Join-String does not produce the same result as -join

[PetSerAl]

Steps to reproduce

class C: System.Collections.IEnumerable {
    [string] ToString() { return 'Correct' }
    [System.Collections.IEnumerator] GetEnumerator() { return ('Not', 'Correct').GetEnumerator() }
}
[C]::new(), [C]::new(), [C]::new() -join ', '
[C]::new(), [C]::new(), [C]::new() | Join-String -Separator ', '

Expected behavior

Correct, Correct, Correct
Correct, Correct, Correct

Actual behavior

Correct, Correct, Correct
Not Correct, Not Correct, Not Correct

Environment data

Name                           Value
----                           -----
PSVersion                      6.2.0-preview.3
PSEdition                      Core
GitCommitId                    6.2.0-preview.3-104-ge399b06cd23a8d9a7df9be5ed329471ba5b8eb82
OS                             Microsoft Windows 10.0.17763
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0, 5.0, 5.1.10032.0, 6.2.0-preview.3}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

[vexx32]

I don't think this is really avoidable. By the time the cmdlet receives input, the GetEnumerator() has already been called by the pipeline logic. 🙁

[PetSerAl]

@vexx32

By the time the cmdlet receives input, the GetEnumerator() has already been called by the pipeline logic.

Any evidence to support this claim?

function My-Join-String { @($input) -join ', ' }
[C]::new(), [C]::new(), [C]::new() | My-Join-String

[vexx32]

@PetSerAl Ah, I see! Okay, I misunderstood the original point.

Looks like it's enumerating each pipeline item, likely in an attempt to make pipeline input behave the same as Join-String -InputObject ([C]::new(), [C]::new(), [C]::new())

@mklement0 and I have discussed this a bit and have a code pattern that could help rectify it outlined in #4242 as you've seen.

[PetSerAl]

@vexx32 If it enumerating each pipeline item, the it should produce Not, Correct, Not, Correct, Not, Correct instead of Not Correct, Not Correct, Not Correct.

[vexx32]

Not quite. It calls the enumerator of your object to enumerate it, and receives back a two element array for each item. It doesn't then enumerate each of those arrays, that would be an extra level of enumeration. 🙂

I would guess that somewhere in its logic it converts these either implicitly or explicitly to string, which presumably forces PS to call ToString() on the resulting array.

[PetSerAl]

That is the problem. Join-String should not do [C] -> [Array] -> [String] instead of [C] -> [String].

[iSazonov]

Are other cmdlets affected by this?

[vexx32]

I don't think a lot of cmdlets will be. Most cmdlets implement -InputObject strictly for pipeline use, so they can make some generally safe assumptions.

There may be some, but I don't know of any affected in the same way as this one is... Yet 😄

[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.