Custom error from parameters in PowerShell

Question

Is it possible to have ValidateScript generate a custom error message when a test fails, like say Test-Path?

Instead of this:

Answer 1

Your ValidateScript should look something like this:

[ValidateScript({
    try {
        $Folder = Get-Item $_ -ErrorAction Stop
    } catch [System.Management.Automation.ItemNotFoundException] {
        Throw [System.Management.Automation.ItemNotFoundException] "${_} Maybe there are network issues?"
    }
    if ($Folder.PSIsContainer) {
        $True
    } else {
        Throw [System.Management.Automation.ValidationMetadataException] "The path '${_}' is not a container."
    }
})]

That will give you a message like this:

Test-Folder : Cannot validate argument on parameter 'Folder'. Cannot find path '\\server\Temp\asdf' because it does not exist. Maybe there are network issues?

Or:

Test-Folder : Cannot validate argument on parameter 'Folder'. The path '\\server\Temp\asdf' is not a container.

If the older versions of PowerShell are throwing a double error, you may need to test inside the function:

Function Test-Folder {
    Param (
        [parameter(Mandatory=$true)]
        [String]$Folder
    )

    try {
        $Folder = Get-Item $_ -ErrorAction Stop
    } catch [System.Management.Automation.ItemNotFoundException] {
        Throw [System.Management.Automation.ItemNotFoundException] "The '${Folder}' is not found, maybe there are network issues?"
    }

    if (-not $Folder.PSIsContainer) {
        Throw [System.Management.Automation.ApplicationFailedException] "The path '${_}' is not a container."
    }

    Write-Host "The folder is: ${Folder}"
}

The part that I always hated in PowerShell was trying to figure out what error to catch; without catching all. Since I finally figure it out, here's how:

PS > Resolve-Path 'asdf'
Resolve-Path : Cannot find path '.\asdf' because it does not exist.
At line:1 char:1
+ Resolve-Path 'asdf'
+ ~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (asdf:String) [Resolve-Path], ItemNotFoundE
   xception
    + FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.ResolvePathCommand

PS > $Error[0].Exception.GetType().FullName
System.Management.Automation.ItemNotFoundException