Constructor

[Zoltan Hernyak]

Hi,

see my classes:

public class One
{
public One(string s) { .... }
}

public class Two:One
{
public class Two(int x)
{
...
}
}

what if I have to make some preparation before call the "base"
constructor from Two? for example:

public class Two(int x)
{
string s = String.Format(" Your int was {0}",x);
base(s); // this is not working !!!!
}

So, the question is: how can I call the base constructor if
I can't call using the ":base( )" syntax. Is there any other
possibility to call the base constructor INSIDE the descendant
class' constructor?

aroan

[Daniel O'Connell]

Re: Constructor


"Zoltan Hernyak" <aroan@ektf.h u> wrote in message
news:es9iov4lq3 4an1m571rhb1jmm sgcq1ihio@4ax.c om...[color=blue]
> Hi,
>
> see my classes:
>
> public class One
> {
> public One(string s) { .... }
> }
>
> public class Two:One
> {
> public class Two(int x)
> {
> ...
> }
> }
>
> what if I have to make some preparation before call the "base"
> constructor from Two? for example:
>
> public class Two(int x)
> {
> string s = String.Format(" Your int was {0}",x);
> base(s); // this is not working !!!!
> }
>
> So, the question is: how can I call the base constructor if
> I can't call using the ":base( )" syntax. Is there any other
> possibility to call the base constructor INSIDE the descendant
> class' constructor?
>[/color]
No, the language does not allow this.
Assuming you have a simple case like the one you posed, you can do
public Two(int x) : base(String.For mat("Your int was {0}",x) {

}

If the case is more complex, then you may be able to use a static method (be
careful not to cause a stack overflow like this):

public class Two : One
{
public Two(int x) : base(Two.Format Int(x))
{

}
static string FormatInt(int x)
{
//make sure you NEVER call new Two() from thsi method, however!
return String.Format(" Your int was {0}",x);
}
}[color=blue]
> aroan
>
>[/color]

[Zoltan Hernyak]

Re: Constructor

On Sun, 12 Oct 2003 10:15:58 GMT, in
microsoft.publi c.dotnet.langua ges.csharp you wrote:
[color=blue]
> public Two(int x) : base(Two.Format Int(x))[/color]

Ok. I understand. But let's suppose One(int a, int b) constuctor needs
two ints, and works only when a<b.

One x = new One(1,2); // will work
One x = new One(2,1); // throws an exception
One x = new One(1,1); // throws an exception

I want to extend this class to "TWO". Two class' constructor
can work when a<b or a=b. When a<b, we needs to change a with b,
when a=b, we can't allow the instance to be created.

public Two(int a, int b):base( ???? )
{
if (a<b) call base(a,b)
else if (a<b) call base(b,a);
else throw new Exception("Wron g attribues!")
}

Two x = new Two(1,2); // will work
Two x = new Two(2,1); // will work
Two x = new Two(1,1); // throws an exception

How can I do that?
[color=blue][color=green]
>> aroan[/color][/color]
On Sun, 12 Oct 2003 10:15:58 GMT, in
microsoft.publi c.dotnet.langua ges.csharp you wrote:
[color=blue]
> public Two(int x) : base(Two.Format Int(x))[/color]

Ok. I understand. But let's suppose One(int a, int b) constuctor needs
two ints, and works only when a<b.

One x = new One(1,2); // will work
One x = new One(2,1); // throws an exception
One x = new One(1,1); // throws an exception

I want to extend this class to "TWO". Two class' constructor
can work when a<b or a=b. When a<b, we needs to change a with b,
when a=b, we can't allow the instance to be created.

public Two(int a, int b):base( ???? )
{
if (a<b) call base(a,b)
else if (a<b) call base(b,a);
else throw new Exception("Wron g attribues!")
}

Two x = new Two(1,2); // will work
Two x = new Two(2,1); // will work
Two x = new Two(1,1); // throws an exception

How can I do that?
[color=blue][color=green]
>> aroan[/color][/color]

[Jon Skeet [C# MVP]]

Re: Constructor

Zoltan Hernyak <aroan@ektf.h u> wrote:[color=blue][color=green]
> > public Two(int x) : base(Two.Format Int(x))[/color]
>
> Ok. I understand. But let's suppose One(int a, int b) constuctor needs
> two ints, and works only when a<b.
>
> One x = new One(1,2); // will work
> One x = new One(2,1); // throws an exception
> One x = new One(1,1); // throws an exception
>
> I want to extend this class to "TWO". Two class' constructor
> can work when a<b or a=b. When a<b, we needs to change a with b,
> when a=b, we can't allow the instance to be created.[/color]

Well, there *are* ways round it - the best is probably to use a factory
method instead, ie:

public static Two CreateTwo (int a, int b)
{
if (a < b)
return new Two (a, b);
if (b < a)
return new Two (b, a);
if (a==b)
throw new ArgumentExcepti on ("a and b must be different");
}

private Two (int a, int b) : base (a, b)
{
}

I can't say I've run into this kind of problem many times though.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too

[Daniel O'Connell]

Re: Constructor


"Jon Skeet [C# MVP]" <skeet@pobox.co m> wrote in message
news:MPG.19f365 64f04be37e98986 1@msnews.micros oft.com...[color=blue]
> Zoltan Hernyak <aroan@ektf.h u> wrote:[color=green][color=darkred]
> > > public Two(int x) : base(Two.Format Int(x))[/color]
> >
> > Ok. I understand. But let's suppose One(int a, int b) constuctor needs
> > two ints, and works only when a<b.
> >
> > One x = new One(1,2); // will work
> > One x = new One(2,1); // throws an exception
> > One x = new One(1,1); // throws an exception
> >
> > I want to extend this class to "TWO". Two class' constructor
> > can work when a<b or a=b. When a<b, we needs to change a with b,
> > when a=b, we can't allow the instance to be created.[/color]
>
> Well, there *are* ways round it - the best is probably to use a factory
> method instead, ie:
>
> public static Two CreateTwo (int a, int b)
> {
> if (a < b)
> return new Two (a, b);
> if (b < a)
> return new Two (b, a);
> if (a==b)
> throw new ArgumentExcepti on ("a and b must be different");
> }
>
> private Two (int a, int b) : base (a, b)
> {
> }
>
> I can't say I've run into this kind of problem many times though.
>[/color]
Ya, me either..
Goes to show, however, always provide an accurate sample case or people may
not understand what you need.[color=blue]
> --
> Jon Skeet - <skeet@pobox.co m>
> http://www.pobox.com/~skeet
> If replying to the group, please do not mail me too[/color]

[Zoltan Hernyak]

Re: Constructor

On Sun, 12 Oct 2003 14:20:12 +0100, Jon Skeet [C# MVP]
<skeet@pobox.co m> wrote:
[color=blue]
>public static Two CreateTwo (int a, int b)
>{
> if (a < b)
> return new Two (a, b);
> if (b < a)
> return new Two (b, a);
> if (a==b)
> throw new ArgumentExcepti on ("a and b must be different");
>}[/color]

Ahh, I see. I worked a lot in Delphi, where constructors behave like
methods, and it is simple to call a base constructor (and many
constructors) inside another constructor... I was just thinking
the old way... thanks for the advices.

Zoltan Hernyak