How to pass a custom function inside a ForEach-Object -Parallel

by

The solution isn’t quite as straightforward as one would hope:

# Sample custom function.
function Get-Custom {
  Param ($A)
  "[$A]"
}

# Get the function's definition *as a string*
$funcDef = ${function:Get-Custom}.ToString()

"Apple", "Banana", "Grape"  | ForEach-Object -Parallel {
  # Define the function inside this thread...
  ${function:Get-Custom} = $using:funcDef
  # ... and call it.
  Get-Custom $_
}

Note: This answer contains an analogous solution for using a script block from the caller’s scope in a ForEach-Object -Parallel script block.

  • Note: If your function were defined in a module that is placed in one of the locations known to the module-autoloading feature, your function calls would work as-is with ForEach-Object -Parallel, without extra effort – but each thread would incur the cost of (implicitly) importing the module.

  • The above approach is necessary, because – aside from the current location (working directory) and environment variables (which apply process-wide) – the threads that ForEach-Object -Parallel creates do not see the caller’s state, notably neither with respect to variables nor functions (and also not custom PS drives and imported modules).

  • As of PowerShell 7.2.x, an enhancement is being discussed in GitHub issue #12240 to support copying the caller’s state to the parallel threads on demand, which would make the caller’s functions automatically available.

Note that redefining the function in each thread via a string is crucial, as an attempt to make do without the aux. $funcDef variable and trying to redefine the function with ${function:Get-Custom} = ${using:function:Get-Custom} fails, because ${function:Get-Custom} is a script block, and the use of script blocks with the $using: scope specifier is explicitly disallowed in order to avoid cross-thread (cross-runspace) issues.

  • However, ${function:Get-Custom} = ${using:function:Get-Custom} would work with Start-Job; see this answer for an example.

  • It would not work with Start-ThreadJob, which currently syntactically allows you to do & ${using:function:Get-Custom} $_, because ${using:function:Get-Custom} is preserved as a script block (unlike with Start-Job, where it is deserialized as a string, which is itself surprising behavior – see GitHub issue #11698), even though it shouldn’t. That is, direct cross-thread use of [scriptblock] instances causes obscure failures, which is why ForEach-Object -Parallel prevents it in the first place.

  • A similar loophole that leads to cross-thread issues even with ForEach-Object -Parallel is using a command-info object obtained in the caller’s scope with Get-Command as the function body in each thread via the $using: scope: this too should be prevented, but isn’t as of PowerShell 7.2.7 – see this post and GitHub issue #16461.

${function:Get-Custom} is an instance of namespace variable notation, which allows you to both get a function (its body as a [scriptblock] instance) and to set (define) it, by assigning either a [scriptblock] or a string containing the function body.

More Related Contents:

  1. What Library for Powershell 6 contains the get-wmiobject command?
  2. How to automate either PowerShell or PowerShell Core for same machine
  3. Need to extract value of attribute from xml from Command promt [closed]
  4. How do I write the value of a single property of a object?
  5. How can I use powershell to run through an installer?
  6. How do I concatenate strings and variables in PowerShell?
  7. How to load assemblies in PowerShell?
  8. Convert a secure string to plain text
  9. How to exit from ForEach-Object in PowerShell
  10. PowerShell – Decode System.Security.SecureString to readable password
  11. Convert a string to datetime in PowerShell
  12. What does $_ mean in PowerShell?
  13. How to pass credentials to the Send-MailMessage command for sending emails
  14. How to get Powershell Invoke-Restmethod to return body of http 500 code response
  15. How to normalize a path in PowerShell?
  16. How to create new clone instance of PSObject object
  17. Arrays and -contains – test for substrings in the elements of an array
  18. How to create a shortcut using PowerShell
  19. Try/catch does not seem to have an effect
  20. How to run an installation in /silent mode with adjusted settings
  21. Powershell Join-Path showing 2 dirs in result instead of 1 – accidental script/function output
  22. Powershell script to see currently logged in users (domain and machine) + status (active, idle, away)
  23. powershell – list local users and their groups
  24. Gracefully stopping in Powershell
  25. Upload BIG files via HTTP
  26. weird delay of the output of an object when followed by start-sleep (or until script end)
  27. Boolean literals in PowerShell
  28. Difference between PowerShell Console and PowerShell ISE
  29. How to parse HTML table with Powershell Core 7?
  30. Unix newlines to Windows newlines (on Windows)

Leave a Comment