VB.Net evaluates 'if' conditions unnecessarily?

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Tim_Mac - 25 Apr 2006 18:23 GMT

hi,
in c# i can run the following code without risk of
NullReferenceException:

if(textNode != null && !String.IsNullOrEmptytextNode.InnerText))

the reason it is safe to run is because if the first condition is
false, then the CLR will not bother to examine further conditions
(false and anything else is still false...) and the second part of the
condition will not evaluate.

however, in VB, i am running the following code:

If Not textNode Is Nothing And Not
String.IsNullOrEmpty(textNode.InnerText) Then ...

and i get a NullReferenceException... what gives?
thanks for any tips.
tim

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Jon Skeet [C# MVP] - 25 Apr 2006 18:27 GMT

> in c# i can run the following code without risk of
> NullReferenceException:
[quoted text clipped - 13 lines]
> and i get a NullReferenceException... what gives?
> thanks for any tips.

I believe you want the AndAlso operator. The equivalent for "or" is
OrElse.

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Tim_Mac - 25 Apr 2006 22:43 GMT

hi jon,
that must be it. many thanks for adding this new word to my vb
vocabulary. it makes you wonder what the normal AND is supposed to do
then...
AndNotAlsoButOnlyIfTheFirstOneIsNotFalse.

i'm sure the docs have a perfectly logical answer. no need to explain.

thanks again
tim

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Scott M. - 26 Apr 2006 00:46 GMT

"And" evaluates both sides of the conditional regardless of the first
conditional actual value. Same is true for "Or". This is for situations
when the second conditional is a method call and you want the method to run
regardless of its return value.

"AndAlso" and "OrElse" were added to solve that issue.

> hi jon,
> that must be it. many thanks for adding this new word to my vb
[quoted text clipped - 6 lines]
> thanks again
> tim

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Jon Skeet [C# MVP] - 26 Apr 2006 06:03 GMT

> "And" evaluates both sides of the conditional regardless of the first
> conditional actual value. Same is true for "Or". This is for situations
> when the second conditional is a method call and you want the method to run
> regardless of its return value.
>
> "AndAlso" and "OrElse" were added to solve that issue.

I would guess it's historical - because And and Or historically didn't
do short-circuiting, the behaviour wasn't changed for VB.NET. I doubt
that many people really *want* the non-short-circuiting behaviour to be
the more common one...

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Greg Young [MVP] - 26 Apr 2006 06:38 GMT

I agree with this completely .. I remember the first time I came accross
this years ago having not come a VB background I found it to be the oddest
thing.

The "interesting" things we bring forward for the sake of "backwards
thought" compatibility :)

>> "And" evaluates both sides of the conditional regardless of the first
>> conditional actual value. Same is true for "Or". This is for situations
[quoted text clipped - 8 lines]
> that many people really *want* the non-short-circuiting behaviour to be
> the more common one...

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Scott M. - 27 Apr 2006 23:44 GMT

Actually, MS did change the non-short circuited functionality of "And" and
"Or" to be short-circuited in the first .NET beta.

Too many existing VB 6.0 developers that had already known about the
original behavior complained that switching the operation of "And" and "Or"
for .NET would make VB 6.0 code harder to port to .NET, so MS switched them
back to non-short circuited and introduced "AndAlso" and "OrElse" as a way
to have the short-circuited approach available.

>I agree with this completely .. I remember the first time I came accross
>this years ago having not come a VB background I found it to be the oddest
[quoted text clipped - 16 lines]
>> that many people really *want* the non-short-circuiting behaviour to be
>> the more common one...

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Ben Voigt - 03 May 2006 00:35 GMT

>> in c# i can run the following code without risk of
>> NullReferenceException:
[quoted text clipped - 16 lines]
> I believe you want the AndAlso operator. The equivalent for "or" is
> OrElse.

C/C++/C# && ..... VB.NET AndAlso (logical) short-circuits
C/C++/C# & ...... VB/VB.NET And (bitwise) always evaluates both operands

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VB.Net evaluates 'if' conditions unnecessarily?

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