: C# poly and method hiding issue

"harrylmh" <minghong@hotpo p.com> wrote in message
news:d0pe70tvmb i8uldnogagoaevq nuqhcf1tp@4ax.c om...[color=blue]
> Hi,
> I'm learning C# and I just don't quite understand the need for
> polymorphism. why do we need to use it? how does a base class variable
> holding a derived class instance do any good?
>
> Also, what's the difference between method hiding and overriding when
> they're both still overriding the base method.
>
> Thanks[/color]


Polymorphism allows you to treat instances of different classes in a uniform
manner. Suppose e.g., that you have a container of objects that are all
subclasses of the type Drawable, say Rectangles and Circles. Suppose also
that class Drawable contains a virtual method Draw that the subclasses
override. You can then draw all the objects on the screen with code like
this:

foreach (Drawable d in drawables)
d.Draw(graphics );

The real power of this approach comes from its extensibility. You can add a
completely new Drawable subtypes and your loop above would work without you
having to change it. Compare that to the C-style code where you would have a
switch on object type and call to a specific drawing function based on the
type. You would have to update this switch statement every time a new class
was introduced - this would be a maintenance problem.

Method hiding, on the other hand, is something that actually breaks
polymorphism. If one of you Drawable subclasses would declare a new Draw
method like so:

public new void Draw(Graphics g)
{
//...
}

Then the loop above would end up calling Drawable.Draw for instances of this
type. In general this is something that you don't want to happen.

Regards,
Sami

[Jon Skeet [C# MVP]]

Re: C# poly and method hiding issue

harrylmh <minghong@hotpo p.com> wrote:[color=blue]
> I'm learning C# and I just don't quite understand the need for
> polymorphism. why do we need to use it? how does a base class variable
> holding a derived class instance do any good?[/color]

I can write a method which (say) reads the contents of one stream,
encrypts it, and writes it out to another stream. I can do that without
knowing what kind of stream I'm writing to (or reading from) at all -
it could be over the network, or a file, or memory, or something
completely different.
[color=blue]
> Also, what's the difference between method hiding and overriding when
> they're both still overriding the base method.[/color]

They're not both overriding the base method - one is only hiding it.

See http://www.pobox.com/~skeet/csharp/faq/#override.new

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

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