Method invocation failed because [System.Collections.ObjectModel.Collection`1[[]] does not contain a method which I added to pscustomobject

Question

We have a json file which contains an array with products with sell price and for some (the products we have to buy) a part number and buy price. For example:

{
"Prices":[
    {
        "Product":"AD User",
        "Price":3.52
    },
    {
        "Product":"Skype for Business Plus SAL",
        "Price":4.68,
        "Part number":"6SH-00002",
        "Buy price":3.90
    },
    {
        "Product":"Citrix CAL",
        "Price":6.40,
        "Part number":"ROYSPLACXABASE",
        "Buy price":5.33
    }
    ]
}

I use the following code to retrieve the contents of the json and for each object with a part number i put an object to an array of PSCustomObjects and add an int OrderCount and a function to increment this OrderCount:

$PurchaseOrder = @()
[object[]]$Prices = Get-Content 'C:\Pricelist.json' | Out-String | ConvertFrom-Json
foreach($_price  in ($Prices.prices | Where-Object {$_."Part Number"})){ 
    $PurchaseOrderItem = $_price.psobject.copy()
    $PurchaseOrderItem | Add-Member -Name "OrderCount" -MemberType NoteProperty -Value 0
    $PurchaseOrderItem | Add-Member -MemberType ScriptMethod -Name IncrementOrderCount {
        param([int]$amount)
        $this.OrderCount = $this.OrderCount + $amount
    }
    $PurchaseOrder += $PurchaseOrderItem
}

If figured out the following function to update the $PurchaseOrder:

function Update-PurchaseOrder{
    [Cmdletbinding()]
    param(
        [ref]$PurchaseOrder,
        [string]$ProductName,
        [int]$Count
    )

    $PurchaseOrder.Value.where({$_.Product -eq $ProductName}).IncrementOrderCount($Count)
}

Instead I get the following error when I run this function

Update-PurchaseOrder -PurchaseOrder ([ref]$PurchaseOrder) -ProductName "Citrix CAL" -Count 5

Method invocation failed because [System.Collections.ObjectModel.Collection`1[[System.Management.Automation.PSObject, System.Management.Automation, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]] does not contain a
method named 'IncrementOrderCount'.
At line:9 char:5
+     $PurchaseOrder.Value.where({$_.Name -eq $ProductName}).IncrementO ...
+     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : MethodNotFound

If i run this code with a $Prices object which is created manually, for example:

$Prices = @([pscustomobject]@{Product="Product1";Price=1},[pscustomobject]@{Product="Product2";Price=2})

It does run without any problem. I've tried to cast several objects to different types, but I'm out of options.

---

I've added an additional piece of code to check if the Product exists in my PurchaseOrder, now it runs without problem.

Write-Verbose "Updating PurchaseOrder for product $Title "
if($PurchaseOrder.Value.where({$_.Product -eq $Title})){
    try{
        $PurchaseOrder.Value.where({$_.Product -eq $Title}).IncrementOrderCount($Count)
    } catch {
        Write-Warning "Error while incrementing OrderCount for product $($Title): $($_.Exception.Message)"
    }
} else {
    Write-Verbose "Product $Title not found in PurchaseOrder."
}

Answer 1

Use ForEach-Object to iterate over the collection and then call the method on each object.

Also the property with the Product name is Product rather than Name.

function Update-PurchaseOrder{
    [Cmdletbinding()]
    param(
        [ref]$PurchaseOrder,
        [string]$ProductName,
        [int]$Count
    )

    $PurchaseOrder.Value.where({$_.Product -eq $ProductName}) | ForEach-Object {$_.IncrementOrderCount($Count)}
}

Also I believe $PurchaseOrder would already be passed by reference so you could skip the extra hassle and just pass $PurchaseOrder rather than ([ref]$PurchaseOrder). And then not need the extra step of unpacking the value in the function.