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Parameter Validation

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I was trying to explain how to validate PowerShell script parameters to my students when I found the number and quality of enum data type examples was woefully inadequate. Here’s a series of examples that show you how to validate input parameters against an enum list of values.

The basic concept requires you to validate an element in an enum type. It uses a literal value

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# Create a enum type.
Add-Type -TypeDefinition @"
  public enum msgTypes
    { moe, larry, curly }
"@
 
# Check whether the value is found in the enum type.
if ([enum]::isDefined(([msgTypes]), [msgTypes]::moe)) {
  Write-Host "Success"
}

You test this testEnum1.ps1 script file with this syntax:

powershell testEnum1.ps1

It prints:

Success

The next sample embeds the validation in a local function:

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# Create a enum type.
Add-Type -TypeDefinition @"
  public enum msgTypes
    { moe, larry, curly }
"@
 
# A local function for verbose reporting.
function Print-Message ($msg) {
  # Check whether the value is found in the enum type.
  if ([enum]::isDefined(([msgTypes]), [msgTypes]::$msg)) {
    Write-Host "Success"
  }
}
 
# Call the function with a literal value.
Print-Message "moe"

You test this testEnum2.ps1 script file with this syntax:

powershell testEnum2.ps1

It also prints:

Success

The next sample testEnum3.ps1 accepts a parameter and passes it to the validation function:

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# Create a enum type.
Add-Type -TypeDefinition @"
  public enum msgTypes
    { moe, larry, curly }
"@
 
# A local function for verbose reporting.
function Print-Message ($msg) {
  # Check whether the value is found in the enum type.
  if ([enum]::isDefined(([msgTypes]), [msgTypes]::$msg)) {
    Write-Host "Success"
  }
}
 
# Wrap the Parameter call to avoid a type casting warning.
try {
  param (
    [Parameter(Mandatory)][hashtable]$args
  )
}
catch {}
 
# Call the function with a literal value.
Print-Message "moe"

You test this testEnum.ps1 script file with this syntax:

powershell testEnum3.ps1 moe

It also prints:

Success

However, if you don’t pass a parameter to the testEnum3.ps1, like this

powershell testEnum3.ps1

It raises the following error:

Exception calling "IsDefined" with "2" argument(s): "Value cannot be null.
Parameter name: value"
At C:\Data\cit225\mysql\test\testEnum3.ps1:9 char:7
+   if ([enum]::isDefined(([msgTypes]), [msgTypes]::$msg)) {
+       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : ArgumentNullException

Rewriting the Print-Message function in the script may appear to fix the problem. At least, a missing parameter won’t cause an error when you wrap the call to the isDefined method inside an if-statement.

Here’s how that wrapping attempt would look:

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# A local function for verbose reporting.
function Print-Message ($msg) {
  if (!($msg -eq $null)) {
    if ([enum]::isDefined(([msgTypes]), [msgTypes]::$msg)) {
      Write-Host "Success"
    }
  }
  else {
  Write-Host "Failure" }
}

While the prior change to the Print-Message function manages the lack of a parameter, it doesn’t prevent a failure when you pass an incorrect parameter. A new variation of the old error occurs when you pass a parameter that is not a member of the enum type, like

powershell testEnum4.ps1 Shem

It now prints:

Failure

So, you need to complete another step. Our woeful tale of parameter validation against a set of possible enum values isn’t quite complete. That’s because any incorrect parameter value raises a null value when isDefined method can’t find a valid value in the enum type. This standard behavior means that the isDefined method returns this error message:

Exception calling "IsDefined" with "2" argument(s): "Value cannot be null.
Parameter name: value"
At C:\Data\cit225\mysql\test\testEnum4.ps1:10 char:9
+     if ([enum]::isDefined(([msgTypes]), [msgTypes]::$msg)) {
+         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : ArgumentNullException

After all the effort to sort through how PowerShell handles the isDefined method and navigating Microsoft’s limited PowerShell examples, we’ve travelled down a rabbit hole. The problem is that the isDefined method isn’t terribly useful.

You need to use another getValues method, which returns the set of member values from the enum type. This requires you to write a new function. Find-EnumMatch seems an appropriate Pascal-like name for that function because that’s what it will do. Writing the new function also simplifies the Print-Message function.

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# Create a enum type for statements, for future use.
Add-Type -TypeDefinition @"
  public enum msgTypes
    { moe, larry, curly }
"@
 
# A local function to find a valid enum member.
function Find-EnumMatch ($typeof, $member) {
  # Set default return value.
  $evaluated = $false
 
  # Check for a not null match to an enum member.
  if (!($msg -eq $null)) {  
    foreach ($msgValue in $msgTypes = [enum]::getValues(([enum]::'[' + $typeof + ']'))) {
      if ($msgValue -eq $member) {
 	    $evaluated = $true
 	    break }
    }
  }
  # Return whether true or false.
  return $evaluated
}   
 
# A local function for verbose reporting.
function Print-Message ($msg) {
  # Check for a not null match to an enum member.
  if (find-enumMatch $msg) { Write-Host "Success" }
  else { Write-Host "Failure" }
}
 
# Wrap the Parameter call to avoid a type casting warning.
try {
  param (
    [Parameter(Mandatory)][hashtable]$args
  )
}
catch {}

Now, if we test the program with a valid, invalid, or null value parameter it works as expected. It prints “Success” when the parameter is found as a member of the enum type, and prints “Failure” when the parameter is null or not found as a member of the enum type. It also never raises an unhandled exception.

There’s an important explicit casting trick required on line #14 to avoid the following error:

Cannot convert argument "enumType", with value: "[msgTypes]", for "GetValues" to type "System.Type": "Cannot convert the "[msgTypes]" value of
type "System.String" to type "System.Type"."
At C:\Data\cit225\mysql\test\testEnum7.ps1:14 char:27
+ ... ($msgValue in $msgTypes = [enum]::getValues(('[' + $typeof + ']'))) {
+                   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodException
    + FullyQualifiedErrorId : MethodArgumentConversionInvalidCastArgument

While many developers might think that you can write the line without explicitly casting the enum name enclosed by square brackets, you actually have to explicitly cast it.

As always, I hope this helps those looking for an arcane bit of knowledge. While handling parameters is routine, it sure appears the method for doing so in PowerShell with an enum type isn’t quite well documented.

Written by maclochlainn

April 24th, 2022 at 1:17 am

Posted in Microsoft,Microsoft Windows 10,Microsoft Windows 11,PowerShell,Windows 10,Windows 11,Windows OS

Tagged with ,

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