powershell mandatory parameter throws error when null

Question

I am confused about how to do error handling in the case that a mandatory variable remains null.

function parse-com{
    [CmdletBinding()]
    Param
    (
        [Parameter(Mandatory=$True)]
        [string[]]$list
     )
...

In this case if no argument is passed for $list then I am prompted for it, but if I just hit enter (pass a null to $list) then I throw an error. What I would rather do is throw a usage statement and/or exit gracefully. Example...

PS C:\Users\memyself> parse-com
cmdlet parse-com at command pipeline position 1
Supply values for the following parameters:
list[0]: 
parse-com : Cannot bind argument to parameter 'list' because it is an empty array.
At line:1 char:1
+ parse-com
+ ~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [parse-com], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorEmptyArrayNotAllowed,parse-com

Look into using a ValidateScript on the parameter instead of merely making it mandatory. — Jeff Zeitlin
– Jeff Zeitlin, Commented Jun 9, 2017 at 18:13

BenH · Accepted Answer · 2017-06-09 18:27:01Z

5

When passing a $null value, that satisfies the Mandatory requirement. If you want the command then to fail prior to execution then you should use validation as @JeffZeitlin suggests.

It sounds like what you need to validate is that the value is not $null nor is it an empty array. For which you could user [ValidateNotNullOrEmpty()]

function parse-com{
    [CmdletBinding()]
    Param (
        [Parameter(Mandatory=$True)]
        [ValidateNotNullOrEmpty()]
        [string[]]$list
    )

answered Jun 9, 2017 at 18:27

BenH

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5 Comments

klaypigeon Over a year ago

I had high hopes, but I cannot really see that it does anything other than throw a different error.

klaypigeon Over a year ago

PS D:\MyCOMTracker> D:\MyCOMTracker\parse-com.ps1 cmdlet parse-com.ps1 at command pipeline position 1 Supply values for the following parameters: list[0]:  D:\MyCOMTracker\parse-com.ps1 : Cannot validate argument on parameter 'list'. The argument is null, empty, or an element of the argument  collection contains a null value. Supply a collection that does not contain any null values and then try the command again.   + CategoryInfo          : InvalidData: (:) [parse-com.ps1], ParentContainsErrorRecordException   + FullyQualifiedErrorId : ParameterArgumentValidationError,parse-com.ps1

BenH Over a year ago

@klaypigeon The point of the Validate is to throw that error before the command executes. If you want to just accept a null value, then take out the mandatory and the validate and you'll have to write your own logic around what to do when the parameter is $null inside the function.

Frederik Winkelsdorf Over a year ago

This answer is wrong and misleading, as Mandatory already implies not $null or empty. That's why the options AllowNull or AllowEmptyString exist for Mandatory parameters. See their description in the docs: learn.microsoft.com/en-us/powershell/module/…. Solving OPs problem needs a different streak, like e.g. making the parameter non-mandatory or check from the calling site.

Frederik Winkelsdorf Over a year ago

But ValidateNotNullOrEmpty can be used to check untyped parameters or parameters which accept $null. See: learn.microsoft.com/en-us/powershell/module/…

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