Using .NET objects within a Powershell (V5) class

Question

Below is the exact code that I am having trouble with.

A brief description:

I am trying to set up a PowerShell class that will hold objects of different types for easy access. I've done this numerous times in C#, so I thought it would be fairly straight forward. The types wanted are [System.Printing] and WMI-Objects.

Originally I had tried to write the class directly to my PowerShell profile for easy usage, but my profile fails to load when I have to class code in it. Saying that it can’t find the type name "System.Printing.PrintServer", or any other explicitly listed types.

After that failed, I moved it to its own specific module and then set my profile to import the module on open. However, even when stored in its own module, if I explicitly list a .NET type for any of the properties, the entire module fails to load. Regardless of whether I have added or imported the type / dll.

The specific problem area is this:

    [string]$Name
    [System.Printing.PrintServer]$Server
    [System.Printing.PrintQueue]$Queue
    [System.Printing.PrintTicket]$Ticket
    [System.Management.ManagementObject]$Unit
    [bool]$IsDefault

When I have it set to this, everything "kind of" works, but then all my properties have the _Object type, which is not helpful.

    [string]$Name
    $Server
    $Queue
    $Ticket
    $Unit
    $IsDefault


Add-Type -AssemblyName System.Printing
Add-Type -AssemblyName ReachFramework
Class PrinterObject
{
    [string]$Name
    [System.Printing.PrintServer]$Server
    [System.Printing.PrintQueue]$Queue
    [System.Printing.PrintTicket]$Ticket
    [System.Management.ManagementObject]$Unit
    [bool]$IsDefault

   PrinterObject([string]$Name)
    {
        #Add-Type -AssemblyName System.Printing
        #Add-Type -AssemblyName ReachFramework
        $this.Server = New-Object System.Printing.PrintServer -ArgumentList [System.Printing.PrintSystemDesiredAccess]::AdministrateServer
        $this.Queue =  New-Object System.Printing.PrintQueue (($this.Server), ($this.Server.GetPrintQueues() |
        Where-Object {$_.Name -match $Name} | Select-Object -ExpandProperty Name))

        $this.Ticket = $this.Queue.UserPrintTicket
        $this.Unit = Get-WmiObject -Query "SELECT * FROM Win32_Printer WHERE Name LIKE `"%$Name%`""
    }

    PrinterObject([string]$Name, [bool]$IsNetwork)
    {
        #Add-Type -AssemblyName System.Printing
        #Add-Type -AssemblyName ReachFramework
        if($IsNetwork -eq $true) {
        $this.Server = New-Object System.Printing.PrintServer ("\\Server")
        $this.Queue =  New-Object System.Printing.PrintQueue (($this.Server), ($this.Server.GetPrintQueues() |
        Where-Object {$_.Name -match $Name} | Select-Object -ExpandProperty Name))

        $this.Ticket = $this.Queue.UserPrintTicket
        $this.Unit = Get-WmiObject -Query "SELECT * FROM Win32_Printer WHERE Name LIKE `"%$Name%`""
        }
        else {
        $This.Server = New-Object System.Printing.PrintServer -argumentList [System.Printing.PrintSystemDesiredAccess]::AdministrateServer
        $this.Queue =  New-Object System.Printing.PrintQueue (($this.Server), ($this.Server.GetPrintQueues() |
        Where-Object {$_.Name -match $Name} | Select-Object -ExpandProperty Name))

        $this.Ticket = $this.Queue.UserPrintTicket
        $this.Unit = Get-WmiObject -Query "SELECT * FROM Win32_Printer WHERE Name LIKE `"%$Name%`"" }
    }
    [void]SetPrintTicket([int]$Copies, [string]$Collation, [string]$Duplex)
    {
        $this.Ticket.CopyCount = $Copies
        $this.Ticket.Collation = $Collation
        $this.Ticket.Duplexing = $Duplex
        $this.Queue.Commit()
    }

    [Object]GetJobs($Option)
    {
            if($Option -eq 1) { return $this.Queue.GetPrintJobInfoCollection() | Sort-Object -Property JobIdentifier | Select-Object -First 1}
            else { return $this.Queue.GetPrintJobInfoCollection() }
    }
    static [Object]ShowAllPrinters()
    {
        Return Get-WmiObject -Class Win32_Printer | Select-Object -Property Name, SystemName
    }

}

Answer 1

To complement PetSerAl's helpful answer, which explains the underlying problem and contains effective solutions, with additional background information:

To recap:

• As of PowerShell 7.3.1, a PowerShell class definition can only reference .NET types that have already been loaded into the session before the script is invoked.

• Because class definitions are processed at parse time of a script, rather than at runtime, Add-Type -AssemblyName calls inside a script execute too late for the referenced assemblies' types to be known to any class definitions inside the same script.

• A using assembly statement should solve this problem, but currently doesn't:

  • using assembly should be the parse-time equivalent of an Add-Type (analogous to the relationship between using module and Import-Module), but this hasn't been implemented yet, because it requires extra work to avoid the potential for undesired execution of arbitrary code when an assembly is loaded.

  • Implementing a solution has been green-lighted in GitHub issue #3641, and the necessary work is being tracked as part of GitHub issue #6652 - but it is unclear when this will happen, given that the issue hasn't received attention in several years.

Answer 2

Every PowerShell script is completely parsed before the first statement in the script is executed. An unresolvable type name token inside a class definition is considered a parse error. To solve your problem, you have to load your types before the class definition is parsed, so the class definition has to be in a separate file. For example:

Main.ps1:

Add-Type -AssemblyName System.Printing
Add-Type -AssemblyName ReachFramework

. $PSScriptRoot\Class.ps1

Class.ps1:

using namespace System.Management
using namespace System.Printing

Class PrinterObject
{
    [string]$Name
    [PrintServer]$Server
    [PrintQueue]$Queue
    [PrintTicket]$Ticket
    [ManagementObject]$Unit
    [bool]$IsDefault
}

The other possibility would be embed Class.ps1 as a string and use Invoke-Expression to execute it. This will delay parsing of class definition to time where types is available.

Add-Type -AssemblyName System.Printing
Add-Type -AssemblyName ReachFramework

Invoke-Expression @'
    using namespace System.Management
    using namespace System.Printing

    Class PrinterObject
    {
        [string]$Name
        [PrintServer]$Server
        [PrintQueue]$Queue
        [PrintTicket]$Ticket
        [ManagementObject]$Unit
        [bool]$IsDefault
    }
'@

Answer 3

A better solution (than just invoking the entire class in a string) would be to just create your objects and pass them to the class as parameters. For example, this runs fine:

Add-Type -AssemblyName PresentationCore,PresentationFramework

class ExampleClass {
    $object

    ExampleClass ($anotherClass) {
        $this.object = $anotherClass
    }

    [void] Show () {
        $this.object::Show('Hello')
    }
}

$y = [ExampleClass]::new([System.Windows.MessageBox])
$y.Show()

However, if you were to do something like this, you can expect Unable to find type [System.Windows.MessageBox].

Add-Type -AssemblyName PresentationCore,PresentationFramework

class ExampleClass2 {
    $object

    ExampleClass () {
        $this.object = [System.Windows.MessageBox]
    }

    [void] Show () {
        $this.object::Show('Hello')
    }
}