Inheriting SharePoint masterpages with Powershell

If you want your subsites to inherit their masterpages from their parents the first thing you need to know is the difference between normal sites and publishing sites: Only publishing sites actually support inheriting masterpages. If you're site is not a publishing site you have to iterate over all the Webs and set the property manually.

If you have a publishing site your in luck SharePoint makes it easy for you but it's not completely obvious what to do. My first attempt at setting these properties to Inherit was by setting the site property. And while this does make the UI say the right thing, it doesn't actually change the property CustomMasterUrl in any way and you'll just get the old masterpage.

$web.AllProperties["__InheritsCustomMasterUrl"] = "True" // This is wrong

What you have to do is update the CustomMasterUrl as well, but then you're getting into unsupported functionality and it's also unnecessary work. Conveniently SharePoint already provides you with a PublishingWeb class that supports inheritance for these properties.

Here is an example:

$isPubWeb = [Microsoft.SharePoint.Publishing.PublishingWeb]::IsPublishingWeb($web)
if ($isPubWeb) {
	$pubWeb = [Microsoft.SharePoint.Publishing.PublishingWeb]::GetPublishingWeb($web)
}

if ($xmlNode.CustomMaster -ne $null) {
	if ($xmlNode.CustomMaster -eq "Inherit") {
		if ($isPubWeb) {
			# https://msdn.microsoft.com/en-us/library/ms562472.aspx
			$pubWeb.CustomMasterUrl.SetInherit($true, $false)
		} else {
			Write-Host -Fore Red "You cannot inherit without a publishing web!"
		}
	} else {
		$url = $($site.ServerRelativeUrl + "/_catalogs/masterpage/" + $xmlNode.CustomMaster)
		if ($isPubWeb) {
			$pubWeb.CustomMasterUrl.SetValue($url, $false)
		} else {
			$web.CustomMasterUrl = $url
		}
	}
}