Re: I don't understand inheritance!

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Date: Fri, 3 Dec 2004 10:33:45 +0100

I guess it's something that has been overlooked in the 1.1 framework.

In 2.0, these methods are declared as protected virtual, so you don't need
to reimplement the interface, just override them if you want to.

HTH,
Stefan

"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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