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:

Test-Folder : Cannot validate argument on parameter 'Folder'. The "Test-Path $_ -Path Type Container" validation script for the argument with value "blabla" did not return a result of True. Determine why the validation script failed, and then try the comma and again.

It would be nice to have it report this instead in the $Error variable:

The 'Folder' is not found, maybe there are network issues?

Code:

Function Test-Folder {
    Param (
        [parameter(Mandatory=$true)]
        [ValidateScript({Test-Path $_ -PathType Container})]
        [String]$Folder
    )
    Write-Host "The folder is: $Folder"
}

Workaround 1:

I could remove the Mandatory=$true and change it as below. But this doesn't give me the correct Get-Help syntax and doesn't do the Test-Path validation, because it only checks if the parameter is present.

Function Test-Folder {
    Param (
        [parameter()]
        [String]$Folder = $(throw "The $_ is not found, maybe there are network issues?")
    )
    Write-Host "The folder is: $Folder"
}

Workaround 2:

I found this workaround on a blog post, but the problem is that it generates two errors instead of one.

Function Test-Folder {
    Param (
        [parameter(Mandatory=$true)]
        [ValidateScript({
            if (Test-Path $_ -PathType Container) {$true}
            else {Throw "The $_ is not found, maybe there are network issues?"}})]
        [String]$Folder
    )
    Write-Host "The folder is: $Folder"
}

Workaround 3:

I could also try to make it more clear by adding a comment section. However, this is still not the desired result as the error needs to be readable to end users.

Function Test-Folder {
    Param (
        [parameter(Mandatory=$true)]
        [ValidateScript({
        # The folder is not found, maybe there are network issues?
        Test-Path $_ -PathType Container})]
        [String]$Folder
    )
    Write-Host "The folder is: $Folder"
}

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