[string]$a = $null; $a -eq $null; $a -eq ""

Mat

[string]$a = $null; $a -eq $null; $a -eq ""

False
True

So a string variable can't be set to $null (which causes problems trying to
work with .NET) - or is there another way to set a .NET string to null/$null?

Cheers,
-Mat

Tao Ma

Hi Mat,

In the '[string] $a = $null', [string] is type attribute of the variable $a.
Unlike other languages, PowerShell will try to convert any object as long as
it is convertible to target type. So $null is converted into an empty string
object. Bruce Payette explains in his book Windows PowerShell In
Action.(page 142)

Is it possible to remove [string] here ? you can use '-is' to check
$variable type.

Tao Ma


"Mat" <> дÈëÏûÏ¢ÐÂÎÅ:...
> [string]$a = $null; $a -eq $null; $a -eq ""
>
> False
> True
>
> So a string variable can't be set to $null (which causes problems trying
> to
> work with .NET) - or is there another way to set a .NET string to
> null/$null?
>
> Cheers,
> -Mat

Roman Kuzmin

See also:


--
Thanks,
Roman Kuzmin


PowerShell and .NET in FAR Manager


"Mat" <> wrote in message
news:...
> [string]$a = $null; $a -eq $null; $a -eq ""
>
> False
> True
>
> So a string variable can't be set to $null (which causes problems trying
> to
> work with .NET) - or is there another way to set a .NET string to
> null/$null?
>
> Cheers,
> -Mat

Mat

Thanks Roman, I added my vote.

Tao: I would expect that $null would convert to null (not "") becasue null
is a valid value for a string type.

Cheers,
-Mat

"Roman Kuzmin" wrote:

> See also:
>
>
> --
> Thanks,
> Roman Kuzmin
>
>
> PowerShell and .NET in FAR Manager
>
>
> "Mat" <> wrote in message
> news:...
> > [string]$a = $null; $a -eq $null; $a -eq ""
> >
> > False
> > True
> >
> > So a string variable can't be set to $null (which causes problems trying
> > to
> > work with .NET) - or is there another way to set a .NET string to
> > null/$null?
> >
> > Cheers,
> > -Mat
>

Mat

Hi Tao, in my scenario I have a .NET object with a string property and I
want to set that property to $null but it apparently sets it to "" instead.
Cheers,
-Mat

"Tao Ma" wrote:

> Hi Mat,
>
> In the '[string] $a = $null', [string] is type attribute of the variable $a.
> Unlike other languages, PowerShell will try to convert any object as long as
> it is convertible to target type. So $null is converted into an empty string
> object. Bruce Payette explains in his book Windows PowerShell In
> Action.(page 142)
>
> Is it possible to remove [string] here ? you can use '-is' to check
> $variable type.
>
> Tao Ma
>
>
> "Mat" <> �´Èë�û�¢�ÂÎÅ:...
> > [string]$a = $null; $a -eq $null; $a -eq ""
> >
> > False
> > True
> >
> > So a string variable can't be set to $null (which causes problems trying
> > to
> > work with .NET) - or is there another way to set a .NET string to
> > null/$null?
> >
> > Cheers,
> > -Mat
>
>
>

Jacob Saaby Nielsen

Hey mat,

well that's perhaps the issue right there.

$null is not a valid value for anything - because $null is not a value.

$null is NOT equal to "".
$null is NOT equal to "0".

$null is more a state of non-existance, than it is an actual value.

"" is a value. It's a value that says "empty", and is of type string.
"0" is a value. It's a value that says "zero", and is of type int or char.
$null just tells you that there isn't a value, or that something doesn't
exist.

So setting a string value to $null, if you follow the logic of e.g. setting
a .NET object to $null (or Nothing),
should actually destroy the string variable, instead of setting it to nothing.

There is no logic reason, at least in my mind, why a string should ever be
$null. Because it it's $null, it's
either non-existant, or you have a variable with an unknown value. When you
have an empty string, you know
its value: Empty.

There's a lot of reason why a string should be just plain empty though.

I hope that contributed to understanding the difference between $null and...
well, everything else

Best Regards,
Jacob Saaby Nielsen


gmail: jacob DOT saaby
hotmail (IM/LinkedIN/Facebook): same as gmail

> Thanks Roman, I added my vote.
>
> Tao: I would expect that $null would convert to null (not "") becasue
> null is a valid value for a string type.
>
> Cheers,
> -Mat
> "Roman Kuzmin" wrote:
>
>> See also:
>>
>> 07821&SiteID=99
>> --
>> Thanks,
>> Roman Kuzmin
>>
>> PowerShell and .NET in FAR Manager
>> "Mat" <> wrote in message
>> news:...
>>
>>> [string]$a = $null; $a -eq $null; $a -eq ""
>>>
>>> False
>>> True
>>> So a string variable can't be set to $null (which causes problems
>>> trying
>>> to
>>> work with .NET) - or is there another way to set a .NET string to
>>> null/$null?
>>> Cheers,
>>> -Mat

Leo Tohill

Still, in the world at large, there exist methods that take string parms that
have special meaning when null. Since we can't change the world, we have to
accommodate it. I think it is important for PS to support passing null to
string parms of methods.

"Jacob Saaby Nielsen" wrote:

> Hey mat,
>
> well that's perhaps the issue right there.
>
> $null is not a valid value for anything - because $null is not a value.
>
> $null is NOT equal to "".
> $null is NOT equal to "0".
>
> $null is more a state of non-existance, than it is an actual value.
>
> "" is a value. It's a value that says "empty", and is of type string.
> "0" is a value. It's a value that says "zero", and is of type int or char.
> $null just tells you that there isn't a value, or that something doesn't
> exist.
>
> So setting a string value to $null, if you follow the logic of e.g. setting
> a .NET object to $null (or Nothing),
> should actually destroy the string variable, instead of setting it to nothing.
>
> There is no logic reason, at least in my mind, why a string should ever be
> $null. Because it it's $null, it's
> either non-existant, or you have a variable with an unknown value. When you
> have an empty string, you know
> its value: Empty.
>
> There's a lot of reason why a string should be just plain empty though.
>
> I hope that contributed to understanding the difference between $null and...
> well, everything else
>
> Best Regards,
> Jacob Saaby Nielsen
>
>
> gmail: jacob DOT saaby
> hotmail (IM/LinkedIN/Facebook): same as gmail
>
> > Thanks Roman, I added my vote.
> >
> > Tao: I would expect that $null would convert to null (not "") becasue
> > null is a valid value for a string type.
> >
> > Cheers,
> > -Mat
> > "Roman Kuzmin" wrote:
> >
> >> See also:
> >>
> >> 07821&SiteID=99
> >> --
> >> Thanks,
> >> Roman Kuzmin
> >>
> >> PowerShell and .NET in FAR Manager
> >> "Mat" <> wrote in message
> >> news:...
> >>
> >>> [string]$a = $null; $a -eq $null; $a -eq ""
> >>>
> >>> False
> >>> True
> >>> So a string variable can't be set to $null (which causes problems
> >>> trying
> >>> to
> >>> work with .NET) - or is there another way to set a .NET string to
> >>> null/$null?
> >>> Cheers,
> >>> -Mat
>
>
>

Matthew Hobbs

"Jacob Saaby Nielsen" wrote:
> There is no logic reason, at least in my mind, why a string should ever be
> $null. [...]

Philosophy aside... System.String is a reference type and can be null. You
are talking about string values. System.String can refer to a value (a
string value) or not (null).

Karl Prosser[MVP]

It is a side effect of how powershell works, something to definately be
aware of, so you don't run into gotchas, but not a huge problem as you
can easily work around it in a variety of ways (i.e not force the
variable to be strongly typed as a string)

but it should be noted and documented a a gotcha

Leo Tohill wrote:
> Still, in the world at large, there exist methods that take string parms that
> have special meaning when null. Since we can't change the world, we have to
> accommodate it. I think it is important for PS to support passing null to
> string parms of methods.
>
> "Jacob Saaby Nielsen" wrote:
>
>> Hey mat,
>>
>> well that's perhaps the issue right there.
>>
>> $null is not a valid value for anything - because $null is not a value.
>>
>> $null is NOT equal to "".
>> $null is NOT equal to "0".
>>
>> $null is more a state of non-existance, than it is an actual value.
>>
>> "" is a value. It's a value that says "empty", and is of type string.
>> "0" is a value. It's a value that says "zero", and is of type int or char.
>> $null just tells you that there isn't a value, or that something doesn't
>> exist.
>>
>> So setting a string value to $null, if you follow the logic of e.g. setting
>> a .NET object to $null (or Nothing),
>> should actually destroy the string variable, instead of setting it to nothing.
>>
>> There is no logic reason, at least in my mind, why a string should ever be
>> $null. Because it it's $null, it's
>> either non-existant, or you have a variable with an unknown value. When you
>> have an empty string, you know
>> its value: Empty.
>>
>> There's a lot of reason why a string should be just plain empty though.
>>
>> I hope that contributed to understanding the difference between $null and...
>> well, everything else
>>
>> Best Regards,
>> Jacob Saaby Nielsen
>>
>>
>> gmail: jacob DOT saaby
>> hotmail (IM/LinkedIN/Facebook): same as gmail
>>
>>> Thanks Roman, I added my vote.
>>>
>>> Tao: I would expect that $null would convert to null (not "") becasue
>>> null is a valid value for a string type.
>>>
>>> Cheers,
>>> -Mat
>>> "Roman Kuzmin" wrote:
>>>
>>>> See also:
>>>>
>>>> 07821&SiteID=99
>>>> --
>>>> Thanks,
>>>> Roman Kuzmin
>>>>
>>>> PowerShell and .NET in FAR Manager
>>>> "Mat" <> wrote in message
>>>> news:...
>>>>
>>>>> [string]$a = $null; $a -eq $null; $a -eq ""
>>>>>
>>>>> False
>>>>> True
>>>>> So a string variable can't be set to $null (which causes problems
>>>>> trying
>>>>> to
>>>>> work with .NET) - or is there another way to set a .NET string to
>>>>> null/$null?
>>>>> Cheers,
>>>>> -Mat
>>
>>

Jacob Saaby Nielsen

Hey Matthew,

yes - I do of course mean the value of the string, thanks for correcting
me

Best Regards,
Jacob Saaby Nielsen


gmail: jacob DOT saaby
hotmail (IM/LinkedIN/Facebook): same as gmail

> "Jacob Saaby Nielsen" wrote:
>
>> There is no logic reason, at least in my mind, why a string should
>> ever be $null. [...]
>>
> Philosophy aside... System.String is a reference type and can be null.
> You are talking about string values. System.String can refer to a
> value (a string value) or not (null).
>