Prerequisites

• Write a descriptive title.
• Make sure you are able to repro it on the latest released version
• Search the existing issues.
• Refer to the FAQ.
• Refer to Differences between Windows PowerShell 5.1 and PowerShell.

Steps to reproduce

Attempting modification of a collection while it is being enumerated should result in an error:

• An error occurred while enumerating through a collection: Collection was modified; enumeration operation may not execute.

This error isn't consistently surfaced - note that this is a regression from Windows PowerShell:

# OK: Reports an error for the attempt to modify the collection being enumerated.
'--- foreach'
$l = [System.Collections.Generic.List[string]] @('one', 'two')
foreach ($e in $l) { $l.Remove($e) }

# !! Fails QUIETLY in PS CORE as of v7.4.0-preview.5
'--- pipeline'
$l = [System.Collections.Generic.List[string]] @('one', 'two')
$l | ForEach-Object { $l.Remove($_) }

Important: The quiet failure only occurs when the code is executed via a script file; if you paste the last two statements into an interactive console, the error does surface.

[Update: NOT PowerShell's fault - see below] The following WinForms example affects Windows PowerShell too (both interactively and in a script):

using namespace System.Windows.Forms
Add-Type -Assembly System.Windows.Forms

$f = [Form]::new()
$f.Controls.AddRange((
  [CheckBox]::new(),
  [CheckBox]::new()
))

# !! Should fail with an error message, but quietly aborts after the first .Remove() call.
$f.Controls | ForEach-Object { $f.Controls.Remove($_) }

$f.Controls.Count # !! still nonzero

Expected behavior

First snippet:

• Both the foreach and the ForEach-Object statement should report the expected error.

Second snippet:

• The expected error should occur.

Actual behavior

First snippet:

• The ForEach-Object statement doesn't report an error and quietly aborts the pipeline after the first .Remove() call

Second snippet:

• The expected error doesn't occur; quietly aborts the pipeline after the first .Remove() call.

Error details

No response

Environment data

PowerShell 7.4.0-preview.5

Visuals

No response