How to load or read an XML file using ConvertTo-Xml and Select-Xml?

How can I accomplish something like this:

PS /home/nicholas/powershell> 
PS /home/nicholas/powershell> $date=(Get-Date | ConvertTo-Xml)                                         
PS /home/nicholas/powershell> 
PS /home/nicholas/powershell> $date

xml                            Objects
---                            -------
version="1.0" encoding="utf-8" Objects

PS /home/nicholas/powershell> 
PS /home/nicholas/powershell> $date.OuterXml
<?xml version="1.0" encoding="utf-8"?><Objects><Object Type="System.DateTime">12/12/2020 2:43:46 AM</Object></Objects>
PS /home/nicholas/powershell>

but, instead, reading in a file?

how do I load/import/read/convert an xml file using ConvertTo-Xml for parsing with Select-Xml using Xpath?

PS /home/nicholas/powershell> 
PS /home/nicholas/powershell> $xml=ConvertTo-Xml ./bookstore.xml
PS /home/nicholas/powershell> 
PS /home/nicholas/powershell> $xml                              

xml                            Objects
---                            -------
version="1.0" encoding="utf-8" Objects

PS /home/nicholas/powershell> 
PS /home/nicholas/powershell> $xml.InnerXml                     
<?xml version="1.0" encoding="utf-8"?><Objects><Object Type="System.String">./bookstore.xml</Object></Objects>
PS /home/nicholas/powershell> 
PS /home/nicholas/powershell> $xml.OuterXml                     
<?xml version="1.0" encoding="utf-8"?><Objects><Object Type="System.String">./bookstore.xml</Object></Objects>
PS /home/nicholas/powershell> 
PS /home/nicholas/powershell> cat ./bookstore.xml

<?xml version="1.0"?>
<!-- A fragment of a book store inventory database -->
<bookstore xmlns:bk="urn:samples">
  <book genre="novel" publicationdate="1997" bk:ISBN="1-861001-57-8">
    <title>Pride And Prejudice</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>24.95</price>
  </book>
  <book genre="novel" publicationdate="1992" bk:ISBN="1-861002-30-1">
    <title>The Handmaid's Tale</title>
    <author>
      <first-name>Margaret</first-name>
      <last-name>Atwood</last-name>
    </author>
    <price>29.95</price>
  </book>
  <book genre="novel" publicationdate="1991" bk:ISBN="1-861001-57-6">
    <title>Emma</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>19.95</price>
  </book>
  <book genre="novel" publicationdate="1982" bk:ISBN="1-861001-45-3">
    <title>Sense and Sensibility</title>
    <author>
      <first-name>Jane</first-name>
      <last-name>Austen</last-name>
    </author>
    <price>19.95</price>
  </book>
</bookstore>

PS /home/nicholas/powershell>

Creating the xml file within the REPL console itself works as expected:

How to parse XML in Powershell with Select-Xml and Xpath?

Solution

Properly reading an XML document in Powershell works like this:

$doc = New-Object xml
$doc.Load( (Convert-Path bookstore.xml) )

XML can come in numerous file encodings, and using the XmlDocument.Load method makes sure the file is read properly without prior knowledge of the encoding.

Not reading a file with the correct encoding will result in mangled data or errors except in very basic or very lucky cases.

The often-seen method of using Get-Content and casting the resulting string to [xml] is the wrong way of dealing with XML for this very reason. So don't do that.

You can get a correct result with Get-Content, but that requires

Prior knowledge of the file encoding (e.g. Get-Content bookstore.xml -Encoding UTF8)
Hard-coding the file encoding into your script (meaning it will break if the XML encoding ever changes unexpectedly)
Limiting yourself to the very few file encodings that Get-Content supports (XML supports more)

It means you put yourself in a position where you have to manually think about and solve a problem that XML has been specifically designed to automatically handle for you.

Doing things correctly with Get-Content is a lot of unnecessary extra work and limitations. And doing things incorrectly is pointless when doing it right is so easy.

Examples, after loading $doc like shown above.

$doc.bookstore.book

prints a list of <book> elements and their properties

genre           : novel
publicationdate : 1997
ISBN            : 1-861001-57-8
title           : Pride And Prejudice
author          : author
price           : 24.95

genre           : novel
publicationdate : 1992
ISBN            : 1-861002-30-1
title           : The Handmaid's Tale
author          : author
price           : 29.95

genre           : novel
publicationdate : 1991
ISBN            : 1-861001-57-6
title           : Emma
author          : author
price           : 19.95

genre           : novel
publicationdate : 1982
ISBN            : 1-861001-45-3
title           : Sense and Sensibility
author          : author
price           : 19.95

$doc.bookstore.book | Format-Table

prints the same thing as a table

genre publicationdate ISBN          title                 author price
----- --------------- ----          -----                 ------ -----
novel 1997            1-861001-57-8 Pride And Prejudice   author 24.95
novel 1992            1-861002-30-1 The Handmaid's Tale   author 29.95
novel 1991            1-861001-57-6 Emma                  author 19.95
novel 1982            1-861001-45-3 Sense and Sensibility author 19.95

$doc.bookstore.book | Where-Object publicationdate -lt 1992 | Format-Table

filters the data

genre publicationdate ISBN          title                 author price
----- --------------- ----          -----                 ------ -----
novel 1991            1-861001-57-6 Emma                  author 19.95
novel 1982            1-861001-45-3 Sense and Sensibility author 19.95

$doc.bookstore.book | Where-Object publicationdate -lt 1992 | Sort publicationdate | select title

sorts and prints only the <title> field

title                
-----                
Sense and Sensibility
Emma

There are many more ways of slicing and dicing the data, it all depends on what you want to do.