Adding a site column and add it to a content type from PowerShell issue

Question

Within a PS script, I'm trying to add a site column and add this column to a custom content type.

Here is my script :

add-pssnapin Microsoft.SharePoint.PowerShell -EA 0

function AddFieldToContentType([Microsoft.SharePoint.SPWeb] $web, [string]$contentTypeName, [string]$xmlSchema)
{

    $fieldName = $web.Fields.AddFieldAsXml($xmlSchema)
    $web.Update()

    $field = $web.Fields[$fieldName]
    $ct = $web.AvailableContentTypes[$contentTypeName]
    $link = new-object Microsoft.SharePoint.SPFieldLink $field
    $ct.FieldLinks.Add($link)
    $ct.Update($true)


}

[Microsoft.SharePoint.SPWeb]$web = get-spweb -Identity http://somewhere
[xml]$xml = 'my field schema truncated for readability'

AddFieldToContentType $web "my content type name" $xml

This fails at the line $ct.Update($true) with this error :

Exception calling "Update" with "1" argument(s): "The collection cannot be modified."

What's wrong with this code ?

Thanks

Answer 1

AvailableContentTypes is readonly, so you need to use ContentTypes instead.

$field = $web.Fields[$fieldName]
$ct = $web.ContentTypes[$contentTypeName]
$link = new-object Microsoft.SharePoint.SPFieldLink $field
$ct.FieldLinks.Add($link)
$ct.Update($true)