Select-Object modifies input objects when using both -ExpandProperty and -Property #7768
Description
Select-Object should never modify its input objects, but it does, when using both the -ExpandProperty and -Property parameters.
This behavior is documented, but wrong.
Steps to reproduce
$NestedObjects =
[pscustomobject]@{a=1; b=[pscustomobject]@{c=2}},
[pscustomobject]@{a=3; b=[pscustomobject]@{c=4}}
$NestedObjects | Out-Host
$NestedObjects | Select-Object a -ExpandProperty b | Out-Host
$NestedObjects | Out-Host
$NestedObjects | Select-Object a -ExpandProperty b | Out-HostExpected behavior
Re-running the same Select-Object statement should produce the same output, and should not modify its input objects, so the example code should produce the following output:
a b
- -
1 @{c=2}
3 @{c=4}
c a
- -
2 1
4 3
a b
- -
1 @{c=2}
3 @{c=4}
c a
- -
2 1
4 3
Actual behavior
Instead of creating new objects with the appropriate fields, Select-Object modifies the objects specified by -ExpandProperty, which causes an error the second time the same code runs:
a b
- -
1 @{c=2}
3 @{c=4}
c a
- -
2 1
4 3
a b
- -
1 @{c=2; a=1}
3 @{c=4; a=3}
Select-Object : The property cannot be processed because the property "a" already exists.
At line:9 char:18
+ $NestedObjects | Select-Object a -ExpandProperty b | Out-Host
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (@{a=1; b=}:PSObject) [Select-Object], PSArgumentException
+ FullyQualifiedErrorId : AlreadyExistingUserSpecifiedPropertyExpand,Microsoft.PowerShell.Commands.SelectObjectCommand
Select-Object : The property cannot be processed because the property "a" already exists.
At line:9 char:18
+ $NestedObjects | Select-Object a -ExpandProperty b | Out-Host
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (@{a=3; b=}:PSObject) [Select-Object], PSArgumentException
+ FullyQualifiedErrorId : AlreadyExistingUserSpecifiedPropertyExpand,Microsoft.PowerShell.Commands.SelectObjectCommand
c a
- -
2 1
4 3
Environment data
> $PSVersionTable
Name Value
---- -----
PSVersion 6.0.1
PSEdition Core
GitCommitId v6.0.1
OS Microsoft Windows 10.0.17134
Platform Win32NT
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
WSManStackVersion 3.0