README.md

PowerShellTraps is a collection of some PowerShell traps and oddities shown by demo scripts, workarounds, and automated tests. On invoking scripts change to their directory. See also TESTS. Some scripts require Invoke-PowerShell.ps1.

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Index

• Basic
• Class
• Clixml
• Cmdlets
• Module
• PowerShell.exe

Basic

• Operators -and and -or have the same precedence
• Invoking native apps with error output
• Automatic variables as parameters or local variables
• Break and Continue with not matching label
• Break and Continue without loop
• Collection as Boolean
• Collection property enumeration
• Comparison operators work differently with scalars and collections
• Compound assignment operators
• Count-and-Length
  • Mixed objects
  • PSCustomObject does not have surrogate Count and Length
  • Count and Length of a scalar fail in the strict mode
• Dictionary Count, Keys, Values may misbehave
• Different kinds of null
• Double quoted strings and sub-expressions with double quotes
• DynamicParam
  • Dynamic switch parameter
  • $MyInvocation.ExpectingInput is false in DynamicParam
• Enums evaluated to true
• Enums without [Flags] attribute
• Errors of unusual type
• FormatEnumerationLimit
• Function invoked like method
• Invocation with odd paths
• LastExitCode
• Local action preference variables
• Magic method Where
• Misleading error location
• Missing ternary operator
• Negative number literal argument
• Null converted to empty string
• Number of returned objects
• Operators -match, -notmatch do not reset $matches
• Counter-intuitive equal precedence of some operators
• Tricky properties of types implementing IDictionary
• Properties of XML nodes
• Provider specific filters
• PSCustomObject
• [ref] may be unwrapped unexpectedly
• Directive #requires
• Runspace pool memory leaks with Close()
• Statements are not expressions
• ErrorRecord formatting may fail in the strict mode in the default host
• String as Boolean
• String constructor
• String equality operators
• switch is a looping construct
• Tempting wrong operators
• Throw may not terminate
• ThrowTerminatingError
  • ThrowTerminatingError does not trigger the immediate catch
  • ThrowTerminatingError produces not true terminating errors
• Too simple function names
• trap creates a new scope
• trap with continue
• trap with no break or continue
• try and trap catch terminating errors
• Unexpected output
• Unexpected overloaded method
• Unrolled collections
• v5 method new()
• ValidateScript and cryptic error messages
• ValueFromPipeline used for several parameters
• Parameters with ValueFromRemainingArguments
• When process block is called
• WildcardPattern.Escape() does not escape some backticks

Class

• Attribute with later defined type parameter
• Cannot derive a class from another defined later
• Cannot derive from dot sourced
• Identical script blocks issue
• Methods do not see implicitly created variables
• Parser fails on custom type

Clixml

• Cannot write to hidden files
• Hashtable case sensitivity
• OrderedDictionary becomes Hashtable
• ScriptBlock becomes String
• No LiteralPath in v2

Cmdlets

• ConvertFrom-Csv
  • ConvertFrom-Csv has undocumented partial comment support
• ConvertFrom-Json
  • ConvertTo-Json different number types
  • ConvertFrom-Json returns an array not unrolled
  • ConvertFrom-Json pipeline input
• ConvertTo-Json
  • ConvertTo-Json Array as PSObject
  • ConvertTo-Json without Compress may change data
• Copy-Item
  • Copy-Item - Exclude is ignored with Recurse
  • Copy-Item inconsistent destination directory
• ForEach-Object
• Get-ChildItem
  • Get-ChildItem gets items converted to strings differently
  • Get-ChildItem wildcard-path fails in a directory with backticks
  • Get-ChildItem -LiteralPath -Recurse gets nothing for a directory with brackets
  • Get-ChildItem -Recurse and missing -Path unexpected search
  • Get-ChildItem -LiteralPath -Recurse ignores -Include
• Get-Content
  • Get-Content with OutVariable and ReadCount
• Get-Event
  • Get-Event erratic failures in v2-4
• Get-Item
  • Get-Item fails to get Cert:
  • Get-Item works incorrectly in some locations
• Get-ItemPropertyValue
  • Get-ItemPropertyValue ignores ErrorAction
• Get-Unique oddities
• Get-WmiObject
  • Get-WmiObject - amended WMI data may fail in the strict mode Latest
• Group-Object
  • Group-Object -AsHashTable and custom expressions
  • Group-Object and properties made by expressions with no values
• Import-Csv may trim leading spaces
• Import-Module
  • Import-Module non-terminating error
• Invoke-Expression
  • Invoke-Expression ErrorAction is ignored in favour of $ErrorActionPreference
• Invoke-RestMethod
  • Invoke-RestMethod returns an array not unrolled
• Join-Path
  • Join-Path fails if the path drive does not exist
  • Join-Path with path separator
• Read-Host
  • v5 output before Read-Host
• Remove-Item
  • Remove-Item -LiteralPath fails in a directory with brackets
  • Remove-Item may ignore items specified by Exclude
• Remove-ItemProperty -Confirm fails on 'No' (Registry)
• Rename-Item
  • Rename-Item has no LiteralPath in v2
• Resolve-Path
  • Resolve-Path -Path with wildcards misses hidden items
• Select-Xml
  • Select-Xml -Content does not support input as stream
• Set-Content
  • Set-Content fails in a directory with brackets
  • Set-Content unexpected output location
• Split-Path with UNC paths: mind the current provider
• Start-Process
• Test-Path
  • Test-Path oddities
  • Test-Path -Path with wildcards misses hidden items
• Wait-Process fails if a process name is not found
• Where-Object
• Write-Debug
• Write-Information writes nothing in ForEach method
• Write-Progress

Module

• Conflict with aliases
• Function parent scope
• Not found function
• Not found function (part 2)
• Not found variable
• Script block scope
• How to set a variable in the caller scope
• Strict mode is not propagated
• v2 does not support the manifest field RootModule

PowerShell.exe

• Exit code 0 with Command syntax error
• Exit code 0 with File script error
• Exit code 5 with File, 1 with Command
• Exit code depends
• Global and script scope
• Not current version
• Different positional parameters in powershell and pwsh
• Switch parameter with value
• Unexpected start location
• v3 interactive issues
• Parameter Version must be the first.
• Version and scriptblock host problem

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