How does Select-Object stop the pipeline in PowerShell v3? - powershell

Question

In PowerShell v2, the following line:
1..3| foreach { Write-Host "Value : $_"; $_ }| select -First 1
Would display:
Value : 1
1
Value : 2
Value : 3
Since all elements were pushed down the pipeline. However, in v3 the above line displays only:
Value : 1
1
The pipeline is stopped before 2 and 3 are sent to Foreach-Object (Note: the -Wait switch for Select-Object allows all elements to reach the foreach block).
How does Select-Object stop the pipeline, and can I now stop the pipeline from a foreach or from my own function?
Edit: I know I can wrap a pipeline in a do...while loop and continue out of the pipeline. I have also found that in v3 I can do something like this (it doesn't work in v2):
function Start-Enumerate ($array) {
do{ $array } while($false)
}
Start-Enumerate (1..3)| foreach {if($_ -ge 2){break};$_}; 'V2 Will Not Get Here'
But Select-Object doesn't require either of these techniques so I was hoping that there was a way to stop the pipeline from a single point in the pipeline.

Answer 1

Check this post on how you can cancel a pipeline:
http://powershell.com/cs/blogs/tobias/archive/2010/01/01/cancelling-a-pipeline.aspx
In PowerShell 3.0 it's an engine improvement. From the CTP1 samples folder ('\Engines Demos\Misc\ConnectBugFixes.ps1'):
# Connect Bug 332685
# Select-Object optimization
# Submitted by Shay Levi
# Connect Suggestion 286219
# PSV2: Lazy pipeline - ability for cmdlets to say "NO MORE"
# Submitted by Karl Prosser
# Stop the pipeline once the objects have been selected
# Useful for commands that return a lot of objects, like dealing with the event log
# In PS 2.0, this took a long time even though we only wanted the first 10 events
Start-Process powershell.exe -Args '-Version 2 -NoExit -Command Get-WinEvent | Select-Object -First 10'
# In PS 3.0, the pipeline stops after retrieving the first 10 objects
Get-WinEvent | Select-Object -First 10

Answer 2

After trying several methods, including throwing StopUpstreamCommandsException, ActionPreferenceStopException, and PipelineClosedException, calling $PSCmdlet.ThrowTerminatingError and $ExecutionContext.Host.Runspace.GetCurrentlyRunningPipeline().stopper.set_IsStopping($true) I finally found that just utilizing select-object was the only thing that didn't abort the whole script (versus just the pipeline). [Note that some of the items mentioned above require access to private members, which I accessed via reflection.]
# This looks like it should put a zero in the pipeline but on PS 3.0 it doesn't
function stop-pipeline {
$sp = {select-object -f 1}.GetSteppablePipeline($MyInvocation.CommandOrigin)
$sp.Begin($true)
$x = $sp.Process(0) # this call doesn't return
$sp.End()
}
New method follows based on comment from OP. Unfortunately this method is a lot more complicated and uses private members. Also I don't know how robust this - I just got the OP's example to work and stopped there. So FWIW:
# wh is alias for write-host
# sel is alias for select-object
# The following two use reflection to access private members:
# invoke-method invokes private methods
# select-properties is similar to select-object, but it gets private properties
# Get the system.management.automation assembly
$smaa=[appdomain]::currentdomain.getassemblies()|
? location -like "*system.management.automation*"
# Get the StopUpstreamCommandsException class
$upcet=$smaa.gettypes()| ? name -like "*upstream*"
filter x {
[CmdletBinding()]
param(
[parameter(ValueFromPipeline=$true)]
[object] $inputObject
)
process {
if ($inputObject -ge 5) {
# Create a StopUpstreamCommandsException
$upce = [activator]::CreateInstance($upcet,#($pscmdlet))
$PipelineProcessor=$pscmdlet.CommandRuntime|select-properties PipelineProcessor
$commands = $PipelineProcessor|select-properties commands
$commandProcessor= $commands[0]
$null = $upce.RequestingCommandProcessor|select-properties *
$upce.RequestingCommandProcessor.commandinfo =
$commandProcessor|select-properties commandinfo
$upce.RequestingCommandProcessor.Commandruntime =
$commandProcessor|select-properties commandruntime
$null = $PipelineProcessor|
invoke-method recordfailure #($upce, $commandProcessor.command)
1..($commands.count-1) | % {
$commands[$_] | invoke-method DoComplete
}
wh throwing
throw $upce
}
wh "< $inputObject >"
$inputObject
} # end process
end {
wh in x end
}
} # end filter x
filter y {
[CmdletBinding()]
param(
[parameter(ValueFromPipeline=$true)]
[object] $inputObject
)
process {
$inputObject
}
end {
wh in y end
}
}
1..5| x | y | measure -Sum
PowerShell code to retrieve PipelineProcessor value through reflection:
$t_cmdRun = $pscmdlet.CommandRuntime.gettype()
# Get pipelineprocessor value ($pipor)
$bindFlags = [Reflection.BindingFlags]"NonPublic,Instance"
$piporProp = $t_cmdRun.getproperty("PipelineProcessor", $bindFlags )
$pipor=$piporProp.GetValue($PSCmdlet.CommandRuntime,$null)
Powershell code to invoke method through reflection:
$proc = (gps)[12] # semi-random process
$methinfo = $proc.gettype().getmethod("GetComIUnknown", $bindFlags)
# Return ComIUnknown as an IntPtr
$comIUnknown = $methinfo.Invoke($proc, #($true))

Answer 3

I know that throwing a PipelineStoppedException stops the pipeline. The following example will simulate what you see with Select -first 1 in v3.0, in v2.0:
filter Select-Improved($first) {
begin{
$count = 0
}
process{
$_
$count++
if($count -ge $first){throw (new-object System.Management.Automation.PipelineStoppedException)}
}
}
trap{continue}
1..3| foreach { Write-Host "Value : $_"; $_ }| Select-Improved -first 1
write-host "after"