Prevent Select-Object with ExpandProperty from updating source PSObjects

[marshallwp]

When using Select-Object to select from a PSObject with both -Property and -ExpandedProperty arguments specified, the source object will be copied instead of referenced.

This PR addresses issue #21308. It ensures that when a PSObject is selected from with both the -ExpandProperty and -Property arguments specified, the source object will not be updated. While technically preventing the source object from being modified is a breaking change, it breaks things in a manner previously approved by the @Powershell/powershell-committee. This PR is needed because the previous attempt at fixing this only works for non-PSObject inputs.

In particular, for operations matching the aforementioned restrictions, this PR does the following: Within the ProcessExpandParameter method, the expandedObject is now set to a copy of the resulting PSObject instead of a reference.

Additionally, Pester tests have been added.

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.ps1 : +36 -1

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[mklement0]

Thank you for tackling this, @BinaryWizard904.

Let me summarize the net effects of this PR:

• It will reliably prevent modification of the object contained in the input object property targeted by -ExpandProperty in combination with -Property arguments, by attaching the -Property values as instance ETS properties directly to a (new) PSObject wrapper around the object (instead of attaching them via the resurrection tables, which is how instance ETS members are normally - and more robustly - attached).

  • For custom objects (PSCustomObject, PowerShell's "property bags", such as created with [pscustomobject] @{ ... }), this effectively creates a (shallow) clone of the input object before the ETS == direct properties are attached.

  • For instances of all other .NET types, it creates a PSObject wrapper with directly attached ETS properties, which are therefore easy to "lose", because - unlike with the resurrection tables - the loss of the wrapper means loss of the ETS properties (e.g., loss of the wrapper / ETS properties occurs in a simple cast of the invisible wrapper to the underlying type or when passing it to a command parameter typed as the underlying type).

    • However, this is the price to pay if modification (in an ETS sense) of the original object is to be avoided.

• It will not solve #7937, which is an edge case, however:

  • If the input object's -ExpandProperty-targeted property value is a (non-custom) object that has preexisting ETS instance members that are resurrection table-based (as is typical), these instance members are effectively eclipsed by the output PSObject wrapper and its directly attached -Property-targeted ETS instance properties.
  • However, you can technically still access them via .psobject.BaseObject on the wrapper.
  • This limitation seems to come down to there currently being no support for a given PSObject instance to directly surface both directly attached ETS members and ETS members present in the resurrection tables for its base object as direct members.

[SteveL-MSFT]

@PowerShell/wg-powershell-cmdlets discussed this. We have concerns about any change in behavior now would likely break existing scripts that may not be aware are depending on this behavior. So, we must reject this change in behavior, but will open a doc issue to explicitly state that the input object is modified and an example of how to achieve the desired result by creating a new object instead of using Select-Object. In addition, given the history of Select-Object, we would consider a new set of cmdlet(s) that exhibit the desired behavior such as Expand-Object (published to PSGallery).

[mklement0]

@SteveL-MSFT, please note that you had previously approved this breaking change; quoting https://github.com/PowerShell/PowerShell/issues/7768#issuecomment-459158766 (which the initial post also links to):

@PowerShell/powershell-committee reviewed this. We believe the original intent is to not modify the original object and this looks like a bug. This is a Breaking Change, but the benefit outweighs the potential impact. We agree that we should just fix the original behavior so that a new object is emitted.