OOP???

[Dean L. Howen]

Ex: I have 2 classes like this:

class Parent
{
private string msg;
public string Message
{
get
{
return this.msg;
}
set
{
this.msg = value;
}
.......
}
};

class Child : Parent
{
Child ()
{
// do something with property "Message"
}
...
}
As the class I describe above, I have some questions. Please help me:
Q1: I just want the property "Message" is used directly when create instance
of class Parent

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
Parent obj = new Parent();
obj.Message = "Parent"; // it's ok
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

but, I don't want it (the property "Message") be used if create instance of
class Child, although it could be used inside class Child
Child obj = new Child();
obj.Message = "Parent"; // it's not allowed

could we do something like those.

Q2: could we override properties in inheritance class. I want override
System.Data.DataTable.Rows in System.Data.DataTable class
ex:

class MyDataRowCollection : System.Data.DataRowCollection
{
.....
}

class MyDataTable : System.Data.DataTable
{
/// I want covert the property: base.Rows from System.Data.DataRowCollection
to MyDataRowCollection. Could we do this?
};

Please give me your ideas.
Thanks so much.

[Jon Skeet [C# MVP]]

Dean L. Howen <co*****@hotpop.com> wrote:

As the class I describe above, I have some questions. Please help me:
Q1: I just want the property "Message" is used directly when create instance
of class Parent
<snip>
but, I don't want it (the property "Message") be used if create instance of
class Child
In that case Child shouldn't derive from Parent, as you don't want them
to be usable in the same way. It breaks Liskov's Substitutability
Principle. Besides, someone could do:

Parent foo = new Child();
foo.Message = ...; and the compiler couldn't know the difference.
Q2: could we override properties in inheritance class.
Yes, so long as the property is declared to be virtual - and so long as
you use the same signature.
I want override
System.Data.DataTable.Rows in System.Data.DataTable class

You can't do that, as the Rows property isn't virtual. You could *hide*
the property, but you can't override it.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

[sadhu]

Q1 : Well, then you really musn't be inheriting from Parent.
Inheritance is valid only if the child modifies/enhances the parent's
behavior. If part of a parent is not valid in the child, then that's a
indication that the inheritance hierarchy is not right.

That said, you have two options.
1. You can make the assignment to Child's Message throw an exception at
runtime. Declare Message as virtual and override it in the child to
throw an exception
2. Hide the Parent's Message by creating a new private Message property
in the Child. Note that the 2nd suggestion will fail if
Child d = new Child();
Parent p = d;
p.Message = "blah";

Q2 : Yes, you can have virtual properties and override them because
properties end up being method calls.

[Dean L. Howen]

ok, I agree to hide property. But could you tell me the method to "hide"
property which I've not known before.

"Jon Skeet [C# MVP]" <sk***@pobox.com> wrote in message
news:MP************************@msnews.microsoft.c om...

Dean L. Howen <co*****@hotpop.com> wrote:

As the class I describe above, I have some questions. Please help me:
Q1: I just want the property "Message" is used directly when create
instance
of class Parent

<snip>

but, I don't want it (the property "Message") be used if create instance
of
class Child

In that case Child shouldn't derive from Parent, as you don't want them
to be usable in the same way. It breaks Liskov's Substitutability
Principle. Besides, someone could do:

Parent foo = new Child();
foo.Message = ...; and the compiler couldn't know the difference.

Q2: could we override properties in inheritance class.

Yes, so long as the property is declared to be virtual - and so long as
you use the same signature.

I want override
System.Data.DataTable.Rows in System.Data.DataTable class

You can't do that, as the Rows property isn't virtual. You could *hide*
the property, but you can't override it.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too

[Jon Skeet [C# MVP]]

Dean L. Howen <co*****@hotpop.com> wrote:

ok, I agree to hide property. But could you tell me the method to "hide"
property which I've not known before.

Just declare it as normal and use the "new" keyword to tell the
compiler that you're aware it's hiding an existing property.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too