PowerShell mandatory parameter depends on another parameter

You could group those parameters by defining a parameter set to accomplish this.

param (    [Parameter(ParameterSetName='One')][switch]$CreateNewChild,    [Parameter(ParameterSetName='One',Mandatory=$true)][string]$PropertyType)

Reference:

https://devblogs.microsoft.com/powershell/powershell-v2-parametersets

http://blogs.technet.com/b/heyscriptingguy/archive/2011/06/30/use-parameter-sets-to-simplify-powershell-commands.aspx

--- Update ---

Here's a snippet that mimics the functionality you're looking for. The "Extra" parameter set will not be processed unless the -Favorite switch is called.

[CmdletBinding(DefaultParametersetName='None')] param(     [Parameter(Position=0,Mandatory=$true)] [string]$Age,     [Parameter(Position=1,Mandatory=$true)] [string]$Sex,     [Parameter(Position=2,Mandatory=$true)] [string]$Location,    [Parameter(ParameterSetName='Extra',Mandatory=$false)][switch]$Favorite,          [Parameter(ParameterSetName='Extra',Mandatory=$true)][string]$FavoriteCar)$ParamSetName = $PsCmdLet.ParameterSetName    Write-Output "Age: $age"Write-Output "Sex: $sex"Write-Output "Location: $Location"Write-Output "Favorite: $Favorite"Write-Output "Favorite Car: $FavoriteCar"Write-Output "ParamSetName: $ParamSetName"

You can also use a dynamic parameter:

New way to create a dynamic parameter