.net 如何在 PowerShell 中检查字符串是否为 null 或为空？
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How can I check if a string is null or empty in PowerShell?
提问by pencilCake
Is there a built-in IsNullOrEmpty-like function in order to check if a string is null or empty, in PowerShell?
IsNullOrEmpty在 PowerShell 中是否有内置的类似函数来检查字符串是 null 还是空？
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.
到目前为止我找不到它，如果有内置方式，我不想为此编写函数。
回答by Keith Hill
You guys are making this too hard. PowerShell handles this quite elegantly e.g.:
你们这太难了。PowerShell 非常优雅地处理了这个问题，例如：
> $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by Shay Levy
You can use the IsNullOrEmptystatic method:
您可以使用IsNullOrEmpty静态方法：
[string]::IsNullOrEmpty(...)
回答by Roman Kuzmin
In addition to [string]::IsNullOrEmptyin order to check for null or empty you can cast a string to a Boolean explicitly or in Boolean expressions:
除了[string]::IsNullOrEmpty检查 null 或 empty 之外，您还可以显式地或在布尔表达式中将字符串转换为布尔值：
$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 ($string) { "string is not null or empty" }
Output:
输出：
False
string is null or empty
False
string is null or empty
True
string is not null or empty
回答by Rubanov
If it is a parameter in a function, you can validate it with ValidateNotNullOrEmptyas you can see in this example:
如果它是函数中的参数，您可以使用ValidateNotNullOrEmpty以下示例对其进行验证：
Function Test-Something
{
Param(
[Parameter(Mandatory=$true)]
[ValidateNotNullOrEmpty()]
[string]$UserName
)
#stuff todo
}
回答by Nico van der Stok
Personally, I do not accept a whitespace ($STR3) as being 'not empty'.
就个人而言，我不接受空格 ($STR3) 为“非空”。
When a variable that only contains whitespaces is passed onto a parameter, it will often error that the parameters value may not be '$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.
当一个只包含空格的变量传递给一个参数时，经常会出错，参数值可能不是'$null'，而不是说它可能不是一个空格，一些remove命令可能会删除一个根文件夹而不是一个subfolder 如果子文件夹名称是“空格”，则在许多情况下不接受包含空格的字符串的所有原因。
I find this is the best way to accomplish it:
我发现这是实现它的最佳方法：
$STR1 = $null
IF ([string]::IsNullOrWhitespace($STR1)){'empty'} else {'not empty'}
Empty
空的
$STR2 = ""
IF ([string]::IsNullOrWhitespace($STR2)){'empty'} else {'not empty'}
Empty
空的
$STR3 = " "
IF ([string]::IsNullOrWhitespace($STR3)){'empty !! :-)'} else {'not Empty :-('}
Empty!! :-)
空的！！:-)
$STR4 = "Nico"
IF ([string]::IsNullOrWhitespace($STR4)){'empty'} else {'not empty'}
Not empty
不是空的
回答by mhenry1384
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.
我有一个 PowerShell 脚本，我必须在一台计算机上运行它，所以它已经过时了，它没有 [String]::IsNullOrWhiteSpace()，所以我自己写了。
function IsNullOrWhitespace($str)
{
if ($str)
{
return ($str -replace " ","" -replace "`t","").Length -eq 0
}
else
{
return $TRUE
}
}
回答by Skatterbrainz
# 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by Nikolay Polyagoshko
PowerShell 2.0 replacement for [string]::IsNullOrWhiteSpace()is string -notmatch "\S"
PowerShell 2.0 替代[string]::IsNullOrWhiteSpace()是string -notmatch "\S"
("\S" = any non-whitespace character)
(" \S" = 任何非空白字符)
> $null -notmatch "\S"
True
> " " -notmatch "\S"
True
> " x " -notmatch "\S"
False
Performance is very close:
性能非常接近：
> Measure-Command {1..1000000 |% {[string]::IsNullOrWhiteSpace(" ")}}
TotalMilliseconds : 3641.2089
> Measure-Command {1..1000000 |% {" " -notmatch "\S"}}
TotalMilliseconds : 4040.8453
回答by Ryan
Another way to accomplish this in a pure PowerShell way would be to do something like this:
以纯 PowerShell 方式完成此操作的另一种方法是执行以下操作：
("" -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.
这将成功评估 null、空字符串和空格。我将传递的值格式化为空字符串以处理空值（否则在调用 Trim 时空值会导致错误）。然后只需使用空字符串评估相等性。我想我仍然更喜欢 IsNullOrWhiteSpace，但是如果您正在寻找另一种方法来做到这一点，这将起作用。
$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 $val).Trim())
>False
In a fit of boredom, I played with this some and made it shorter (albeit more cryptic):
在一阵无聊中，我玩了一些并缩短了它的时间（尽管更神秘）：
!!(("$val").Trim())
or
或者
!(("$val").Trim())
depending on what you're trying to do.
取决于你想做什么。
回答by Steve Friedl
Note that the "if ($str)"and "IsNullOrEmpty"tests don't work comparably in all instances: an assignment of $str=0produces false for both, and depending on intended program semantics, this could yield a surprise.
请注意，"if ($str)"和"IsNullOrEmpty"测试并非在所有情况下都具有可比性：$str=0两者的赋值都会产生错误，并且根据预期的程序语义，这可能会产生意外。
