Script calling Get-Volume does not throw an exception although ErrorActionPreference is set to "Stop"

Question

This is my script in file test.ps1:

$ErrorActionPreference = "Stop" # make it throw an exception
try
{
  Get-Volume -FileSystemLabel "doesnotexist"
}
catch [Microsoft.PowerShell.Cmdletization.Cim.CimJobException]
{
  Write-Host "exception thrown"
}

I run it from a PowerShell terminal like this:

./test.ps1

But it does not throw the exception and instead prints out a red error message.

If you're familiar with PowerShell you will understand that it should throw an exception instead.

Why doesn't it throw an exception as expected?

It looks like the reason is that `Get-Volume` is implemented as a script function, rather than a compiled cmdlet, in Windows PowerShell. There is a known issue with cmdlets implemented as script functions - preference variables like `$ErrorActionPreference` are not respected by default, unless they are global variables. See https://stackoverflow.com/a/55841499/7571258 — zett42, May 26 '23 at 17:19

score 2 · Accepted Answer · answered May 26 '23 at 15:48

You need to do one of the following:

Set $ErrorActionPreference = "Stop" before calling the script
Use $Global:ErrorActionPreference = "Stop" in the script
Execute the script as powershell.exe -file test.ps1
Use Get-Volume -FileSystemLabel "notreal" -ErrorAction Stop in the script

mklement0 · Answer 2 · 2023-05-26T19:18:02.047

zett42 has provided the crucial pointer:

The Get-Volume command is implemented as a function that is part of the Storage module, which is a (CDXML-based) script module, i.e. implemented in PowerShell code.
Due to an unfortunate design limitation - see GitHub issue #4568 - module-based functions do NOT see the preference variables of outside callers - they only see such variables if they're defined in the global scope (which is the only scope that modules and non-module code share).
- Note: Binary cmdlets - i.e. those implemented as .NET assemblies (typically compiled from C# code) - are not affected.
Therefore, your script-scoped $ErrorActionPreference = "Stop" definition had no effect.

Workarounds:

[Suboptimal]: Set $ErrorActionPreference in the global scope:
- $global:ErrorActionPreference = 'Stop'
- Important: Given that global definitions apply session-wide and linger after your script exits, be sure to restore the previous value of $global:ErrorActionPreference before exiting your script.
Preferably, use the common -ErrorAction parameter with Get-Volume, which is a command-scoped way of setting $ErrorActionPreference, which module-based functions honor as well.^[1]

try
{
  # -ErrorAction Stop promotes the *non-terminating* error that Get-Volume
  # emits to a (script)-*terminating* one, which triggers the `catch` block.
  Get-Volume -ErrorAction Stop -FileSystemLabel "doesnotexist"
}
catch [Microsoft.PowerShell.Cmdletization.Cim.CimJobException]
{
  Write-Host "exception thrown"
}

^{[1] Behind the scenes, when -ErrorAction is used with an (of necessity advanced) function - whether module-based or not - PowerShell translates the value into a function-local $ErrorActionPreference variable.

Even though this should be a mere implementation detail, it can unfortunately have side effects - see GitHub issue #19500.}

Script calling Get-Volume does not throw an exception although ErrorActionPreference is set to "Stop"

2 Answers2