Be Careful With “Throw” Statements (Part 2)

In the previous tip we explained how the throw statement can be influenced by $ErrorActionPreference set to “SilentlyContinue”, and that throw will not exit function code properly. Here is again the example we used:

function Copy-Log
{
  "Doing prerequisites"
  "Testing whether target path exists"
  "If target path does not exist, bail out"
  throw "Target path does not exist"
  "Copy log files to target path"
  "Delete log files in original location"
}

 
PS> Copy-Log
Doing prerequisites
Testing whether target path exists
If target path does not exist, bail out
Target path does not exist
In Zeile:8 Zeichen:3
+   throw "Target path does not exist"
+   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : OperationStopped: (Target path does not exist:String) [], RuntimeExceptio 
   n
    + FullyQualifiedErrorId : Target path does not exist


PS> $ErrorActionPreference = 'SilentlyContinue'

PS> Copy-Log
Doing prerequisites
Testing whether target path exists
If target path does not exist, bail out
Copy log files to target path
Delete log files in original location

While throw exists the function with the default ErrorAction, it continues the code when ErrorAction is set to “SilentlyContinue”. This most likely is a bug because the only difference between the ErrorAction settings “Continue” and “SilentlyContinue” should be whether error messages are visible or not. These settings should not affect the code that is actually executed.

The bug in throw only occurs when the terminating error emitted by throw is not handled. Once you use try…catch or even add a simple (and completely empty) trap statement, all is fine, and throw works as expected:

# add a trap to fix
trap {}

$ErrorActionPreference = "SilentlyContinue"
Copy-Log
$ErrorActionPreference = "Continue"
Copy-Log 

Once the trap is in place, the code will exit at the throw statement regardless of $ErrorActionPreference setting. You might want to add the empty trap statement to your PowerShell profile script to be protected from this bug, or reconsider the use of throw altogether.

Your learning points:

• Throw is part of an error handling system, and the exception emitted by throw needs to be caught by try…catch or trap. If the exception is not caught, throw may not work as expected.
• Because of this, try not to use throw to exit function code for functions that are exposed to end users. For end users, rather than emitting an (ugly) exception, emit a human-friendly error message using Write-Warning or Write-Host, and exit the code using the “return” statement
• If you must emit an exception so that callers can catch it in their error handlers, but also want to make sure your code definitely exists regardless of $ErrorActionPreference, use Write-Error in combination with the “return” statement:

function Copy-Log
{
  "Doing prerequisites"
  "Testing whether target path exists"
  "If target path does not exist, bail out"
  Write-Error "Target path does not exist" return
  "Copy log files to target path"
  "Delete log files in original location"
}

Since “return” is not affected by the $ErrorActionPreference, your code will always exit. Let’s test:

 
PS> Copy-Log
Doing prerequisites
Testing whether target path exists
If target path does not exist, bail out
Copy-Log : Target path does not exist
In Zeile:1 Zeichen:1
+ Copy-Log
+ ~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,Copy-Log  

With $ErrorActionPreference set to SilentlyContinue, the error message is suppressed as expected, but the code will reliably exit:

 
PS> $ErrorActionPreference = 'SilentlyContinue'

PS> Copy-Log
Doing prerequisites
Testing whether target path exists
If target path does not exist, bail out

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