<!-- file.xml -->
<people>
    <person id="101">
        <name>Jon Lajoie</name>
        <age>22</age>
    </person>
    <person id="102">
        <name>Lord Gaben</name>
        <age>65</age>
    </person>
    <person id="103">
        <name>Gordon Freeman</name>
        <age>29</age>
    </person>
</people>

---

Loading an XML File

To load an XML file, you can use any of these:

# First Method
$xdoc = New-Object System.Xml.XmlDocument
$file = Resolve-Path(".\file.xml")
$xdoc.load($file)

# Second Method
[xml] $xdoc = Get-Content ".\file.xml"

# Third Method
$xdoc = [xml] (Get-Content ".\file.xml")

---

Accessing XML as Objects

PS C:\> $xml = [xml](Get-Content file.xml)
PS C:\> $xml

PS C:\> $xml.people

person
--------
{Jon Lajoie, Lord Gaben, Gordon Freeman}

PS C:\> $xml.people.person

id                                      name                                    age
--                                      ----                                    ---
101                                     Jon Lajoie                              22
102                                     Lord Gaben                              65
103                                     Gordon Freeman                          29

PS C:\> $xml.people.person[0].name
Jon Lajoie

PS C:\> $xml.people.person[1].age
65

PS C:\> $xml.people.person[2].id
103

---

Accessing XML with XPath

PS C:\> $xml = [xml](Get-Content file.xml)
PS C:\> $xml

PS C:\> $xml.SelectNodes("//people")

person
--------
{Jon Lajoie, Lord Gaben, Gordon Freeman}

PS C:\> $xml.SelectNodes("//people//person")

id                                      name                                    age
--                                      ----                                    ---
101                                     Jon Lajoie                              22
102                                     Lord Gaben                              65
103                                     Gordon Freeman                          29

PS C:\> $xml.SelectSingleNode("people//person[1]//name")
Jon Lajoie

PS C:\> $xml.SelectSingleNode("people//person[2]//age")
65

PS C:\> $xml.SelectSingleNode("people//person[3]//@id")
103

Accessing XML containing namespaces with XPath

PS C:\> [xml]$xml = @"
<ns:people xmlns:ns="http://schemas.xmlsoap.org/soap/envelope/">
    <ns:person id="101">
        <ns:name>Jon Lajoie</ns:name>
    </ns:person>
    <ns:person id="102">
        <ns:name>Lord Gaben</ns:name>
    </ns:person>
    <ns:person id="103">
        <ns:name>Gordon Freeman</ns:name>
    </ns:person>
</ns:people>
"@

PS C:\> $ns = new-object Xml.XmlNamespaceManager $xml.NameTable
PS C:\> $ns.AddNamespace("ns", $xml.DocumentElement.NamespaceURI)
PS C:\> $xml.SelectNodes("//ns:people/ns:person", $ns)

id                                      name
--                                      ----
101                                     Jon Lajoie
102                                     Lord Gaben
103                                     Gordon Freeman

# Set The Formatting
$xmlsettings = New-Object System.Xml.XmlWriterSettings
$xmlsettings.Indent = $true
$xmlsettings.IndentChars = "    "

# Set the File Name Create The Document
$XmlWriter = [System.XML.XmlWriter]::Create("C:\YourXML.xml", $xmlsettings)

# Write the XML Decleration and set the XSL
$xmlWriter.WriteStartDocument()
$xmlWriter.WriteProcessingInstruction("xml-stylesheet", "type='text/xsl' href='style.xsl'")

# Start the Root Element
$xmlWriter.WriteStartElement("Root")
  
    $xmlWriter.WriteStartElement("Object") # <-- Start <Object>

        $xmlWriter.WriteElementString("Property1","Value 1")
        $xmlWriter.WriteElementString("Property2","Value 2")

        $xmlWriter.WriteStartElement("SubObject") # <-- Start <SubObject> 
            $xmlWriter.WriteElementString("Property3","Value 3")
        $xmlWriter.WriteEndElement() # <-- End <SubObject>

    $xmlWriter.WriteEndElement() # <-- End <Object>

$xmlWriter.WriteEndElement() # <-- End <Root> 

# End, Finalize and close the XML Document
$xmlWriter.WriteEndDocument()
$xmlWriter.Flush()
$xmlWriter.Close()

Output XML File

<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet type='text/xsl' href='style.xsl'?>
<Root>
    <Object>
        <Property1>Value 1</Property1>
        <Property2>Value 2</Property2>
        <SubObject>
            <Property3>Value 3</Property3>
        </SubObject>
    </Object>
</Root>

Sample Data

XML Document

First, let's define a sample XML document named "books.xml" in our current directory:

<?xml version="1.0" encoding="UTF-8"?>
<books>
    <book>
        <title>Of Mice And Men</title>
        <author>John Steinbeck</author>
        <pageCount>187</pageCount>
        <publishers>
            <publisher>
                <isbn>978-88-58702-15-4</isbn>
                <name>Pascal Covici</name>
                <year>1937</year>
                <binding>Hardcover</binding>
                <first>true</first>
            </publisher>
            <publisher>
                <isbn>978-05-82461-46-8</isbn>
                <name>Longman</name>
                <year>2009</year>
                <binding>Hardcover</binding>
            </publisher>
        </publishers>
        <characters>
            <character name="Lennie Small" />
            <character name="Curley's Wife" />
            <character name="George Milton" />
            <character name="Curley" />
        </characters>
        <film>True</film>
    </book>
    <book>
        <title>The Hunt for Red October</title>
        <author>Tom Clancy</author>
        <pageCount>387</pageCount>
        <publishers>
            <publisher>
                <isbn>978-08-70212-85-7</isbn>
                <name>Naval Institute Press</name>
                <year>1984</year>
                <binding>Hardcover</binding>
                <first>true</first>
            </publisher>
            <publisher>
                <isbn>978-04-25083-83-3</isbn>
                <name>Berkley</name>
                <year>1986</year>
                <binding>Paperback</binding>
            </publisher>
            <publisher>
                <isbn>978-08-08587-35-4</isbn>
                <name>Penguin Putnam</name>
                <year>2010</year>
                <binding>Paperback</binding>
            </publisher>
        </publishers>
        <characters>
            <character name="Marko Alexadrovich Ramius" />
            <character name="Jack Ryan" />
            <character name="Admiral Greer" />
            <character name="Bart Mancuso" />
            <character name="Vasily Borodin" />
        </characters>
        <film>True</film>
    </book>
</books>

New Data

What we want to do is add a few new books to this document, let's say Patriot Games by Tom Clancy (yes, I'm a fan of Clancy's works ^__^) and a Sci-Fi favourite: The Hitchhiker's Guide to the Galaxy by Douglas Adams mainly because Zaphod Beeblebrox is just fun to read.

Somehow we've acquired the data for the new books and saved them as a list of PSCustomObjects:

$newBooks = @(
    [PSCustomObject] @{
        "Title" = "Patriot Games";
        "Author" = "Tom Clancy";
        "PageCount" = 540;
        "Publishers" = @(
            [PSCustomObject] @{
                "ISBN" = "978-0-39-913241-4";
                "Year" = "1987";
                "First" = $True;
                "Name" = "Putnam";
                "Binding" = "Hardcover";
            }
        );
        "Characters" = @(
            "Jack Ryan", "Prince of Wales", "Princess of Wales",
            "Robby Jackson", "Cathy Ryan", "Sean Patrick Miller"
        );
        "film" = $True;
    },
    [PSCustomObject] @{
        "Title" = "The Hitchhiker's Guide to the Galaxy";
        "Author" = "Douglas Adams";
        "PageCount" = 216;
        "Publishers" = @(
            [PSCustomObject] @{
                "ISBN" = "978-0-33-025864-7";
                "Year" = "1979";
                "First" = $True;
                "Name" = "Pan Books";
                "Binding" = "Hardcover";
            }
        );
        "Characters" = @(
            "Arthur Dent", "Marvin", "Zaphod Beeblebrox", "Ford Prefect",
            "Trillian", "Slartibartfast", "Dirk Gently"
        );
        "film" = $True;
    }
);

Templates

Now we need to define a few skeleton XML structures for our new data to go into. Basically, you want to create a skeleton/template for each list of data. In our example, that means we need a template for the book, characters, and publishers. We can also use this to define a few default values, such as the value for the film tag.

$t_book = [xml] @'
<book>
    <title />
    <author />
    <pageCount />
    <publishers />
    <characters />
    <film>False</film>
</book>
'@;

$t_publisher = [xml] @'
<publisher>
    <isbn/>
    <name/>
    <year/>
    <binding/>
    <first>false</first>
</publisher>
'@;

$t_character = [xml] @'
<character name="" />
'@;

We're done with set-up.

Adding the new data

Now that we're all set-up with our sample data, let's add the custom objects to the XML Document Object.

# Read the xml document
$xml = [xml] Get-Content .\books.xml;

# Let's show a list of titles to see what we've got currently:
$xml.books.book | Select Title, Author, @{N="ISBN";E={If ( $_.Publishers.Publisher.Count ) { $_.Publishers.publisher[0].ISBN} Else { $_.Publishers.publisher.isbn}}};;

# Outputs:
# title                                author         ISBN
# -----                                ------         ----
# Of Mice And Men                      John Steinbeck 978-88-58702-15-4
# The Hunt for Red October             Tom Clancy     978-08-70212-85-7

# Let's show our new books as well:
$newBooks | Select Title, Author, @{N="ISBN";E={$_.Publishers[0].ISBN}};

# Outputs:
# Title                                Author        ISBN
# -----                                ------        ----
# Patriot Games                        Tom Clancy    978-0-39-913241-4
# The Hitchhiker's Guide to the Galaxy Douglas Adams 978-0-33-025864-7

# Now to merge the two:

ForEach ( $book in $newBooks ) {
    $root = $xml.SelectSingleNode("/books");
    
    # Add the template for a book as a new node to the root element
    [void]$root.AppendChild($xml.ImportNode($t_book.book, $true));
    
    # Select the new child element
    $newElement = $root.SelectSingleNode("book[last()]");
    
    # Update the parameters of that new element to match our current new book data
    $newElement.title     = [String]$book.Title;
    $newElement.author    = [String]$book.Author;
    $newElement.pageCount = [String]$book.PageCount;
    $newElement.film      = [String]$book.Film;
    
    # Iterate through the properties that are Children of our new Element:
    ForEach ( $publisher in $book.Publishers ) {
        # Create the new child publisher element
        # Note the use of "SelectSingleNode" here, this allows the use of the "AppendChild" method as it returns
        # a XmlElement type object instead of the $Null data that is currently stored in that leaf of the
        # XML document tree
        [void]$newElement.SelectSingleNode("publishers").AppendChild($xml.ImportNode($t_publisher.publisher, $true));
        
        # Update the attribute and text values of our new XML Element to match our new data
        $newPublisherElement = $newElement.SelectSingleNode("publishers/publisher[last()]");
        $newPublisherElement.year = [String]$publisher.Year;
        $newPublisherElement.name = [String]$publisher.Name;
        $newPublisherElement.binding = [String]$publisher.Binding;
        $newPublisherElement.isbn = [String]$publisher.ISBN;
        If ( $publisher.first ) {
            $newPublisherElement.first = "True";
        }
    }
    
    ForEach ( $character in $book.Characters ) {
        # Select the characters xml element
        $charactersElement = $newElement.SelectSingleNode("characters");
        
        # Add a new character child element
        [void]$charactersElement.AppendChild($xml.ImportNode($t_character.character, $true));
        
        # Select the new characters/character element
        $characterElement = $charactersElement.SelectSingleNode("character[last()]");
        
        # Update the attribute and text values to match our new data
        $characterElement.name = [String]$character;
    }
}

# Check out the new XML:
$xml.books.book | Select Title, Author, @{N="ISBN";E={If ( $_.Publishers.Publisher.Count ) { $_.Publishers.publisher[0].ISBN} Else { $_.Publishers.publisher.isbn}}};

# Outputs:
# title                                author         ISBN
# -----                                ------         ----
# Of Mice And Men                      John Steinbeck 978-88-58702-15-4
# The Hunt for Red October             Tom Clancy     978-08-70212-85-7
# Patriot Games                        Tom Clancy     978-0-39-913241-4
# The Hitchhiker's Guide to the Galaxy Douglas Adams  978-0-33-025864-7

We can now write our XML to disk, or screen, or web, or wherever!

Profit

While this may not be the procedure for everyone I found it to help avoid a whole bunch of [void]$xml.SelectSingleNode("/complicated/xpath/goes[here]").AppendChild($xml.CreateElement("newElementName") followed by $xml.SelectSingleNode("/complicated/xpath/goes/here/newElementName") = $textValue

I think the method detailed in the example is cleaner and easier to parse for normal humans.

Improvements

It may be possible to change the template to include elements with children instead of breaking out each section as a separate template. You just have to take care to clone the previous element when you loop through the list.