Powershell exception when updating ContentType

Question

I have a strange issue when using PowerShell to modify a library ContentType. The following script works correctly:

$web = Get-SPWeb $url   
$lib = $web.Lists[$libName]
$lib.ContentTypes["KPI"].FieldLinks["KPI_x0020_Type"].Hidden = $true
$lib.ContentTypes["KPI"].Update()

I want to tidy the code up a little but when I assign the ContentType to a variable it throws System.Management.Automation.RuntimeException: Cannot index into a null array:

$web = Get-SPWeb $url   
$lib = $web.Lists[$libName]
$contentType = $lib.ContentTypes["KPI"]
if ($contentType) {
    $contentType.FieldLinks["KPI_x0020_Type"].Hidden = $true # <-- throws here
    $contentType.Update()
}

Answer 1

Your script is breaking in the $contentType line. When you call out the hash table index like that the variable becomes flat like a regular text string. Try this:

$web = Get-SPWeb $url   
$lib = $web.Lists[$libName]
$contentType = $lib.ContentTypes
if ($contentType["KPI"]) {
    $contentType["KPI"].FieldLinks["KPI_x0020_Type"].Hidden = $true
    $contentType.Update()
}

By not calling out the hash table key you preserve the hash table nature going into your if statement. That should get you past the null error. Then as ReTech mentioned, you have to make sure $contentType["KPI"].FieldLinks["KPI_x0020_Type"] exists and is editable/settable. Which should be the case since your first version was working correctly. Hope this helps.

Answer 2

Thanks for feedback, it ultimately led to the correct answer below, which was to tell PowerShell the type of the variable used to contain the SPContentType.

$web = Get-SPWeb $url   
$lib = $web.Lists[$libName]
[Microsoft.SharePoint.SPContentType]$contentType = $lib.ContentTypes["KPI"]
if ($contentType) {
    $contentType.FieldLinks["KPI_x0020_Type"].Hidden = $true # <-- no longer throws here
    $contentType.Update()
}