Wrong catch block execution in PowerShell - powershell

Question

I'm a C# developer and sometimes PowerShell is just driving me mad.
I have the following code:
$script:ErrorActionPreference = 'Stop'
try {
# Some code here
}
catch [Microsoft.PowerShell.Commands.WriteErrorException] {
# Print error messages (without stacktrace)
Write-Host -ForegroundColor Red $_.Exception.Message
exit 1
}
catch [System.Management.Automation.RuntimeException] {
# A thrown string
Write-Host -ForegroundColor Red $_.Exception.Message
Write-Host -ForegroundColor Red $_.ScriptStackTrace
exit 1
}
catch {
# Print proper exception message (including stack trace)
Write-Host -ForegroundColor Red "$($_.Exception.GetType().Name): $($_.Exception.Message)"
Write-Host -ForegroundColor Red $_.ScriptStackTrace
exit 1
}
The idea is basically:
If the exception comes from a call to Write-Error use the first catch block.
If a string is thrown directly, use the second catch block.
For any other exception, use the last catch block.
Now, my problem is with Write-Error and the first catch block:
If I call Write-Error within the try block, the second catch block is executed (even though the first one should be executed).
If I remove the second catch block and then call Write-Error, the correct (first) catch block is used.
Why is that?
I've checked whether WriteErrorException and RuntimeException are inheriting from each other: They don't (both inherit from SystemException but this shouldn't matter).
I've also verified that this behavior is the same in both PowerShell 5.1 and PowerShell Core (6.0).

Answer 1

Write-Error won't throw a terminating error by default, but it will with ErrorActionPreference set to Stop as you've mentioned. However, this changes the exception thrown to ActionPreferenceStopException which does inherit RuntimeException
You can still catch the WriteErrorException without the RuntimeException clause because the inner error record for the ActionPreferenceStopException contains the WriteErrorException
You can see what I mean by running this:
Write-Error 'this is a test' -ErrorAction Stop
$error[0].ErrorRecord.Exception.GetType()
# IsPublic IsSerial Name BaseType
# -------- -------- ---- --------
# True True WriteErrorException System.SystemException
But with the RuntimeException clause, it will get picked up first because RuntimeException is the closest matching exception type.
To workaround this you'd need to either throw a more specific exception or test $_ within the RuntimeException clause. Here's the latter
$script:ErrorActionPreference = 'Stop'
try {
# Some code here
}
catch [Microsoft.PowerShell.Commands.WriteErrorException] {
# Print error messages (without stacktrace)
Write-Host -ForegroundColor Red $_.Exception.Message
exit 1
}
catch [System.Management.Automation.RuntimeException] {
if ($_.Exception -is [Microsoft.PowerShell.Commands.WriteErrorException]) {
# Print error messages (without stacktrace)
Write-Host -ForegroundColor Red $_.Exception.Message
exit 1
}
# A thrown string
Write-Host -ForegroundColor Red $_.Exception.Message
Write-Host -ForegroundColor Red $_.ScriptStackTrace
exit 1
}
catch {
# Print proper exception message (including stack trace)
Write-Host -ForegroundColor Red "$($_.Exception.GetType().Name): $($_.Exception.Message)"
Write-Host -ForegroundColor Red $_.ScriptStackTrace
exit 1
}
You could also add a ActionPreferenceStopException clause and test for $_ there.
Edit: Actually, unless you really want to use Write-Error, you'd be better off just throwing an exception similar to how you would in C#. So instead of Write-Error, use:
throw [System.InvalidOperationException]::new('This is my message')