Validate parameter value using another parameter value - powershell

Question

I'm trying to validate whether a path exists before running a function.
There is no default for the folder path, but the file name default should be template.csv. Is there a way, through the ValidateScript attribute, to validate a parameter value based on another parameter value?
The below code returns the error that the variable $TemplateDir has not been set. I'm also not entirely sure if it would test for the default file name value.
function get-Template {
[CmdletBinding()]
Param (
[Parameter(Mandatory = $true, Position = 0)]
[ValidateScript({Test-Path $_})]
[string]$TemplateDir,
[Parameter(Mandatory = $false)]
[ValidateScript({Test-Path ($TemplateDir + "\" + $_)})]
[string]$TemplateFile = "template.csv"
)
...
}
Any advice?

Answer 1

You can set up a dynamic parameter with the DynamicParam block, that depends on the value of another mandatory parameter:
function Get-FilePath
{
Param (
[Parameter(Mandatory = $true, Position = 0)]
[ValidateScript({Test-Path $_})]
[string]$TemplateDir
)
DynamicParam {
# Set up parameter attribute
$fileParamAttribute = New-Object System.Management.Automation.ParameterAttribute
$fileParamAttribute.Position = 3
$fileParamAttribute.Mandatory = $false
$fileParamAttribute.HelpMessage = "Please supply a file name"
# Set up ValidateSet param with actual file name values
$fileValidateParam = New-Object System.Management.Automation.ValidateSetAttribute #(Get-ChildItem $TemplateDir -File |Select-Object -ExpandProperty Name)
# Add the parameter attributes to an attribute collection
$attributeCollection = New-Object System.Collections.ObjectModel.Collection[System.Attribute]
$attributeCollection.Add($fileParamAttribute)
$attributeCollection.Add($fileValidateParam)
# Create the actual $TemplateFile parameter
$fileParam = New-Object System.Management.Automation.RuntimeDefinedParameter('TemplateFile', [string], $attributeCollection)
# Push the parameter(s) into a parameter dictionary
$paramDictionary = New-Object System.Management.Automation.RuntimeDefinedParameterDictionary
$paramDictionary.Add('TemplateFile', $fileParam)
# Return the dictionary
return $paramDictionary
}
begin{
# Check if a value was supplied, otherwise set it
if(-not $PSBoundParameters.ContainsKey('TemplateFile'))
{
$TemplateFile = 'template.csv'
}
$myPath = Join-Path $TemplateDir -ChildPath $TemplateFile
}
end {
return $myPath
}
}
This will also give you automatic tab completion for the arguments to -TemplateFile
You can read more about DynamicParam with Get-Help about_Functions_Advanced_Parameters