Suppressing error messages in PowerShell ErrorVariable

Question

Using the snippet below, I'm trying to find a way to suppress the "Attempted to divide by zero" error, and be left with just the custom error message in $err. I've tried combinations of try/catch, Invoke-Command/Expression, redirecting 2>$null, but in every case the error stream still contains the "attempted to divide by zero" message.

function func1 {
  [CmdletBinding()]
  param()

  func2 -ErrorAction SilentlyContinue -ErrorVariable er
  if ($er.Count -ge 1) {
    Write-Error -Message "Custom error message!"
  }
}

function func2 {
  [CmdletBinding()]
  param()

  1/0
}

func1 -ErrorAction SilentlyContinue -ErrorVariable err
$err

populates $err with two items:

Attempted to divide by zero.
At line:15 char:3
+   1/0
+   ~~~
    + CategoryInfo          : NotSpecified: (:) [], RuntimeException
    + FullyQualifiedErrorId : RuntimeException
 
func1 : Custom error message!
At line:18 char:1
+ func1 -ErrorAction SilentlyContinue -ErrorVariable err
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,func1

Thanks so much for your thoughts!

mklement0 · Accepted Answer · 2022-10-21 15:54:50Z

1

Unfortunately the solution is somewhat tricky, because the error that occurs in func2 is invariably recorded in the $err variable passed to -ErrorVariable in the func1 invocation, and is already stored there when the func2 call returns.

Therefore you need to manually remove it:

Replace:

if ($er.Count -ge 1) {
  Write-Error -Message "Custom error message!"
}

with:

if ($er.Count -ge 1) {
  if ($PSBoundParameters.ContainsKey('ErrorVariable')) {
    # Remove the most recently added error from the $err variable.
    $PSCmdlet.GetVariableValue($PSBoundParameters['ErrorVariable']).RemoveAt(0)
  }
  Write-Error -Message "Custom error message!"
}

The automatic $PSBoundParameters variable allows you to check if the common -ErrorAction parameter was specified and with what value (i.e., what target variable name).
The automatic $PSCmdlet variable's .GetVariableValue() method allows access to variables defined in the caller's scope, which enables retrieving the target variable value, which is a [System.Collections.ArrayList] instance whose .RemoveAt() method can be called to remove the most recently added error, which is at index 0.

Asides:

1/0 causes a statement-terminating error, whereas the common -ErrorAction parameter only acts on non-terminating errors (to catch terminating errors you need try / catch). However, because this statement-terminating error occurs inside a function, execution inside that function (potentially) continues, and to the caller the error is in effect a non-terminating one.
For a comprehensive overview of PowerShell's bewilderingly complex error handling, see GitHub docs issue #1583.

edited Oct 21, 2022 at 15:54

answered Oct 20, 2022 at 21:46

mklement0

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3 Comments

cobside Over a year ago

Thanks! That's useful... but I don't believe addresses the original problem. If I encapsulate the call to func1 in a try/catch block, it does not help me to suppress the "Attempted to divide my zero" error contained in $err.

Fitzgery Over a year ago

In your catch block use throw “<your custom message here” to perform a terminating error and stop the script or use write-error “your message”

mklement0 Over a year ago

@Fitzgery, it turns out that no try / catch was ultimately needed, and the real goal (which I initially missed) was to prevent the 1/0 error from surfacing in the $err variable passed to the outer function call's -ErrorVariable parameter. Doing so requires tricky manual manipulation of this variable, unfortunately.

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