239 lines
6.7 KiB
Markdown
239 lines
6.7 KiB
Markdown
---
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title: 'Parameter Validation with PSTypeName'
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date: 2022-03-16T09:24:56+01:00
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#draft: true
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categories: ['PowerShell']
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tags: ['parameter', 'validation', 'PSTypeName']
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# lastmod: 2022-03-16T09:24:56+01:00
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---
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 on [Unsplash](https://unsplash.com)')
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## 🖼️ Intro
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This post explains how to use `PSCustomObject`s as function parameters. We compare the basic usage with an
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advanced one using the `[PSTypeName()]` parameter attribute.
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## 🗑️ Well-Known Workflow
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So let's start with a common object definition how it is used with a function:
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```powershell
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$Rocinante = [PSCustomObject]@{
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Owner = 'Martian Congressional Republic Navy'
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Type = 'Light Frigate'
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Class = 'Corvette'
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Registry = 'ECF-270'
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HullNumber = '158'
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LengthInMeter = 46
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Name = 'Rocinante'
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}
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```
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As you can see, a `PSCustomObject` has still the the same class type and just differs by its note properties.
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```bash
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> $Rocinante | Get-Member
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TypeName: System.Management.Automation.PSCustomObject
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Name MemberType Definition
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---- ---------- ----------
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Equals Method bool Equals(System.Object obj)
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GetHashCode Method int GetHashCode()
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GetType Method type GetType()
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ToString Method string ToString()
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Class NoteProperty string Class=Corvette
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HullNumber NoteProperty string HullNumber=158
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LengthInMeter NoteProperty int LengthInMeter=46
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Name NoteProperty string Name=Rocinante
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Owner NoteProperty string Owner=Martian Congressional Republic Navy
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Registry NoteProperty string Registry=ECF-270
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Type NoteProperty string Type=Light Frigate
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> $Rocinante.PSObject.TypeNames
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System.Management.Automation.PSCustomObject
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System.Object
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```
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So we can use the out object as an function parameter.
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```powershell
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function Invoke-Launch {
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[CmdletBinding()]
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param (
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[Parameter(Mandatory = $true)]
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[ValidateNotNullOrEmpty()]
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[PSCustomObject]$Ship
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)
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begin {}
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process {
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# Manual input validation for $Ship
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# test if all needed properties are present.
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$DockLength = 50
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if ($Ship.LengthInMeter -gt $DockLength) {
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Write-Error -Message "Ship doesn't fit in the docking station." -ErrorAction 'Stop'
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}
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# ...
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# ...
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}
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end {}
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}
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```
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This common pattern could fail whenever someone changes your object properties. If the _LengthInMeter_ property is missing you ran into an error. E.g.:
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```console
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> $Rocinante = [PSCustomObject]@{ foo = 'bar' }
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> Invoke-Launch -Ship $Rocinante
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```
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{{< note >}}
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Keep in mind - Because we are using here custom objects and not class instances, we can not use `Rocinante` as a parameter type like `[Rocinante]$Ship` which would solve this immediately.
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{{< /note >}}
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To fix this we can use the `[PSTypeName()]` parameter attribute, to ensure an object with the correct type name is used. This doesn't verify your parameters but minimize the risk for using invalid parameter objects.
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## 🛡️ PSTypeName Usage
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Let's first modify the object creation and use a custom type name.
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```powershell
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$Rocinante = [PSCustomObject]@{
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# You can use special property 'PSTypeName'
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# to set it implicit within the creation.
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PSTypeName = 'Ship.Corvette.LightFrigate'
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Owner = 'Martian Congressional Republic Navy'
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Type = 'Light Frigate'
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Class = 'Corvette'
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Registry = 'ECF-270'
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HullNumber = '158'
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LengthInMeter = 46
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Name = 'Rocinante'
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}
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# Legacy syntax for injection a custom type name
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# $Rocinante.PSObject.TypeNames.insert(0,'Ship.Corvette.LightFrigate')
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```
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```bash
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> $Rocinante | Get-Member
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TypeName: Ship.Corvette.LightFrigate
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Name MemberType Definition
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---- ---------- ----------
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Equals Method bool Equals(System.Object obj)
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GetHashCode Method int GetHashCode()
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GetType Method type GetType()
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ToString Method string ToString()
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Class NoteProperty string Class=Corvette
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HullNumber NoteProperty string HullNumber=158
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LengthInMeter NoteProperty int LengthInMeter=46
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Name NoteProperty string Name=Rocinante
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Owner NoteProperty string Owner=Martian Congressional Republic Navy
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Registry NoteProperty string Registry=ECF-270
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Type NoteProperty string Type=Light Frigate
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> $Rocinante.PSObject.TypeNames
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Ship.Corvette.LightFrigate
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System.Management.Automation.PSCustomObject
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System.Object
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```
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Now we can replace the `[PSCustomObject]` parameter type by `[PSTypeName()]`
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```powershell
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function Invoke-Launch {
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[CmdletBinding()]
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param (
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[Parameter(Mandatory = $true)]
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[ValidateNotNullOrEmpty()]
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[PSTypeName('Ship.Corvette.LightFrigate')]$Ship
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)
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begin {}
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process {
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$DockLength = 50
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if ($Ship.LengthInMeter -gt $DockLength) {
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Write-Error -Message "Ship doesn't fit in the docking station." -ErrorAction 'Stop'
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}
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# ...
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# ...
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}
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end {}
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}
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```
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## 💭 Final Thoughts
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Over time, your PowerShell functions become more and more complex. You will reach a point where you start using
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objects as parameters. This is where the PSTypeName parameter attribute shown can help you.
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In my experience, the ability to create custom classes _(introduced in PowerShell 5)_ is rarely used for this.
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Most PowerShell users I know have a SysOp or DevOps background. Few come from software development and try to use
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OOP paradigms and patterns.
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Therefore I would also avoid using complex classes, especially if they use not only properties but also methods.
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Like already mentioned `PSTypeName` just tests the used type name and not your definition details.
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You should consider creating a your objects within a wrapper function to mimic a class constructor:
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```powershell
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function New-LightFrigate {
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[CmdletBinding()]
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[OutputType('Ship.Corvette.LightFrigate')]
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param (
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[Parameter(Mandatory = $true)]
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[ValidateNotNullOrEmpty()]
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[string]$Registry,
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[Parameter(Mandatory = $true)]
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[ValidateNotNullOrEmpty()]
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[string]$HullNumber,
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[Parameter(Mandatory = $true)]
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[ValidateNotNullOrEmpty()]
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[string]$Name
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)
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begin {}
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process {
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$Ship = [PSCustomObject]@{
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PSTypeName = 'Ship.Corvette.LightFrigate'
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Owner = 'Martian Congressional Republic Navy'
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Type = 'Light Frigate'
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Class = 'Corvette'
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Registry = $Registry
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HullNumber = $HullNumber
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LengthInMeter = 46
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Name = $Name
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}
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Write-Output $Ship
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}
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end {}
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}
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$Rocinante = New-LightFrigate -Name 'Rocinante' -Registry 'DE-MB2' -HullNumber '158'
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```
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