$Error variable is $Null but $_ contains error in Catch

Note: The following applies to Windows PowerShell as well as to PowerShell (Core) versions up to a least v7.3.7 (current as of this writing).

• Accessing the error at hand inside a catch block should indeed be done via the automatic $_ variable.

• Inside a module, $Error isn't $null, but surprisingly is an always-empty collection (of type System.Collections.ArrayList); therefore, $Error[0] - which in catch blocks outside modules is the same as $_ - is unexpectedly $null in modules:

  • What technically happens is that modules have an unused, module-local copy of $Error, which shadows (hides) the true $Error variable that is located in the global scope.

    • When an error is automatically recorded from inside a module, however, it is added to the global $Error collection - just like in code running outside of modules.

    • As it turns out, the unused, useless module-level copy of $Error is an accidental remnant of functionality that was never implemented: see this answer for the history, and GitHub issue #20458 for a discussion about fixing the problem.

  • Therefore, a workaround is to use $global:Error instead (which is only necessary if you need access to previous errors; as stated, for the current one, use $_).

The following sample code illustrates the problem:

$Error.Clear()

# Define a dynamic module that exports sample function 'foo'.
$null = New-Module {

  Function foo {
    try {
      1 / 0     # Provoke an error that can be caught.
    }
    catch {
      $varNames =   '$Error', '$global:Error', '$_'
      $varValues =   $Error,   $global:Error,   $_
      foreach ($i in 0..($varNames.Count-1)) {
        [pscustomobject] @{
          Name = $varNames[$i]
          Type = $varValues[$i].GetType().FullName
          Value = $varValues[$i]
        }
      }
    }
  }

}

foo

Output; note how the value of $Error is {}, indicating an empty collection:

Name          Type                                     Value
----          ----                                     -----
$Error        System.Collections.ArrayList             {}
$global:Error System.Collections.ArrayList             {Attempted to divide by zero.}
$_            System.Management.Automation.ErrorRecord Attempted to divide by zero.

$error and try / catch are different beasts in PowerShell.

try / catch catches terminating errors, but won't catch a Write-Error (cause it's non terminating).

$error is a list of all errors encountered (including ones swallowed up when -ErrorAction silentlycontinue is used).

$_ is the current error in a try/catch block.

I'd guess that your underlying function calls Write-Error, and you want that to go cleanly into a try/catch. To make this be a terminating error as well, use -ErrorAction Stop.