Crosspost (Inheritance and Interfaces)

[Vlad Ivanov]

I am just starting with C# (i guess same prinicples apply to VB.Net ) so i
don't know how things work very well, but...

Let me add a query to the knowledgable: If class A Inherits a class B that
implements multiple interfaces - does my class A, now based on class B
automatically also implements the interfaces of the base B?

To illustrate: (pseudocode)

CB implements IC and ID
CA inherits CB

Can CA be cast to IC and ID?

And if it can, when i call a method defined by IC that was implemented by CB
(and in turn inherited by CA) then the code executes in CB? Right?

Thanks guys

[Rob Teixeira]

The following is a quote from the C# language guide. Hope it helps.

-Rob

A class inherits all interface implementations provided by its base classes.

Without explicitly re-implementing an interface, a derived class cannot in
any way alter the interface mappings it inherits from its base classes. For
example, in the declarations

interface IControl
{
void Paint();
}
class Control: IControl
{
public void Paint() {...}
}
class TextBox: Control
{
new public void Paint() {...}
}

the Paint method in TextBox hides the Paint method in Control, but it does
not alter the mapping of Control.Paint onto IControl.Paint, and calls to
Paint through class instances and interface instances will have the following
effects

Control c = new Control();
TextBox t = new TextBox();
IControl ic = c;
IControl it = t;
c.Paint(); // invokes Control.Paint();
t.Paint(); // invokes TextBox.Paint();
ic.Paint(); // invokes Control.Paint();
it.Paint(); // invokes Control.Paint();

However, when an interface method is mapped onto a virtual method in a class,
it is possible for derived classes to override the virtual method and alter
the implementation of the interface. For example, rewriting the declarations
above to

interface IControl
{
void Paint();
}
class Control: IControl
{
public virtual void Paint() {...}
}
class TextBox: Control
{
public override void Paint() {...}
}
the following effects will now be observed

Control c = new Control();
TextBox t = new TextBox();
IControl ic = c;
IControl it = t;
c.Paint(); // invokes Control.Paint();
t.Paint(); // invokes TextBox.Paint();
ic.Paint(); // invokes Control.Paint();
it.Paint(); // invokes TextBox.Paint();

Since explicit interface member implementations cannot be declared virtual,
it is not possible to override an explicit interface member implementation.
It is however perfectly valid for an explicit interface member implementation
to call another method, and that other method can be declared virtual to
allow derived classes to override it. For example

interface IControl
{
void Paint();
}
class Control: IControl
{
void IControl.Paint() { PaintControl(); }
protected virtual void PaintControl() {...}
}
class TextBox: Control
{
protected override void PaintControl() {...}
}

Here, classes derived from Control can specialize the implementation of IControl.Paint
by overriding the PaintControl method.



"Vlad Ivanov" <vivanov@polarisconsulting.com> wrote:
>I am just starting with C# (i guess same prinicples apply to VB.Net ) so
i
>don't know how things work very well, but...
>
>Let me add a query to the knowledgable: If class A Inherits a class B that
>implements multiple interfaces - does my class A, now based on class B
>automatically also implements the interfaces of the base B?
>
>To illustrate: (pseudocode)
>
>CB implements IC and ID
>CA inherits CB
>
>Can CA be cast to IC and ID?
>
>And if it can, when i call a method defined by IC that was implemented by
CB
>(and in turn inherited by CA) then the code executes in CB? Right?
>
>Thanks guys
>
>

[Vlad Ivanov]

Oh thanx so much!

"Rob Teixeira" <RobTeixeira@@msn.com> wrote in message
news:3a3a6cc2$1@news.devx.com...
>
> The following is a quote from the C# language guide. Hope it helps.
>
> -Rob
>
>