New closure on scriptblock

Question

Consider this code:

PS> $timer = New-Object Timers.Timer
PS> $timer.Interval = 1000
PS> $i = 1;
PS> Register-ObjectEvent $timer Elapsed -Action { write-host 'i: ' $i }.GetNewClosure()
PS> $timer.Enabled = 1
i:  1
i:  1
i:  1
 ...
# wait a couple of seconds and change $i
PS> $i = 2
i:  2
i:  2
i:  2

I assumed that when I create new closure ({ write-host 'i: ' $i }.GetNewClosure()) value of $i will be tied to this closure. But not in this case. Afer I change the value, write-host takes the new value.

On the other side, this works:

PS> $i = 1;
PS> $action = { write-host 'i: ' $i }.GetNewClosure()
PS> &$action
i:  1
PS> $i = 2
PS> &$action
i:  1

Why it doesn't work with the Register-ObjectEvent?

It would help if you specified what result you expected and what result you got. — Richard, Mar 10 '10 at 09:58
It looks like a bug to me or at least we should have a way to configure this. I found it already submitted: https://connect.microsoft.com/PowerShell/feedback/details/541754/getnewclosure-doesnt-work-on-register-objectevent# — Roman Kuzmin, May 04 '10 at 11:33
Yes, I submitted it as a bug, because I haven't found any solution and the behaviour looks strange. — stej, May 04 '10 at 12:15

score 3 · Accepted Answer · answered Jun 29 '10 at 23:21

3

Jobs are executed in a dynamic module; modules have isolated sessionstate, and share access to globals. PowerShell closures only work within the same sessionstate / scope chain. Annoying, yes.

-Oisin

p.s. I say "jobs" because event handlers are effectively local jobs, no different than script being run with start-job (local machine only, implicit, not using -computer localhost)

answered Jun 29 '10 at 23:21

x0n

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Makes sense, accepted. How did you find that info about jobs and event handlers? Other source than Reflector? :) – stej Jun 30 '10 at 05:56
1

No, I don't have source code. Only microsoft staff have that. Event handlers are jobs, because get-job will return them. – x0n Jun 30 '10 at 15:31

score 0 · Answer 2 · answered Feb 07 '14 at 16:52

Use global variables in that case:

PS> $global:i = 1
PS> $timer = New-Object Timers.Timer
PS> $timer.Interval = 1000
PS> Register-ObjectEvent $timer Elapsed -Action { write-host 'i: ' $global:i }.GetNewClosure()
PS> $timer.Enabled = 1
i:  1
i:  1
i:  1

PS> Set-Variable -Name i -Value 2 -Scope Global

i:  2
i:  2
i:  2

Source:

http://stackoverflow.com/q/12535419/1287856

score 0 · Answer 3 · answered Mar 10 '10 at 10:02

0

I think you are making assumptions that don't hold. PSH is interpreted, so when a code block is created it just holds the source code. When it is later evaluated any variables it uses will be looked up in the normal PSH way: first in the current scope, and then in each outer scope until a variable with a matching name if found.

When the timer fires its event, it executes the code block and thus looks up $i. Which is found in the outer scope with a value of 2.

In the second case, if you just use the code block directly (remove call to GetNewClosure) then the second execution gives 2.

answered Mar 10 '10 at 10:02

Richard

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And that's what I want - in the second example I want to get 1, not 2. It works there, but not in the first example. – stej Mar 10 '10 at 10:25
GetNewClosure is supposed to change the behavior you mention in your first paragraph. – Segfault Mar 10 '10 at 10:28

New closure on scriptblock

3 Answers3

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