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

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"