How do I use a parameterized script block with Where-Object in Powershell?

Question

Is there a better approach for passing information into script blocks in where-object filter scripts than using parent-scoped variables?

Background:

I have a script that looks for un-checked-in and/or modified source files vs. source control and has a parameter that allows it to do a more exhaustive search. I use where-object in a couple of places with a script block object contained in a script-scoped variable that I customize based on the input parameters to the script.

So, if you ask for a thorough search, the filter will compare the candidate file against all TFS files to see if the file isn't in source control, if you choose the less-thorough search, the filter will only compare against checked-out files to see if the file is modified but not checked out.

The customized script blocks refer to script-scoped variables containing the results of doing queries against source control.

So my problem is that I'd like to get rid of a global (script-level) variable and pass all the necessary information into the script blocks as parameters to the script blocks. If I was using invoke-command, I'd use the ArgumentList parameter to do this. Where-Object doesn't seem to have that. One downside of using parent-scoped variable references in the script blocks is that I can't change those variables, so I can't do lazy initialization (or at least I haven't figured out how yet, not being an expert on the scoping rules for Powershell.)

x0n · Accepted Answer · 2011-02-10 18:34:39Z

7

Just to expand a bit on what Keith mentioned, you could do it like this:

ps> $x=2
ps> 1..5 | where (& {param($v); { $_ -eq $v }.getnewclosure() } $x )
2

I tried closing over implict $args to save the param declaration but $args seems exempt from capture. More likely it is being captured but just getting stomped on.

$x could easily be replaced with another function call like (get-x).

Essentially I'm calling a scriptblock that returns a scriptblock which closes over the outer scriptblocks parameter. Same implementation as Keiths essentially, just a little more succint [and obtuse.] Lambdas for the win.

I only wish there was a more pithy way to get closure semantics. That said, I'm glad the method got put in rather than not.

-Oisin

answered Feb 10, 2011 at 18:34

x0n

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5 Comments

Keith Hill Over a year ago

Heheh, yeah that just occurred to me over lunch. I was also thinking that the FilterProcs method would take an -ArgumentList parameter which would get feed to the filter scriptblock passed in.

mjolinor Over a year ago

Is $args exempt from capture, or does creating a new scope for the script block automatically initialize a new $args for that scope, making $args from the parent scope no longer visible? I ran into similar behaviour with hash table expressions with select-object. If $args was used in the hash table expression script block, it had to be explicilty scoped as $script:args. The local $args was null.

x0n Over a year ago

"More likely it is being captured but just getting stomped on." so yeah.

JasonMArcher Over a year ago

@Oisin, you could use type extension to add a method with a shorter name that calls GetNewClosure().

x0n Over a year ago

@jasonmarcher true, but i'm not looking for methods. I'm thinking of native language support for it.

Keith Hill · Accepted Answer · 2011-02-10 20:57:53Z

You can create a temp scope and use the GetNewClosure() method on a scriptblock to have it create a closure around the variables in use in the scriptblock e.g.:

function FilterProcs([scriptblock]$scriptblock)
{
    Get-Process | Where $scriptblock
}

$name = 'Notepad'

& {
    $name = 'PowerShell'
    FilterProcs {$_.Name -eq $name}.GetNewClosure()
}

$name

In this case, the inner modification of $name is seen only locally and not at the script scope. There may be a better way but I think this would work for you.

BTW here's another approach that filters using the Foreach-Object cmdlet:

function FilterProcs([scriptblock]$scriptblock, [object[]]$argumentList)
{
    Get-Process | Foreach {if (&$scriptblock @argumentList){$_}}
}

FilterProcs {param($name, $id) ($_.Name -match $name -and $_.Id -eq $id)} `
            -ArgumentList 'PowerShell_ISE',$pid

P.S. Nice to see you're keeping up the PowerShellin'. :-)

mjolinor · Accepted Answer · 2011-02-12 14:10:26Z

3

You can't close an exsiting scriptblock around $args, but you can create a new scripblock with it:

$x=2
 1..5 | where (& {[scriptblock]::create('$_ -eq ' + $args) } $x )


 2

Oddly, creating a brand new script block seems to be faster than calling getnewclosure on an existing one. I ran both scenarios through 10000 reps, and creating a new script block with $args took 5.3 seconds and getnewclosure with a parameter declaration took 7.9.

edited Feb 12, 2011 at 14:10

answered Feb 12, 2011 at 13:49

mjolinor

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