I need to write a function in powershell that tells apart a 'parameter not being passed' from one passed with string empty (or any other string)
I wrote it like this:
function Set-X {
param(
[AllowNull()][string]$MyParam = [System.Management.Automation.Language.NullString]::Value
)
if ($null -ne $MyParam) { write-host 'oops' }
else { write-host 'ok' }
}
If I call Set-X without parameters from ISE, it works as I expect and prints 'ok'.
But if I do that from the normal console, it prints 'oops'.
What is going on? What is the proper way to do it?
Allowing the user to pass in a parameter argument value of $null does not change the fact that powershell will attempt to convert it to a [string].
Converting a $null value in powershell to a string results in an empty string:
$str = [string]$null
$null -eq $str # False
'' -eq $str # True
(same goes for $null -as [string] and "$null")
Remove the type constraint on the MyParam parameter if you not only want to allow $null but also accept $null as a parameter value:
function Set-X {
param(
[AllowNull()]$MyParam = [System.Management.Automation.Language.NullString]::Value
)
if ($null -ne $MyParam) { write-host 'oops' }
else { write-host 'ok' }
}
As Mathias and BenH have written, the culprit is casting $null to the [string] type, which results in an empty string:
[string]$null -eq '' #This is True
But for the sample code in Mathias answer to work correctly we also have to replace
[System.Management.Automation.Language.NullString]::Value
with $null
function Set-X {
param(
[AllowNull()]$MyParam = $null
)
if ($null -ne $MyParam) { write-host 'oops' }
else { write-host 'ok' }
}
I have function that updates object in WMI. I want user to be able to specify in parameters only values that he wants to update. How can I do it?
function UpdateObject ([bool] $b1, [bool] $b2, [int] $n1, [string] $s1)
{
$myObject = GetObjectFromWmi #(...)
#(...)
#This is bad. As it overrides all the properties.
$myObject.b1 = $b1
$myObject.b2 = $b2
$myObject.n1 = $n1
$myObject.s1 = $s1
#This is what I was thinking but don't kwow how to do
if(IsSet($b1)) { $myObject.b1 = $b1 }
if(IsSet($b2)) { $myObject.b2 = $b2 }
if(IsSet($n1)) { $myObject.n1 = $n1 }
if(IsSet($s1)) { $myObject.s1 = $s1 }
#(...) Store myObject in WMI.
}
I tried passing $null as as parameter but it get's automaticly converted to $false for bool, 0 for int and empty string for string
What are your suggestions?
Check $PSBoundParameters to see if it contains a key with the name of your parameter:
if($PSBoundParameters.ContainsKey('b1')) { $myObject.b1 = $b1 }
if($PSBoundParameters.ContainsKey('b2')) { $myObject.b2 = $b2 }
if($PSBoundParameters.ContainsKey('n1')) { $myObject.n1 = $n1 }
if($PSBoundParameters.ContainsKey('s1')) { $myObject.s1 = $s1 }
$PSBoundParameters acts like a hashtable, where the keys are the parameter names, and the values are the parameters' values, but it only contains bound parameters, which means parameters that are explicitly passed. It does not contain parameters filled in with a default value (except for those passed with $PSDefaultParameterValues).
Building on briantist's answer, if you know that all the parameters exist as properties on the target object you can simply loop through the $PSBoundParameters hashtable and add them one by one:
foreach($ParameterName in $PSBoundParameters.Keys){
$myObject.$ParameterName = $PSBoundParameters[$ParameterName]
}
If only some of the input parameters are to be passed as property values, you can still specify the list just once, with:
$PropertyNames = 'b1','b2','n1','s1'
foreach($ParameterName in $PSBoundParameters.Keys |Where-Object {$PropertyNames -contains $_}){
$myObject.$ParameterName = $PSBoundParameters[$ParameterName]
}
To save yourself having to create a parameter for each property you may want to change, consider using a hashtable or other object to pass this information to your function.
For example:
function UpdateObject ([hashtable]$properties){
$myObject = GetObjectFromWmi
foreach($property in $properties.Keys){
# without checking
$myObject.$property = $properties.$property
# with checking (assuming members of the wmiobject have MemberType Property.
if($property -in (($myObject | Get-Member | Where-Object {$_.MemberType -eq "Property"}).Name)){
Write-Output "Updating $property to $($properties.$property)"
$myObject.$property = $properties.$property
}else{
Write-Output "Property $property not recognised"
}
}
}
UpdateObject -properties {"b1" = $true; "b2" = $false}
If you want a [Boolean] parameter that you want the user to specify explicitly or omit (rather than a [Switch] parameter which can be present or not), you can use [Nullable[Boolean]]. Example:
function Test-Boolean {
param(
[Nullable[Boolean]] $Test
)
if ( $Test -ne $null ) {
if ( $Test ) {
"You specified -Test `$true"
}
else {
"You specified -Test `$false"
}
}
else {
"You did not specify -Test"
}
}
In this sample function the $Test variable will be $null (user did not specify the parameter), $true (user specified -Test $true), or $false (user specified -Test $false). If user specifies -Test without a parameter argument, PowerShell will throw an error.
In other words: This gives you a tri-state [Boolean] parameter (missing, explicitly true, or explicitly false). [Switch] only gives you two states (present or explicitly true, and absent or explicitly false).
I have a bunch of strings, in the form of:
'3m 36s', '24m 38s', '59s'
, to be converted to timespans. My current "solution" is:
'3m 36s', '24m 38s', '59s' |ForEach-Object {
$s = 0
$m = 0
$h = 0
$PSItem.Split(' ') |ForEach-Object {
$item = $PSItem
switch ($PSItem[-1])
{
's'
{
$s = $item.TrimEnd('s')
}
'm'
{
$m = $item.TrimEnd('m')
}
'h'
{
$h = $item.TrimEnd('h')
}
Default
{
Write-Error 'Ooops...' -ErrorAction Stop
}
}
}
$timespan = New-TimeSpan -Hours $h -Minutes $m -Seconds $s
# ToString() is used just to get some easy to read output
$timespan.ToString()
}
While it seems to work for me, I have two issues with the above:
Is the general approach
ForEach -> Split(' ') -> ForEach -> switch
OK-ish? Are there any alternative/better ways of doing the conversion?
I tried using $PSItem in the switch
It seems that the switch construct has it's "own pipeline"
# $item = $PSItem
switch ($PSItem[-1])
{
's'
{
$PSItem
}
}
-- in the above $PSItem evaluates to 's'(, 'm', the value matched). What is actually going on? (internaly?)
I would take one ForEach loop out of things by performing that loop with the Switch command. Here's what I'd end up with:
'3m 36s', '59s', '24m 38s' |%{
$TSParams = #{}
Switch($_.Split()){
{$_[-1] -eq 's'}{$TSParams.Add('Seconds', ([int]$_.trim('s')))}
{$_[-1] -eq 'm'}{$TSParams.Add('Minutes', ([int]$_.trim('m')))}
{$_[-1] -eq 'h'}{$TSParams.Add('Hours', ([int]$_.trim('h')))}
}
New-TimeSpan #TSParams
}
For each string it creates an empty hashtable, then loops through each item of the Split() method, adding the appropriate time to the hashtable. Then it splats that to the New-TimeSpan command, and moves to the next item in the ForEach loop. I tried it locally and had some issues initially when the numbers did not cast as an int, and it tried to convert them to a DateTime, which is why I type cast them in the above code.
I am trying to output the value which checks if the condition matches, but instead its returning value whether it is true or not.
$result = "VM name"
$name= get-content C:\monitor\Serverlist\Serverlist2.txt
foreach($nam1 in $name)
{
#Write-output $nam1
$l=Get-Vm -computername $nam1 |out-string
foreach ($l2 in $l)
{
if ( $l2 = $result )
{
Write-Output $input "is in" $nam1
}
else
{
""
}
}
}
but output i am getting is true in all case, where as VM name is present in comp1 only.
VM name is in comp1
VM name is in comp2
VM name is in comp3
If statement should return condition if it matches? I also tried looping if statement outside 1st foreach loop but that gives me result of last comp "comp3" even the matching result is in comp1.
Any Idea where I am going wrong?
The = sign not for conditional testing, it's for setting the value:
if ( $l2 = $result )
{
Write-Output $input "is in" $nam1
}
Should be... (note that I'm using -eq to test equality)
if ( $l2 -eq $result )
{
Write-Output $input "is in" $nam1
}
But even that wouldn't work. Your conditional test between a string and a VirtualMachine object won't work. Here's what you want...
if ( $l2.Name -eq $result )
{
Write-Output $input "is in" $nam1
}
In the above snippet, I'm doing a conditional test on VirtualMachine.Name (type string) with your variable.
How can you test if an object has a specific property?
Appreciate I can do ...
$members = Get-Member -InputObject $myobject
and then foreach through the $members, but is there a function to test if the object has a specific property?
Additional Info:
The issue is I'm importing two different sorts of CSV file, one with two columns, the other with three. I couldn't get the check to work with "Property", only with "NoteProperty" ... whatever the difference is
if ( ($member.MemberType -eq "NoteProperty" ) -and ($member.Name -eq $propertyName) )
Like this?
[bool]($myObject.PSobject.Properties.name -match "myPropertyNameToTest")
You can use Get-Member
if (Get-Member -inputobject $var -name "Property" -Membertype Properties) {
#Property exists
}
This is succinct and readable:
"MyProperty" -in $MyObject.PSobject.Properties.Name
We can put it in a function:
function HasProperty($object, $propertyName)
{
$propertyName -in $object.PSobject.Properties.Name
}
For me MyProperty" -in $MyObject.PSobject.Properties.Name didn't work, however
$MyObject.PSobject.Properties.Name.Contains("MyProperty")
works
There are a number of solutions to this question that work in strict mode, but some are better than others.
Solutions that do not appear to iterate through every property are the fastest solutions.
Bernie White's solution and
esskar's solution (modified)
Solutions that look as though they iterate through every property are slower.
sebke CCU's solution and
dan-gph's solution
The solution that appears to iterate through every property and uses a regular expression is a little slower than the previous two solutions (because compiling and executing the regular expression takes more time)
CB.'s solution
The solution that uses GetMethod appears to iterate through every property, but its use of GetMethod makes it significantly slower.
Paul's GetMethod solution
The following script was used to compare the previously mentioned solutions in strict mode:
# Tested in PowerShell core 7.2.0
Set-StrictMode -Version Latest
$propertyExistsMethods = New-Object System.Collections.Generic.Dictionary'[string,scriptblock]'
# Fastest
$propertyExistsMethods.Add(
"PSObject.Properties (Bernie White's solution)",
{
Param( [PSObject] $Object, [string] $Property )
[bool]$Object.PSObject.Properties[$Property]
})
$propertyExistsMethods.Add(
"PSObject.Properties.Item (esskar's solution (modified))",
{
Param( [PSObject] $Object, [string] $Property )
[bool]$Object.PSObject.Properties.Item($property)
})
# Not as fast
$propertyExistsMethods.Add(
"Contains (sebke CCU's solution)",
{
Param( [PSObject] $Object, [string] $Property )
$Object.PSobject.Properties.Name.Contains($Property)
})
$propertyExistsMethods.Add(
"-in (dan-gph's solution)",
{
Param( [PSObject] $Object, [string] $Property )
$Property -in $Object.PSobject.Properties.Name
})
# Slower than the previously mentioned solutions
$propertyExistsMethods.Add(
"-match (CB.'s solution)",
{
Param( [PSObject] $Object, [string] $Property )
[bool]($Object.PSobject.Properties.name -match $Property)
})
# Slowest
$propertyExistsMethods.Add(
"GetMember (Paul's solution)",
{
Param( [PSObject] $Object, [string] $Property )
Get-Member -inputobject $Object -name $Property -Membertype Properties
})
foreach ($method in $propertyExistsMethods.Keys) {
$propertyExists = $propertyExistsMethods[$method]
$o = #{}
foreach ($i in 1..100000) {
$o[$i] = "p$i"
}
Write-Host $method
$measure = Measure-Command {
foreach ($i in 1..100000) {
# Always check for a property that does NOT exist
& $propertyExists -Object $o -Property 'p'
}
}
Write-Host $measure | % { $_.Milliseconds }
Write-Host ''
}
The output is as follows:
PSObject.Properties (Bernie White's solution)
00:00:03.1437587
PSObject.Properties.Item (esskar's solution)
00:00:03.5833642
Contains (sebke CCU's solution)
00:00:04.4812702
-in (dan-gph's solution)
00:00:04.6507811
-match (CB.'s solution)
00:00:05.1107066
GetMember (Paul's solution)
00:00:14.5305115
Try this for a one liner that is strict safe.
[bool]$myobject.PSObject.Properties[$propertyName]
For example:
Set-StrictMode -Version latest;
$propertyName = 'Property1';
$myobject = [PSCustomObject]#{ Property0 = 'Value0' };
if ([bool]$myobject.PSObject.Properties[$propertyName]) {
$value = $myobject.$propertyName;
}
I've been using the following which returns the property value, as it would be accessed via $thing.$prop, if the "property" would be to exist and not throw a random exception. If the property "doesn't exist" (or has a null value) then $null is returned: this approach functions in/is useful for strict mode, because, well, Gonna Catch 'em All.
I find this approach useful because it allows PS Custom Objects, normal .NET objects, PS HashTables, and .NET collections like Dictionary to be treated as "duck-typed equivalent", which I find is a fairly good fit for PowerShell.
Of course, this does not meet the strict definition of "has a property".. which this question may be explicitly limited to. If accepting the larger definition of "property" assumed here, the method can be trivially modified to return a boolean.
Function Get-PropOrNull {
param($thing, [string]$prop)
Try {
$thing.$prop
} Catch {
}
}
Examples:
Get-PropOrNull (Get-Date) "Date" # => Monday, February 05, 2018 12:00:00 AM
Get-PropOrNull (Get-Date) "flub" # => $null
Get-PropOrNull (#{x="HashTable"}) "x" # => "HashTable"
Get-PropOrNull ([PSCustomObject]#{x="Custom"}) "x" # => "Custom"
$oldDict = New-Object "System.Collections.HashTable"
$oldDict["x"] = "OldDict"
Get-PropOrNull $d "x" # => "OldDict"
And, this behavior might not [always] be desired.. ie. it's not possible to distinguish between x.Count and x["Count"].
Just check against null.
($myObject.MyProperty -ne $null)
If you have not set PowerShell to StrictMode, this works even if the property does not exist:
$obj = New-Object PSObject;
Add-Member -InputObject $obj -MemberType NoteProperty -Name Foo -Value "Bar";
$obj.Foo; # Bar
($obj.MyProperty -ne $null); # False, no exception
If you are using StrictMode and the psobject might be empty, it will give you an error.
For all purposes this will do:
if (($json.PSobject.Properties | Foreach {$_.Name}) -contains $variable)
I find this method more strict and faster when checking multiple properties
$null -ne $myobject.PSObject.Properties.Item("myPropertyNameToTest")
Real similar to a javascript check:
foreach($member in $members)
{
if($member.PropertyName)
{
Write $member.PropertyName
}
else
{
Write "Nope!"
}
}
Just to clarify
given the following object
$Object
With the following properties
type : message
user : john.doe#company.com
text :
ts : 11/21/2016 8:59:30 PM
The following are true
$Object.text -eq $NULL
$Object.NotPresent -eq $NULL
-not $Object.text
-not $Object.NotPresent
So the earlier answers that explicitly check for the property by name is the most correct way to verify that that property is not present.
I ended up with the following function ...
function HasNoteProperty(
[object]$testObject,
[string]$propertyName
)
{
$members = Get-Member -InputObject $testObject
if ($members -ne $null -and $members.count -gt 0)
{
foreach($member in $members)
{
if ( ($member.MemberType -eq "NoteProperty" ) -and `
($member.Name -eq $propertyName) )
{
return $true
}
}
return $false
}
else
{
return $false;
}
}
I recently switch to set strict-mode -version 2.0 and my null tests failed.
I added a function:
#use in strict mode to validate property exists before using
function exists {
param($obj,$prop)
try {
if ($null -ne $obj[$prop]) {return $true}
return $false
} catch {
return $false
}
return $false
}
Now I code
if (exists $run main) { ...
rather than
if ($run.main -ne $null) { ...
and we are on our way. Seems to work on objects and hashtables
As an unintended benefit it is less typing.
for me this work
Set-StrictMode -Version Latest
$TMP = ...
$HAS_SERVERS=($TMP | Select-Object Servers)
if (-not $HAS_SERVERS.Servers){
echo "No servers. Abort."
} else {
...
}
I just started using PowerShell with PowerShell Core 6.0 (beta) and following simply works:
if ($members.NoteProperty) {
# NoteProperty exist
}
or
if (-not $members.NoteProperty) {
# NoteProperty does not exist
}
You could check with:
($Member.PropertyNames -contains "Name") this will check for the Named property
For identifying which of the objects in an array have a property
$HasProperty = $ArrayOfObjects | Where-Object {$_.MyProperty}