I don't understand inheritance!

[Francois]

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.

[Matt Berther]

Hello francois,

Look up explicit interface implementation to answer the majority of your
questions.

In theory, the way to get around this is to provide a virtual method that
the explicit implementation calls, which your subclasses can override. However,
it looks like the ASP.NET team did not do that here.

--
Matt Berther
http://www.mattberther.com

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.