Re: I don't understand inheritance!
From: Ollie (ollie_riches_at_hotmail.com)
Date: 12/03/04
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Date: Fri, 3 Dec 2004 09:40:30 -0000
you can't override Interface methods that aren't marked as
virtual\abstract...
e.g.
public abstract baseCondition : ICloneable
{
public abstract void DoSomething();
public abstract object Clone();
}
public SimpleCondition : BaseCondition
{
public override object Clone()
{
....
}
public override void DoSomething()
{
....
}
}
You might what to chekc out the 'new' keyword as in:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/csref/html/vclrfoverridepg.asp
"Francois" <francois@bettinghouses.com_NOSPAM> wrote in message
news:Opf$0WP2EHA.936@TK2MSFTNGP12.phx.gbl...
> Hi,
>
> I am trying to extend a .NET class for overriding some methods.
>
> The class that I try to override is : System.Web.UI.WebControls.CheckBox.
> Its full definition is : public class CheckBox : WebControl,
> IPostBackDataHandler (notice the interface that is implemented)
>
> I want to override 3 methods belonging to that class:
>
> 1. protected virtual void OnCheckedChanged(EventArgs e) //Inherited from
> the class itself
> 2. IPostBackDataHandler.LoadPostData(string postDataKey,
NameValueCollection
> postCollection) // defined in the IPostBackDataHandler interface
> 3. IPostBackDataHandler.RaisePostDataChangedEvent() // defined in the
> IPostBackDataHandler interface
>
> Methods 2 and 3 are not defined by the class CheckBox class itself but as
> the class implements the interface IPostBackDataHandler then it must
> implements those 2 methods.
> Then a first question arise is: Why those 2 methods do not appear in the
> class reference of the CheckBox? Should the reference not display all
> methods that are implemented by the interface? Or can a class implements
an
> interface and make the methods defined in the interface private?
>
> Anyway I try to extends the checkBox class with the following code but it
> does not compile:
>
> using System;
> using System.Collections;
> using System.Collections.Specialized;
> using System.Web.UI;
>
> namespace Bos.CustomControls
> {
> public class CheckBoxExtension : System.Web.UI.WebControls.CheckBox
> // i also tried like this but it does not work neither:
> // public class CheckBoxExtension : System.Web.UI.WebControls.CheckBox,
> IPostBackDataHandler
> {
> public CheckBoxExtension()
> {
> }
>
> protected override void OnCheckedChanged(EventArgs e)
> {
> //this method compiles
> base.OnCheckedChanged(e);
> }
>
> override bool IPostBackDataHandler.LoadPostData(string postDataKey,
> NameValueCollection postCollection)
> {
> //this does not compile, compiler says: The modifier 'override' is not
> valid for this item
> return true;
> }
>
> override void IPostBackDataHandler.RaisePostDataChangedEvent()
> {
> //this does not compile, compiler says: The modifier 'override' is not
> valid for this item
> }
> }
> }
>
> To make the code compile and work, I have to do like the following:
>
> public class CheckBoxValue : System.Web.UI.WebControls.CheckBox,
> IPostBackDataHandler
> {
>
> protected override void OnCheckedChanged(EventArgs e)
> {
> // here i can override a method inherited from the base class
> }
>
> bool IPostBackDataHandler.LoadPostData(string postDataKey,
> NameValueCollection postCollection)
> {
> /* here i cannot override this method that the base class implements (as
> the base class also implement the interface IPostBackDataHandler. I have a
> to re implement the interface like if it was never implemented by the base
> class. I do not use the override keyword
> */
> }
>
> void IPostBackDataHandler.RaisePostDataChangedEvent()
> {
> /* here i cannot override this method that the base class implements (as
> the base class also implement the interface IPostBackDataHandler. I have a
> to re implement the interface like if it was never implemented by the base
> class. I do not use the override keyword
> */
> }
> }
>
> For me, this is a problem for 2 reasons:
> - First, I cannot call the method base.LoadPostData neither
> base.RaisePostDataChangedEvent. What about the case I would like to keep
the
> default behavior of those methods but just adding some extra functionality
> that I need for my app? When pple override a method, it can very likely be
> for adding a new functionality, not to replace the functionality.
> - Second, as those methods must exist (as the base class implements the
> interface) why can i not override them? And what I am doing in the second
> code that compiles seems to me more like bypassing the implementation of
the
> interface of the base class and re-implement it again int the derived
class.
> Like if the interface was never implemented at all at first! This seems a
> little bit dangerous as people can reimplement interfaces inherited by
their
> base class without knowing it! And without having to use the "override"
> keyword. Thus it may bring buggy code! Also it just does not make sense to
> me, if a base class implements this or that interface, how comes I can
> reimplement it again in a derived class like if it was never implemented
at
> the first place?
>
> I would really appreciate if someone could enlight me on this.
>
> Tx a lot in advance,
> Best regards,
>
> Francois.
>
>
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