PowerShell : use parameter validation without throwing exception

Question

I'd like to call a PowerShell script this way :
script.ps1 -path mypath\to\files\ -days 6 -hours 0

To validate the command line arguments, I can either do it by hand, either rely on the param syntax :

Param (
    [Parameter(Mandatory=$true )] [string] $path,
    [Parameter(Mandatory=$false)] [int]    $days,
    [Parameter(Mandatory=$false)] [int]    $hours
)

If I use the param syntax :

the param definition must be the first line in the script (excluding comments). Okay, not a problem for me
in case of incorrect parameters, I can't catch the error (for example to display a custom error message)

I'd like to display a custom error message when the script is called with wrong parameters.
Is it possible, and how, to catch the exception in case of parameter error ?

Gregory MOUSSAT · Accepted Answer · 2012-11-03 14:04:01Z

3

So okay, it is not possible to use param AND to catch the related exceptions.

answered Nov 3, 2012 at 14:04

Gregory MOUSSAT

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2 Comments

Roman Kuzmin Over a year ago

It still seems to be possible, see my answer.

Roman Kuzmin Over a year ago

My vote was +1 because I understood the question in a different way and in this way it was interesting. Perhaps I should have abstained, the question is not clear. You may try to edit it and explain the problem better. I'll try to help again.

Roman Kuzmin · Accepted Answer · 2012-11-03 16:03:01Z

1

The examples are for functions (simple and advanced) but the same idea should work for scripts with param as well:

# Simple function.
# Everything not declared in `param` goes to $args.
# If $args is not empty then there are "invalid" parameters or "unexpected" arguments
function test {
    param (
        [string]$path,
        [int]$days,
        [int]$hours
    )
    # check $args and throw an error (in here we just write a warning)
    if ($args) { Write-Warning "Unknown arguments: $args" }
}

Or

# Advanced function.
# For an advanced function we can use an extra argument $args
# with an attribute `[Parameter(ValueFromRemainingArguments=$true)]`
function test {
    param (
        [Parameter(Mandatory=$true )] [string] $path,
        [Parameter(Mandatory=$false)] [int]    $days,
        [Parameter(Mandatory=$false)] [int]    $hours,
        [Parameter(ValueFromRemainingArguments=$true)] $args
    )
    # check $args and throw an error (in this test we just write a warning)
    if ($args) { Write-Warning "Unknown arguments: $args" }
}

The following test:

# invalid parameter
test -path p -invalid -days 5

# too many arguments
test -path p 5 5 extra

in both cases produces the same output:

WARNING: Unknown arguments: -invalid
WARNING: Unknown arguments: extra

edited Nov 3, 2012 at 16:03

answered Nov 3, 2012 at 15:37

Roman Kuzmin

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1 Comment

Gregory MOUSSAT Over a year ago

This don't address the question : how to catch the exception in case of parameter error ? If you have an error on the mandatory paramaters, the script just crash and you can do nothing against it.

Community · Accepted Answer · 2017-05-23 12:10:38Z

1

A possible workaround is to wrap your actual function in another one. Something similar to a private/public relation. Example:

function Example-Private
{
    [CmdletBinding()]
    Param
    (
        [ValidateNotNullOrEmpty()]
        [string]$Arg1,
        [ValidateNotNullOrEmpty()]
        [string]$Arg2   
   )

   # Do what you need
}

function Example-Public
{
    [CmdletBinding()]
    Param
    (
        [string]$Arg1,
        [string]$Arg2   
    )

    try
    {
       Example-Private $Arg1 $Arg2
    }
    catch
    {
       # Display a user-friendly message, save exception into a log file, etc.
    }
}

If you are working on a module you could take a look here how to export your public functions and hide the private ones: Export Powershell Functions

edited May 23, 2017 at 12:10

CommunityBot

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answered Jul 12, 2016 at 21:04

Rosen Petrov

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1 Comment

Gostega Over a year ago

This is the only thing that worked for me. Powershell v5

Keith Hill · Accepted Answer · 2012-09-18 17:36:33Z

-1

In the Begin block you can always do further validation on the parameters but if the parameter is wrong, I think you wouldn't want to continue execution. That is, you'd want to throw a terminating error. Here's an example:

param (
    [Parameter(Mandatory=$true )] [string] $path,
    [Parameter(Mandatory=$false)] [int]    $days,
    [Parameter(Mandatory=$false)] [int]    $hours
)

Begin {
    if ($hours -lt 0 -or $hours -gt 23) {
        throw "Hours parameter must be between 0 and 23"
    }
}

That said, I'm not sure that's any better than using PowerShell's built-in parameter validation functionality e.g.:

param (
    [Parameter(Mandatory=$true )] [string] $path,
    [Parameter(Mandatory=$false)] [int]    $days,
    [Parameter(Mandatory=$false)]
    [ValidateRange(0,23)]
    [int]
    $hours
)

answered Sep 18, 2012 at 17:36

Keith Hill

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5 Comments

Gregory MOUSSAT Over a year ago

This don't work. It don't allow the code to catch any exception during the 'param' validation, for exemple if I add an extra parameter.

Keith Hill Over a year ago

Which this doesn't work? The first example, which is doing manual validation or the second example which uses parameter validation. I suspect you mean the latter didn't work. Did you try the approach in the first example - validating in the Begin block?

Gregory MOUSSAT Over a year ago

I tested the first example. I run with script.ps1 -path xxx -dummy yyy and an exception is fired without executing 'begin'.

Keith Hill Over a year ago

-Dummy is not a declared parameter. If you want to handled invalid parameters then don't use [Parameter(...)].

Gregory MOUSSAT Over a year ago

This is the question. If you know this is not possible, why give a wrong answer ?

チーズパン · Accepted Answer · 2015-12-15 14:35:09Z

-1

you can do that with the param syntax, if you add a string-type dummy-parameter with the property "ValueFromRemainingArguments". Then you can check this dummy-parameter in your script and take appropriate actions, e.g.:

Param(
    [Parameter(Mandatory=$false)]
    [SWITCH]$myparam1,

    [Parameter(Mandatory=$false)]
    [SWITCH]$myparam2,

    [parameter(Mandatory=$false,ValueFromRemainingArguments=$true)]
    [STRING]$dummy     
    )

if ($dummy -eq anythingYouDontLike) throwAMessageOrSomething.

edited Dec 15, 2015 at 14:35

チーズパン

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answered Dec 15, 2015 at 13:35

Herbert

1

1 Comment

Gostega Over a year ago

As far as I understand this doesn't address the question, which is how to avoid the script crashing if one of the param validation fails, which happens before even reaching your "ValueFromRemainingArguments"

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