Powershell custom class creation question

Question

As I'm ramping up on PowerShell and exploring custom classes, I keep hitting this weird Exception that persists 'til I make any change.

> PS C:\Users\Purpl_000> C:\PS\Node\NodeTest.ps1 Cannot convert the
> "Node" value of type "Node" to type "Node". At
> C:\PS\Node\NodeTest.ps1:5 char:1
> + [Node]$node1 = New-Object Node -Property @{Next=$null; Value=3};
> + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     + CategoryInfo          : MetadataError: (:) [], ArgumentTransformationMetadataException
>     + FullyQualifiedErrorId : RuntimeException   Exception setting "Next": "Cannot convert the "Node" value of type "Node" to type
> "Node"." At C:\PS\Node\NodeTest.ps1:9 char:1
> + $node2.Next = $node1;
> + ~~~~~~~~~~~~~~~~~~~~
>     + CategoryInfo          : NotSpecified: (:) [], SetValueInvocationException
>     + FullyQualifiedErrorId : ExceptionWhenSetting   New-Object : The value supplied is not valid, or the property is read-only. Change the
> value, and then try again. At C:\PS\Node\NodeTest.ps1:13 char:16
> + [Node]$node3 = New-Object Node -Property @{Next=$node2; Value=9};
> +                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     + CategoryInfo          : InvalidData: (:) [New-Object], Exception
>     + FullyQualifiedErrorId : SetValueException,Microsoft.PowerShell.Commands.NewObjectCommand  
> Cannot find an overload for "new" and the argument count: "2". At
> C:\PS\Node\NodeTest.ps1:22 char:1
> + [Node]$node4 = [Node]::new($node3, 12);
> + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     + CategoryInfo          : NotSpecified: (:) [], MethodException
>     + FullyQualifiedErrorId : MethodCountCouldNotFindBest

Here's Node.ps1

class Node
{
    [Node]$Next;
    [int]$Value;

    # .ctor
    Node([Node]$next, [int]$value)
    {
        $this.Next = $next;
        $this.Value = $value;
    }

    # default .ctor
    Node() {}


    static [int] Method1([Node]$n1)
    {
        return $n1.Value;
    }

    static [Node] Method2([Node]$n2)
    {
        $n2.Value = $n2.Value + 5;
        return $n2.Value;
    }
}

Here's NodeTest.ps1

. 'C:\PS\Node\Node.ps1';

[Node]$node1 = New-Object Node -Property @{Next=$null; Value=3};
[Node]$node2 = [Node]::new();
$node2.Next = $node1;
$node2.Value = 5;

[Node]$node3 = New-Object Node -Property @{Next=$node2; Value=9};

[Node]$node4 = [Node]::new($node3, 12);

I'm working in the PowerShell ISE. Node.ps1 is saved and ran successfully. NodeTest.ps1 is also saved but blows sometimes up until I do any seeming irrelevant change to the file, save, then Run Script again. At this point, it works fine again. Examples of changes that has resolved the issue: (1) Adding an extra line of blank space, (2) Removing an extra line of blank space, (3) Adding a comment, etc..

I understand

New-Object : The value supplied is not valid, or the property is read-only. Change the value, and then try again.

through my testing of what happens when you use the incorrect types but not quite sure why I'm intermittently seeing this. An easy example of intentionally repro'ing that error by passing an int where a Node is expected. :)

[Node]$node3 = New-Object Node -Property @{Next=5; Value=9};

The follow does confuse me. How can a Node not be a Node then magically a Node again after adding a blank line to NodeTest.ps1?

Exception setting "Next": "Cannot convert the "Node" value of type "Node" to type "Node"."

Any information, explanation, or education would be extremely appreciated!

Thanks all! Michael

Answer 1

I think the issue stems from having objects of type [node] in the current powershell session, then redefining the [node] type. It seems there are distinct definitions of [node] based on file content.

What may help make this a bit clearer is using [node].guid to see the distinct definitions of your class.

Here is a simple reproduction that demonstrates the issue:

classa1.ps1:

#class A def 1
class A {
    [A] $next
}

testclassa.ps1:

. ./classa1.ps1
$aobj1 = new-object A
Write-Host "Current A definition GUID: $([A].guid)"

" " | out-file  -Append ./classa1.ps1
. ./classa1.ps1
$aobj2 = new-object A
Write-Host "New A definition GUID: $([A].guid)"

# throws conversion exception because the type definition of 
# $aobj1.next now differs from the current [A] definition
$aobj1.next = $aobj2

Exception setting "next": "Cannot convert the "A" value of type "A" to type "A"."  
At /home/veefu/src/throwaway/testclassa.ps1:13 char:1
+ $aobj1.next = $aobj2
+ ~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : NotSpecified: (:) [], SetValueInvocationException
+ FullyQualifiedErrorId : ExceptionWhenSetting

It seems the type system treats the two versions of [A] as distinct types without conversion between them, despite the actual definition of [A] being identical.

If tinkering is your style of learning, familiarity with the .gettype() method and | format-list * expression will serve you well. These lend insight into the types and properties of objects PS creates for you. Opaque errors coming from code that should "just work" are often clarified by checking the type of the object being returned. I recall many times my scripts expect a single object and had some strange behavior or error, where .gettype() clarified that my script was actually being given a collection of objects.