PowerShell: Pipeline objects into pipeline objects

Question

I'm using Powershell 5 if that's relevant

When adding a property to the current item in a pipeline:

I cannot refer to $_ until I have fed it into another pipeline. Why?

# make some objects to pass around
$test = (0..3) | %{[pscustomobject]@{key1="v$_.1";key2="v$_.2"} }
#    GM = TypeName: System.Management.Automation.PSCustomObject
# trying to use the current pipeline object
$test | Add-Member @{key3=$_.key1}
$test | ft *

key1 key2 key3
---- ---- ----
v0.1 v0.2     
v1.1 v1.2     
v2.1 v2.2     
v3.1 v3.2     

# try again, this time taking the current pipeline object...
#... and then putting it into another identical looking pipeline
$test | %{ $_ | Add-Member @{key4=$_.key1} }
$test | ft *

key1 key2 key3 key4
---- ---- ---- ----
v0.1 v0.2      v0.1
v1.1 v1.2      v1.1
v2.1 v2.2      v2.1
v3.1 v3.2      v3.1

I suspect it might be that the first one is implying something automatically and I haven't told the invisible/implied function to pass on $PSItem.

All arguments evaluated before pipeline is started. [pscustomobject]@{ key1='something else' } | % { $test | Add-Member @{key3=$_.key1} }. — user4003407
– user4003407, Commented Feb 24, 2017 at 7:11

woxxom · Accepted Answer · 2017-02-24 10:10:36Z

3

All arguments are evaluated in the current scope before the command starts, regardless of being used syntactically in a pipeline. So $test | Add-Member @{key3=$_.key1} uses $_ from the current scope, meaning it's not an element in $test.

To have $_ evaluated for each element in the pipeline, a new scope should be created for each element via ScriptBlock (the code in curly braces) in ForEach, Where like 1..2 | ForEach { $_ }, or in ScriptBlock parameters like Expression in select @{N='calculated property'; E={$_.foo}}.

edited Feb 24, 2017 at 10:10

answered Feb 24, 2017 at 7:23

woxxom

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1 Comment

Joey Over a year ago

What you call context is usually named scope. Script blocks open a new scope, in which variables may be defined that are not there in the outer scope.

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