Provide an implementation of a C# abstract class in PowerShell

Question

I'm trying to create a PowerShell-class implementation of an abstract class' function:

class ReadList : Intacct.SDK.Functions.AbstractFunction {

    [string]$ObjectName
    
    ReadList ([string]$ObjectName) {
        $this.ObjectName = $ObjectName
    }

    [void] WriteXml ( [Intacct.SDK.Xml.IaXmlWriter]$xml ) {

        $xml.WriteStartDocument()
        
        $xml.WriteStartElement("get_list")
        $xml.WriteAttributeString("object",$this.ObjectName)    
        $xml.WriteEndElement() # </get_list> 
    
        $xml.WriteEndDocument()
        $xml.Flush()
        $xml.Close()

    }

}

When I attempt to use it, I get a run-time exception:

Error during creation of type "ReadList". Error message: Method 'WriteXml' in type 'ReadList' from assembly 'PowerShell Class Assembly, Version=1.0.0.1, | Culture=neutral, PublicKeyToken=null' does not have an implementation.

I've tried adding the ref tag:

[void] WriteXml ( [Intacct.SDK.Xml.IaXmlWriter][ref]$xml ) {

But I get a different error:

Multiple type constraints are not allowed on a method parameter.

Am I doing something wrong, or is what I'm trying to do not supported?

Is WriteXml defined in Intacct.SDK.Functions.AbstractFunction as well? — G42
– G42, Commented Jan 25, 2021 at 18:29
No, it's defined in the IFunction interface: github.com/Intacct/intacct-sdk-net/blob/master/Intacct.SDK/… — craig
– craig, Commented Jan 25, 2021 at 20:49

G42 · Accepted Answer · 2021-01-26 20:01:13Z

As far as I can tell this is not supported. I have not been able to find a good workaround, or a resource that succinctly explains it.

Technically the second implementation is correct, but as per the error PowerShell does not support multiple type constraints on a method parameter^[1].
Normally this isn't the end of the world. The problem is when inheriting from an abstract class, the class must define all of the abstract methods ^[2], so a method with signature WriteXml (ref IaXmlWriter <ParameterName>) must be defined. Anything else gives the Method 'WriteXml' ... does not have an implementation.

Even if PowerShell supported multiple type constraints, I don't think this would work as intended because the c# is not the same as PowerShell's [ref] - [ref] was created to support COM Objects and the documentation explicitly states it can't be used to type-cast class members^[3].

The only workaround I'm aware of that might work here is writing the ReadList class definition in c#, and adding to PowerShell via Add-Type^[4]... not great.

All this is beyond the scope of my knowledge; maybe there is a good solution and someone with more know-how can correct me.

References
[1] SO post that touches on this and [ref] (same as below)

Looking at $Error.Exception.StackTrace, System.Management.Automation.ScriptBlock.Create is raising the exception. Couldn't go further with PowerShell 5.1 but PowerShell Core files include a check for class method parameters here which points to the MultipleTypeConstraintsOnMethodParam exception that gets raised.

Scripting.Classes.BasicParsing.Tests.ps1 checks multiple types will raise a MultipleTypeConstraintsOnMethodParam exception.

[2] Derived classes of the abstract class must implement all abstract methods.
Abstract and Sealed Classes and Class Members

[3] SO post that touches on this and multiple type constraints (same as above)

The PSReference type is not supported with class members
about_Classes

[4] Add a .NET type to a session

Edit
To elaborate on the Add-Type solution

Use ReferencedAssemblies to pass in Intacct.SDK.dll and dependent assemblies.
Load these assemblies using [Reflection.Assembly]

Use Load() when specifying version or LoadFromPartialName() with just the name, LoadFrom() when specifying the path.

$Assemblies = @(
    'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'
    'System.Xml.ReaderWriter, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
) |
foreach {
    [Reflection.Assembly]::Load($_)
}

$Assemblies += @(
    "$pwd/Microsoft.Extensions.Logging.Abstractions.dll"
    "$pwd/Intacct.SDK.dll"
) | foreach {
    [Reflection.Assembly]::LoadFrom($_)
}

Add-Type -TypeDefinition $Source -ReferencedAssemblies $Assemblies

craig · Accepted Answer · 2021-01-26 18:46:03Z

I came to the same conclusion that I would need to create a C# class definition and import it.

Push-Location ~/Desktop

$Source = @"
using Intacct.SDK.Functions;
using Intacct.SDK.Xml;

public class GetList : Intacct.SDK.Functions.AbstractFunction
{
    public string ObjectName;

    public GetList (string objectName) 
    {
        this.ObjectName = objectName;
    }

    public override void WriteXml( ref Intacct.SDK.Xml.IaXmlWriter xml )
    {
        xml.WriteStartDocument();        
        xml.WriteStartElement("get_list");
        xml.WriteEndDocument();
        xml.Flush();
    }
}
"@
    
# netstandard and ReaderWriter are located in `$PSHOME`; others on Desktop
$Assemblies = @(
    'netstandard, Version=2.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51'
    'System.Xml.ReaderWriter, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
    "$pwd/Microsoft.Extensions.Logging.Abstractions.dll"
    "$pwd/Intacct.SDK.dll"
)

# using ReferenceAssemblies based on a suggestion from the PowerShell Slack channel
$Type = Add-Type -TypeDefinition $Source -ReferencedAssemblies $Assemblies -PassThru

$GetList = [GetList]::new('something')
$GetList

which generates errors:

Add-Type: getlist.ps1:37:9
Line |
  37 |  $Type = Add-Type -TypeDefinition $Source -ReferencedAssemblies $Assem …
     |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | Unable to load one or more of the requested types. Could not load file or assembly 'Intacct.SDK, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. The parameter is incorrect.  (0x80070057 (E_INVALIDARG))

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