Get data from elements within multiple XML files for output to another, single XML file using Powershell

Question

I\'ll begin by confessing that I\'m a Powershell (and coding) noob. I\'ve stumbled my way through a few scripts, but I make no claims to anything even approaching competence

Answer 1

String interpolation (expansion) in "..." and @"<newline>...<newline>"@ strings happens instantly, with the values that the referenced variables contain at that time getting used.
As a result, the same string - whose value was determined before the loop - is output in each iteration of your foreach loop.

Your use case calls for a templating approach, where string interpolation is deferred, to be invoked on demand with the then-current variable values, using $ExecutionContext.InvokeCommand.ExpandString()^[1]:

# Define the *template* string as a *literal* - with *single* quotes.
$xmlData = @'
    <Invoice>
        <InvoiceId>$InvID</InvoiceId>
        <Timestamp>$Timestamp</Timestamp>
    </Invoice>
'@

 # ...
 # ForEach ($file in $files) { ...
   # Perform interpolation *on demand* with $ExecutionContext.InvokeCommand.ExpandString()
   Add-Content $Manifest -Value $ExecutionContext.InvokeCommand.ExpandString($xmlData)
 # }

Note:

• Variable references can also be embedded by explicitly delineating the variable names via enclosure in {...}, e.g., ${InvID}, which may situationally be required for disambiguation.

• In order to embed expressions / command output, use $(), the subexpression operator, as demonstrated below.

• In order to embed verbatim $ instances, escape them as `$.

---

A simple example:

# Define a template string, *single-quoted*, with *literal contents*:
#  - '$InvID' is simply literally part of the string, not a variable reference (yet).
#  - Ditto for $((Get-Date).TimeOfDay)
$strTempl = 'Invoice ID $InvID extracted at $((Get-Date).TimeOfDay).'

# Echo the template string as-is - unexpanded - ...
$strTempl

# ... and expand it on demand
$InvID = 1
$ExecutionContext.InvokeCommand.ExpandString($strTempl)

# ... and again, after assigning a different value to $InvID
$InvID = 2
$ExecutionContext.InvokeCommand.ExpandString($strTempl)

The above yields something like:

Invoice ID $InvID extracted at $((Get-Date).TimeOfDay).  # template literal
Invoice ID 1 extracted at 11:38:12.2719300.              # first on-demand expansion
Invoice ID 2 extracted at 11:38:12.2766010.              # second on-demand expnsion

---

^{[1] Surfacing the functionality of the $ExecutionContext.InvokeCommand.ExpandString() method in a more discoverable way via an Expand-String cmdlet is the subject of this GitHub feature request.}