Overriding == and testing to null

[EL OSO]

Hi, let's suppose I have a

class A
{
int x;
...
public static bool operator == (A a1 , A a2)
{
return (a1.x==a2.x);
}
}

But, outside the class, I have a problem when I try to test a probably not
initialized class.
For instance, think about this code:
public void myFunc()
{
A a1;
A a2;
.....
if (a1 == null)
{
//some null value handling
}
....
}

The problem is that testing a1==null creates recursive calls to the operator
== of class A.
How do I test if something is null without having to invoke the == operator
for that "something" ?


Tnx in advance.

[EL OSO]

Re: Overriding == and testing to null

Dumb of me, I mixed two questions.
What I was trying to say is that the invoked operator == for class A creates
a null reference when it tries to access a2.x when a2 is null (as I wrote on
MyFunc)

I tried in the definition of operator == something like
if (a1==null) or (a2==null)
return false
else
return (a1.x == (a2.x)

BUT, comparing a1 == null creates the recursive call to operator == in class
A

Do I make myself more clear this time? :p





"EL OSO" <noone@nospam.c om> escribió en el mensaje
news:OaE65zyYFH A.3280@TK2MSFTN GP09.phx.gbl...[color=blue]
> Hi, let's suppose I have a
>
> class A
> {
> int x;
> ...
> public static bool operator == (A a1 , A a2)
> {
> return (a1.x==a2.x);
> }
> }
>
> But, outside the class, I have a problem when I try to test a probably not
> initialized class.
> For instance, think about this code:
> public void myFunc()
> {
> A a1;
> A a2;
> ....
> if (a1 == null)
> {
> //some null value handling
> }
> ...
> }
>
> The problem is that testing a1==null creates recursive calls to the[/color]
operator[color=blue]
> == of class A.
> How do I test if something is null without having to invoke the ==[/color]
operator[color=blue]
> for that "something" ?
>
>
> Tnx in advance.
>
>[/color]

[Mattias Sjögren]

Re: Overriding == and testing to null

[color=blue]
>BUT, comparing a1 == null creates the recursive call to operator == in class
>A[/color]

Cast to object first to prevent that.

if ( (object)a1 == null )



Mattias

--
Mattias Sjögren [MVP] mattias @ mvps.org
http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com
Please reply only to the newsgroup.

[Helge Jensen]

Re: Overriding == and testing to null



EL OSO wrote:
[color=blue]
> I tried in the definition of operator == something like
> if (a1==null) or (a2==null)
> return false
> else
> return (a1.x == (a2.x)[/color]

That's not really what you want, is it, if (object)a1 == null &&
(object)a2 == null ;)

I use the pattern:

public static bool operator==(A a1, A a2) {
if ( (object)a1 == null )
return (object)a2 == null;
else if ((object)a2 == null)
return false;
else
return a1.x == a2.x
}
[color=blue]
> BUT, comparing a1 == null creates the recursive call to operator == in class
> A[/color]

Yes, you just overloaded operator==(A,A) , so that gets called to compare
a1 and null.

The "trick" is to make a1 staticly typed as something that compares by
reference with operator==: object.
[color=blue]
> Do I make myself more clear this time? :p[/color]

mostly,... I'm not that good at being clear myself.

--
Helge Jensen
mailto:helge.je nsen@slog.dk
sip:helge.jense n@slog.dk
-=> Sebastian cover-music: http://ungdomshus.nu <=-

[Ted Miller]

Re: Overriding == and testing to null

You could use object.Referenc eEquals().

"EL OSO" <noone@nospam.c om> wrote in message
news:OaE65zyYFH A.3280@TK2MSFTN GP09.phx.gbl...[color=blue]
> Hi, let's suppose I have a
>
> class A
> {
> int x;
> ...
> public static bool operator == (A a1 , A a2)
> {
> return (a1.x==a2.x);
> }
> }
>
> But, outside the class, I have a problem when I try to test a probably not
> initialized class.
> For instance, think about this code:
> public void myFunc()
> {
> A a1;
> A a2;
> ....
> if (a1 == null)
> {
> //some null value handling
> }
> ...
> }
>
> The problem is that testing a1==null creates recursive calls to the
> operator
> == of class A.
> How do I test if something is null without having to invoke the ==
> operator
> for that "something" ?
>
>
> Tnx in advance.
>
>[/color]