The .ForEach() array method misbehaves with XmlElement instances if the name of the property to extract is passed as a string

[mklement0]

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Steps to reproduce

The forms $collection.ForEach('someProp') and $collection.ForEach({ $_.someProp }) should be equivalent and generally are (the former being faster).

However, with an (adapted) XmlElement enumerating the child elements with .ForEach('someProp') does not work as expected: it returns the child elements as an array in the first element of the returned collection.

[xml] $xml = '<foo><child>A</child><child>B</child></foo>'
$hash = @{ foo = [pscustomobject] @{ child = 'A'}, [pscustomobject] @{ child = 'B' } } # control
$r = $hash.foo.ForEach('child'); [pscustomobject] @{ Count = $r.Count; TypeOfFirstElem = $r[0].GetType().Name; ValueOfFirstElem = $r[0] }
$r = $xml.foo.ForEach('child'); [pscustomobject] @{ Count = $r.Count; TypeOfFirstElem = $r[0].GetType().Name; ValueOfFirstElem = $r[0] }
$r = $xml.foo.ForEach({ $_.child }); [pscustomobject] @{ Count = $r.Count; TypeOfFirstElem = $r[0].GetType().Name; ValueOfFirstElem = $r[0] }

Expected behavior

Count TypeOfFirstElem ValueOfFirstElem
----- --------------- ----------------
    2 String          A
    2 String          A
    2 String          A

Actual behavior

Count TypeOfFirstElem ValueOfFirstElem
----- --------------- ----------------
    2 String          A
    1 Object[]        {A, B}
    2 String          A

Error details

No response

Environment data

PowerShell Core 7.2.0-preview.9

Visuals

No response

[rkeithhill]

Just a guess but it looks like the ForEach('child') invocation is outputting a collection without the enumerate option.

PS> [xml] $xml = '<foo><child>A</child><child>B</child></foo>'
PS> $xml.foo.ForEach('child').GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Collection`1                             System.Object

PS> $xml.foo.ForEach('child')
A
B

PS> $xml.foo.ForEach('child') | % {$_}
A
B

PS> $xml.foo.ForEach('child') | % {$_} | % GetType

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     String                                   System.Object
True     True     String                                   System.Object

[mklement0]

@rkeithhill, note that .ForEach() always returns a System.Collections.ObjectModel.Collection<T> instance, even for a single output object (e.g. ('foo').foreach('length').GetType())

The problem here, as demonstrated in the OP, is that the single output object is itself an array (object[]), unexpectedly, whereas $xml.foo.child enumerates as usual.

[rkeithhill]

> $xml.foo.ForEach('child')[0].GetType()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
True     True     Object[]                                 System.Array

Ah, yup.

[daxian-dbw]

I dug a little under the debugger and here are what I found:

1. When running $xml.foo.ForEach(...), $xml.foo is treated as a single element XmlElement foo. So the operation done within ForEach will be targeting $xml.foo. However, $hash.foo is an array of 2 elements, so it's not really comparable to $xml.foo and thus we can ignore it in this context.

2. $xml.foo.child always return an array of 2 elements. The difference between .ForEach('child') and .ForEach({$_.child}) is:

   • for the latter, the array is automatically unwrapped because it goes through the property access in the language way
   • for the former, it retrieves the PSPropertyInfo and directly add its value to the collection, without unwrapping.

This is why single output object is itself an array.
Not super sure how to fix it though. One option is to always unwrap collection when accessing a property or calling a method via the .ForEach(<member-name>) syntax.

[microsoft-github-policy-service]

This issue has not had any activity in 6 months, if this is a bug please try to reproduce on the latest version of PowerShell and reopen a new issue and reference this issue if this is still a blocker for you.

[microsoft-github-policy-service]

This issue has not had any activity in 6 months, if this is a bug please try to reproduce on the latest version of PowerShell and reopen a new issue and reference this issue if this is still a blocker for you.

[microsoft-github-policy-service]

This issue has not had any activity in 6 months, if this is a bug please try to reproduce on the latest version of PowerShell and reopen a new issue and reference this issue if this is still a blocker for you.

[microsoft-github-policy-service]

This issue has been marked as "No Activity" as there has been no activity for 6 months. It has been closed for housekeeping purposes.