"Cannot convert the "System.Object[]" value" when reading and parsing JSON file in Powershell

Question

I am trying to read and parse a JSON file in PowerShell and I cannot seem convert a specific array. It works fine with other arrays and I am close to banging my head on the desk.

Here is a very simplified version of what I have. Note that I use exactly the same logic in other places and it works just fine:

Powershell script:

using namespace System.Collections.Generic

class Group{
    [string]$name
}

$json = Get-Content -Raw -Path .\config.json | ConvertFrom-Json
$groups = [List[Group]]$json.groups
Write-Output $json

JSON file:

{
    "groups":[
        {
            "name": "test"
        }
    ]
}

Can anybody please tell me what is going on here?

Mathias R. Jessen · Accepted Answer · 2022-09-08 10:53:27Z

2

It's the direct conversion from [object[]] to [List[Group]] that fails in this statement:

[List[Group]]$json.groups

You can solve this with an intermediate conversion from [object[]] to [Group[]] - PowerShell will gladly handle this conversion natively - after which the array satisfies the parameter constraints of one of the List constructors, since [Group[]] implements [IEnumerable[Group]]:

[List[Group]]::new([Group[]]$json.groups)

answered Sep 8, 2022 at 10:53

Mathias R. Jessen

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5 Comments

LeonidasFett Over a year ago

Ok this worked. But why is it that other arrays work just fine using the exact same logic? I have a users array which contains name and password and Powershell manages to parse this just fine. I don't do anything differently, only the class name and the properties are different.

Mathias R. Jessen Over a year ago

@LeonidasFett Update your question with the example you're describing if you want me to explain why it doesn't fail in the same way :)

LeonidasFett Over a year ago

That's weird, I swear it used to work before with the users array. Now, when I execute my script with the users array, I get the same error. This is giving me a headache. XD

zett42 Over a year ago

Even works without new(), using only cast syntax: [List[Group]] [Group[]] $json.groups

Mathias R. Jessen Over a year ago

It does indeed, although PowerShell ends up doing the exact same thing :)

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