How can I check if a string is null or empty in Po

问题:

Is there a built-in IsNullOrEmpty-like function in order to check if a string is null or empty, in PowerShell?

I could not find it so far and if there is a built-in way, I do not want to write a function for this.

回答1:

You can use the IsNullOrEmpty static method:

[string]::IsNullOrEmpty(...)

回答2:

You guys are making this too hard. PowerShell handles this quite elegantly e.g.:

> $str1 = $null
> if ($str1) { 'not empty' } else { 'empty' }
empty

> $str2 = ''
> if ($str2) { 'not empty' } else { 'empty' }
empty

> $str3 = ' '
> if ($str3) { 'not empty' } else { 'empty' }
not empty

> $str4 = 'asdf'
> if ($str4) { 'not empty' } else { 'empty' }
not empty

> if ($str1 -and $str2) { 'neither empty' } else { 'one or both empty' }
one or both empty

> if ($str3 -and $str4) { 'neither empty' } else { 'one or both empty' }
neither empty

回答3:

In addition to [string]::IsNullOrEmpty in order to check for null or empty you can cast a string to a Boolean explicitly or in Boolean expressions:

$string = $null
[bool]$string
if (!$string) { "string is null or empty" }

$string = ''
[bool]$string
if (!$string) { "string is null or empty" }

$string = 'something'
[bool]$string
if ($string) { "string is not null or empty" }

Output:

False
string is null or empty

False
string is null or empty

True
string is not null or empty

回答4:

If it is a parameter in a function, you can validate it with ValidateNotNullOrEmpty as you can see in this example:

Function Test-Something
{
    Param(
        [Parameter(Mandatory=$true)]
        [ValidateNotNullOrEmpty()]
        [string]$UserName
    )

    #stuff todo
}

回答5:

Personally, I do not accept a whitespace ($STR3) as being 'not empty'.

When a variable that only contains whitespaces is passed onto a parameter, it will often error that the parameters value may not be '$null', instead of saying it may not be a whitespace, some remove commands might remove a root folder instead of a subfolder if the subfolder name is a "white space", all the reason not to accept a string containing whitespaces in many cases.

I find this is the best way to accomplish it:

$STR1 = $null
IF ([string]::IsNullOrWhitespace($STR1)){'empty'} else {'not empty'}

Empty

$STR2 = ""
IF ([string]::IsNullOrWhitespace($STR2)){'empty'} else {'not empty'}

Empty

$STR3 = " "
IF ([string]::IsNullOrWhitespace($STR3)){'empty !! :-)'} else {'not Empty :-('}

Empty!! :-)

$STR4 = "Nico"
IF ([string]::IsNullOrWhitespace($STR4)){'empty'} else {'not empty'}

Not empty

回答6:

I have a PowerShell script I have to run on a computer so out of date that it doesn't have [String]::IsNullOrWhiteSpace(), so I wrote my own.

function IsNullOrWhitespace($str)
{
    if ($str)
    {
        return ($str -replace " ","" -replace "`t","").Length -eq 0
    }
    else
    {
        return $TRUE
    }
}

回答7:

# cases
$x = null
$x = ''
$x = ' '

# test
if ($x -and $x.trim()) {'not empty'} else {'empty'}
or
if ([string]::IsNullOrWhiteSpace($x)) {'empty'} else {'not empty'}

回答8:

Note that the "if ($str)" and "IsNullOrEmpty" tests don't work comparably in all instances: an assignment of `$str=0 produces false for both, and depending on intended program semantics, this could yield a surprise.

回答9:

Another way to accomplish this in a pure PowerShell way would be to do something like this:

("" -eq ("{0}" -f $val).Trim())

This evaluates successfully for null, empty string, and whitespace. I'm formatting the passed value into an empty string to handle null (otherwise a null will cause an error when the Trim is called). Then just evaluate equality with an empty string. I think I still prefer the IsNullOrWhiteSpace, but if you're looking for another way to do it, this will work.

$val = null    
("" -eq ("{0}" -f $val).Trim())
>True
$val = "      "
("" -eq ("{0}" -f $val).Trim())
>True
$val = ""
("" -eq ("{0}" -f $val).Trim())
>True
$val = "not null or empty or whitespace"
("" -eq ("{0}" -f $val).Trim())
>False

In a fit of boredom, I played with this some and made it shorter (albeit more cryptic):

!!(("$val").Trim())

or

!(("$val").Trim())

depending on what you're trying to do.

回答10:

Check the length. If the object exists, it will have a length.

Null objects have no length, do not exist, and cannot be checked.

String object have a length.

The question was: IsNull or IsEmpty, NOT IsNull or IsEmpty or IsWhiteSpace

#Null
$str1 = $null
$str1.length
($str1 | get-member).TypeName[0]
# Returns big red error

#Empty
$str2 = ""
$str2.length
($str2 | get-member).TypeName[0]
# Returns 0

## Whitespace
$str3 = " "
$str3.length
($str3 | get-member).TypeName[0]
## Returns 1