How to differentiate not set parameter from $false, 0, empty string? - powershell

Question

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?

Answer 1

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).

Answer 2

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]
}

Answer 3

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}

Answer 4

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).