When should I use Write-Error vs. Throw? Terminating vs. non-terminating errors

Solution 1:

Write-Error should be used if you want to inform the user of a non-critical error. By default all it does is print an error message in red text on the console. It does not stop a pipeline or a loop from continuing. Throw on the other hand produces what is called a terminating error. If you use throw, the pipeline and/or current loop will be terminated. In fact all execution will be terminated unless you use a trap or a try/catch structure to handle the terminating error.

There is one thing to note, if you set $ErrorActionPreference to "Stop" and use Write-Error it will produce a terminating error.

In the script you linked to we find this:

if ($url.Contains("http")) {
       $request = [System.Net.HttpWebRequest]::Create($url)
}
else {
       $URL_Format_Error = [string]"Connection protocol not specified. Recommended action: Try again using protocol (for example 'http://" + $url + "') instead. Function aborting..."
       Write-Error $URL_Format_Error
    return
   }

It looks like the author of that function wanted to stop the execution of that function and display an error message on screen but did not want the entire script to stop executing. The script author could have used throw however it would mean you would have to use a try/catch when calling the function.

return will exit the current scope which can be a function, script, or script block. This is best illustrated with code:

# A foreach loop.
foreach ( $i in  (1..10) ) { Write-Host $i ; if ($i -eq 5) { return } }

# A for loop.
for ($i = 1; $i -le 10; $i++) { Write-Host $i ; if ($i -eq 5) { return } }

Output for both:

1
2
3
4
5

One gotcha here is using return with ForEach-Object. It will not break processing like one might expect.

More information:

• $ErrorActionPreference: about_Preference_Variables
• try/catch: about_Try_Catch_Finally
• trap: about_Trap
• throw: about_Throw
• return: about_Return

Solution 2:

Important: There are 2 types of terminating errors, which the current help topics unfortunately conflate:

• statement-terminating errors, as reported by cmdlets in certain non-recoverable situations and by expressions in which a .NET exception / a PS runtime error occurs; only the statement is terminated, and script execution continues by default.

• script-terminating errors (more accurately: runspace-terminating), as either triggered by Throw or by escalating one of the other error types via the common -ErrorAction parameter, -ErrorAction Stop, or via the $ErrorActionPreference preference variable, $ErrorActionPreference = 'Stop'.
  Unless caught, they terminate the current runspace (thread); that is, they terminate not just the current script, but all its callers too, if applicable).

For a comprehensive overview of PowerShell's error handling, see GitHub documentation issue #1583.

The remainder of this post focuses on non-terminating vs. statement-terminating errors.

To complement the existing helpful answers with a focus on the core of the question: How do you choose whether to report a statement-terminating or non-terminating error?

Cmdlet Error Reporting contains helpful guidelines; let me attempt a pragmatic summary:

The general idea behind non-terminating errors is to allow "fault-tolerant" processing of large input sets: failure to process a subset of the input objects should not (by default) abort the - potentially long-running - process as a whole, allowing you to inspect the errors and reprocess only the failed objects later - as reported via the error records collected in automatic variable $Error.

• Report a NON-TERMINATING error, if your cmdlet / advanced function:

  • accepts MULTIPLE input objects, via the pipeline input and/or array-valued parameters, AND
  • errors occur for SPECIFIC input objects, AND
  • these errors DO NOT PREVENT PROCESSING of FURTHER input objects IN PRINCIPLE (situationally, there may be no input objects left and/or previous input objects may already have been processed successfully).
    • In advanced functions, use $PSCmdlet.WriteError() to report a non-terminating error (Write-Error, unfortunately, doesn't cause $? to be set to $False in the caller's scope - see GitHub issue #3629).
    • Handling a non-terminating error: $? tells you whether the most recent command reported at least one non-terminating error.
      • Thus, $? being $False can either mean that any (nonempty) subset of input objects weren't properly processed, possibly the entire set.
      • Preference variable $ErrorActionPreference and/or common cmdlet parameter -ErrorAction can modify the behavior of non-terminating errors (only) in terms of error output behavior and whether non-terminating errors should be escalated to script-terminating ones.

• Report a STATEMENT-TERMINATING error in all other cases.

  • Notably, if an error occurs in a cmdlet / advanced function that only accepts a SINGLE or NO input object and outputs NO or a SINGLE output object or takes parameter input only and the parameter values given prevent meaningful operation.
    • In advanced functions, you must use $PSCmdlet.ThrowTerminatingError() in order to generate a statement-terminating error.
    • Note that, by contrast, the Throw keyword generates a script-terminating error that aborts the entire script (technically: the current thread).
    • Handling a statement-terminating error: A try/catch handler or trap statement may be used (which cannot be used with non-terminating errors), but note that even statement-terminating errors by default do not prevent the rest of the script from running. As with non-terminating errors, $? reflects $False if the previous statement triggered a statement-terminating error.

Sadly, not all of PowerShell's own core cmdlets play by these rules:

• While unlikely to fail, New-TemporaryFile (PSv5+) would report a non-terminating error if it failed, despite not accepting pipeline input and only producing one output object - this has been corrected as of at least PowerShell [Core] 7.0, however: see GitHub issue #4634.

• Resume-Job 's help claims that passing an unsupported job type (such as a job created with Start-Job, which is not supported, because Resume-Job only applies to workflow jobs) causes a terminating error, but that's not true as of PSv5.1.