PowerShell changes return object's type

Question

I am using PowerShell v3 and the Windows PowerShell ISE. I have the following function that works fine:

function Get-XmlNode([xml]$XmlDocument, [string]$NodePath, [string]$NamespaceURI = "", [string]$NodeSeparatorCharacter = '.')
{
    # If a Namespace URI was not given, use the Xml document's default namespace.
    if ([string]::IsNullOrEmpty($NamespaceURI)) { $NamespaceURI = $XmlDocument.DocumentElement.NamespaceURI }   

    # In order for SelectSingleNode() to actually work, we need to use the fully qualified node path along with an Xml Namespace Manager, so set them up.
    [System.Xml.XmlNamespaceManager]$xmlNsManager = New-Object System.Xml.XmlNamespaceManager($XmlDocument.NameTable)
    $xmlNsManager.AddNamespace("ns", $NamespaceURI)

    [string]$fullyQualifiedNodePath = Get-FullyQualifiedXmlNodePath -NodePath $NodePath -NodeSeparatorCharacter $NodeSeparatorCharacter

    # Try and get the node, then return it. Returns $null if the node was not found.
    $node = $XmlDocument.SelectSingleNode($fullyQualifiedNodePath, $xmlNsManager)
    return $node
}

Now, I will be creating a few similar functions, so I want to break the first 3 lines out into a new function so that I don't have to copy-paste them everywhere, so I have done this:

function Get-XmlNamespaceManager([xml]$XmlDocument, [string]$NamespaceURI = "")
{
    # If a Namespace URI was not given, use the Xml document's default namespace.
    if ([string]::IsNullOrEmpty($NamespaceURI)) { $NamespaceURI = $XmlDocument.DocumentElement.NamespaceURI }   

    # In order for SelectSingleNode() to actually work, we need to use the fully qualified node path along with an Xml Namespace Manager, so set them up.
    [System.Xml.XmlNamespaceManager]$xmlNsManager = New-Object System.Xml.XmlNamespaceManager($XmlDocument.NameTable)
    $xmlNsManager.AddNamespace("ns", $NamespaceURI)
    return $xmlNsManager
}

function Get-XmlNode([xml]$XmlDocument, [string]$NodePath, [string]$NamespaceURI = "", [string]$NodeSeparatorCharacter = '.')
{
    [System.Xml.XmlNamespaceManager]$xmlNsManager = Get-XmlNamespaceManager -XmlDocument $XmlDocument -NamespaceURI $NamespaceURI
    [string]$fullyQualifiedNodePath = Get-FullyQualifiedXmlNodePath -NodePath $NodePath -NodeSeparatorCharacter $NodeSeparatorCharacter

    # Try and get the node, then return it. Returns $null if the node was not found.
    $node = $XmlDocument.SelectSingleNode($fullyQualifiedNodePath, $xmlNsManager)
    return $node
}

The problem is that when "return $xmlNsManager" executes the following error is thrown:

Cannot convert the "System.Object[]" value of type "System.Object[]" to type "System.Xml.XmlNamespaceManager".

So even though I have explicitly cast my $xmlNsManager variables to be of type System.Xml.XmlNamespaceManager, when it gets returned from the Get-XmlNamespaceManager function PowerShell is converting it to an Object array.

If I don't explicitly cast the value returned from the Get-XmlNamespaceManager function to System.Xml.XmlNamespaceManager, then the following error is thrown from the .SelectSingleNode() function because the wrong data type is being passed into the function's 2nd parameter.

Cannot find an overload for "SelectSingleNode" and the argument count: "2".

So for some reason PowerShell is not maintaining the data type of the return variable. I would really like to get this working from a function so that I don't have to copy-paste those 3 lines all over the place. Any suggestions are appreciated. Thanks.

Answer 1

What's happening is PowerShell is converting your namespace manager object to a string array.

I think it has to do with PowerShell's nature of "unrolling" collections when sending objects down the pipeline. I think PowerShell will do this for any type implementing IEnumerable (has a GetEnumerator method).

As a work around you can use the comma trick to prevent this behavior and send the object as a whole collection.

function Get-XmlNamespaceManager([xml]$XmlDocument, [string]$NamespaceURI = "")
{
    ...
    $xmlNsManager.AddNamespace("ns", $NamespaceURI)
    return ,$xmlNsManager
}

Answer 2

More specifically, what is happening here is that your coding habit of strongly typing $fullyQualifiedModePath is trying to turn the result of the Get (which is a list of objects) into a string.

[string]$foo

will constrain the variable $foo to only be a string, no matter what came back. In this case, your type constraint is what is subtly screwing up the return and making it Object[]

Also, looking at your code, I would personally recommend you use Select-Xml (built into V2 and later), rather than do a lot of hand-coded XML unrolling. You can do namespace queries in Select-Xml with -Namespace @{x="..."}.