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

I just figured out another way using get-command, which works with the call operator. $a ends up being a FunctionInfo object. EDIT: I'm told this isn't thread safe, but I don't understand why.

function hi { 'hi' }
$a = get-command hi
1..3 | foreach -parallel { & $using:a }

hi
hi
hi

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).

  • Update: js2010's helpful answer shows a more straightforward solution that passes a System.Management.Automation.FunctionInfo instance, obtained via Get-Command, which can be invoked directly with &. The only caveat is that the original function should be side-effect-free, i.e. should operate solely based on parameter or pipeline inputs, without relying on the caller's state, notably its variables, as that could lead to thread-safety issues. The stringification technique above implicitly prevents any problematic references to the caller's state, because the function body is rebuilt in each thread's context.

• As of PowerShell 7.1, an enhancement is being discussed in GitHub issue #12240 to support copying the caller's state to the threads on demand, which would make the caller's functions available.

Note that making do without the aux. $funcDef variable and trying to redefine the function with ${function:Get-Custom} = ${using:function:Get-Custom} is tempting, but ${function:Get-Custom} is a script block, and the use of script blocks with the $using: scope specifier is explicitly disallowed.

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

• It would also work with Start-ThreadJob, where you could even 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). However, it is unclear whether this behavior is supported by design, because it is subject to the same potential cross-thread issues noted above.

${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.