Convert a SharePoint Online list into JSON using arrays - arrays

Question

I'm trying to convert a set of SharePoint list items (and associated data) into a JSON object. To do this I'm trying to create a multi-dimensional array and then iterate over my SharePoint objects to populate it.
This is the relevant code so far:
#Lookup Source Address
$rootWeb = $Context.Web
$List = $rootWeb.lists.getByTitle($ListName)
$fields = $List.Fields;
$ListItems = $List.GetItems([Microsoft.SharePoint.Client.CamlQuery]::CreateAllItemsQuery())
#Load the List
$Context.Load($rootWeb)
$Context.Load($List)
$Context.Load($ListItems)
$context.Load($fields)
$Context.ExecuteQuery()
$listArray = #()
$listArray["DisplayTitle"] = #()
$listArray["Description"] = #()
$listArray["Setting"] = #()
$listArray["HealthAreas"] = #()
$listArray["ResourceType"] = #()
$listArray["ExternalURL"] = #()
$listArray["Active"] = #()
Write-Host "List items are"
foreach ($item in $ListItems)
{
$listArray["DisplayTitle"].Add($item["Title"])
$listArray["Description"].Add($item["File Description"])
$listArray["Setting"].Add($item["Setting"])
$listArray["HealthAreas"].Add($item["Health_x0020_Area"])
$listArray["ResourceType"].Add($item["Resource_x0020_Type"])
$listArray["ExternalURL"].Add($item["External_x0020_file_x0020_path"])
$listArray["Active"].Add($item["Currently_x0020_active_x003f_"])
}
Write-Host "############################"
Write-Host $listArray | ConvertTo-Json
I know there's a gap in my thinking here (maybe I need a hashtable) but just can't see it. The error I'm receiving is:
You cannot call a method on a null-valued expression.
However I can't see where my null variable may be originating from as I've confirmed each item in the loop does contain data (by writing to console).

Answer 1

The error that you receive is not related to SharePoint but to PowerShell. You created the PowerShell array and tried to access its elements like it was associative array/hashtable.
Please try this code (I've tested it with my own list with different column names and it works fine):
#Lookup Source Address
$rootWeb = $Context.Web
$List = $rootWeb.lists.getByTitle($ListName)
$fields = $List.Fields;
$ListItems = $List.GetItems([Microsoft.SharePoint.Client.CamlQuery]::CreateAllItemsQuery())
#Load the List
$Context.Load($rootWeb)
$Context.Load($List)
$Context.Load($ListItems)
$context.Load($fields)
$Context.ExecuteQuery()
$listArray = New-Object System.Collections.Generic.List[System.Object]
Write-Host "List items are"
foreach ($item in $ListItems)
{
$listArray.Add([hashtable]#{
DisplayTitle=$item["Title"];
Description= $item["File Description"];
Setting= $item["Setting"];
HealthAreas= $item["Health_x0020_Area"];
ResourceType= $item["Resource_x0020_Type"];
ExternalURL= $item["External_x0020_file_x0020_path"];
Active= $item["Currently_x0020_active_x003f_"];
}
)
}
Write-Host "############################"
$json = $listArray | ConvertTo-Json
Write-Host $json