Powershell: ConvertFrom-Json does not respect -ErrorAction

[alastairs]

Steps to reproduce

Run the following expressions:

"some invalid JSON string" | ConvertFrom-Json -ErrorAction Ignore
"some invalid JSON string" | ConvertFrom-Json -ErrorAction SilentlyContinue

Expected behavior

Both expressions write no error to the console, and return either $null or the empty string.

Actual behavior

The expressions write an error to the console and throw the following exception:

ConvertFrom-Json : Invalid JSON primitive: some invalid JSON string.
At line:1 char:12
+ "some invalid JSON string" | ConvertFrom-Json -ErrorAction Ignore
+            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [ConvertFrom-Json], ArgumentException
    + FullyQualifiedErrorId : System.ArgumentException,Microsoft.PowerShell.Commands.ConvertFromJsonCommand

Environment data

> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      5.1.14393.206
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.14393.206
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

[alastairs]

If this is not the appropriate place to report bugs against v5.x, please let me know where I can.

[iSazonov]

@alastairs Could you please test this with recent Powershell 6 build?

[alastairs]

@iSazonov The behaviour is the same with 6.0.0-alpha13

> $PSVersionTable

Name                           Value
----                           -----
PSEdition                      Core
PSVersion                      6.0.0-alpha
PSRemotingProtocolVersion      2.3
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
WSManStackVersion              3.0
CLRVersion
GitCommitId                    v6.0.0-alpha.13
BuildVersion                   3.0.0.0
SerializationVersion           1.1.0.1

BTW, I _love_ that trying this out was just a case of extracting a zip file somewhere and not a full-blow installer. Great work! 👍

[alastairs]

Some further information in case it's useful: my $ErrorActionPreference variable is set to "Stop".

[joeyaiello]

@PowerShell/powershell-committee (subset of @HemantMahawar @lzybkr @khansen00 and myself) agree that the original issue (#2860) is by design. It is unreasonable for ConvertFrom-Json to emit anything but an error when receiving malformed input, and terminating errors are not respected by -ErrorAction. If a developer absolutely wants to make a conscious choice to ignore that terminating error, they should use try/catch.

[joeyaiello]

Okay, now I actually don't know what's going on. It would appear that this is currently a non-terminating error:

ConvertFrom-Json : Conversion from JSON failed with error: Unexpected character encountered while parsing value: s.
Path '', line 0, position 0.
At line:1 char:18
+ "some invalid" | ConvertFrom-Json -ErrorAction SilentlyContinue; 2+2
+                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [ConvertFrom-Json], ArgumentException
    + FullyQualifiedErrorId : System.ArgumentException,Microsoft.PowerShell.Commands.ConvertFromJsonCommand

4

Note: the 2+2 still gets executed

If that's the case, then -ErrorAction should be respected, right?

[dantraMSFT]

Joey,
The proposed fix for this was not accepted and resolved as by-design. Under the hood, Json.Net throws an ArgumentException for malformed json content.

[joeyaiello]

Then I'm not understanding why it's not terminating the statement above....my understanding of PS errors (which may be busted, but please help me understand):

• terminating errors (usually surfaced from internal exceptions) should terminate the script block or statement. In this case, if the ArgumentException is terminating at the PS level, the 2+2 should then not be executed.
• Non-terminating errors respect -ErrorAction. If ArgumentException was caught as a non-terminating error, -ErrorAction SilentlyContinue should silence the error.

What am I missing here?

[dantraMSFT]

This appears to be a side-effect of Cmdlet.ThrowTerminatingError; -ErrorAction is apparently ignored. It's not clear if it is intentional. I see the same behavior on Windows PowerShell.
Here's a couple of PS functions that illustrate the behavior difference:

function TerminatingError
{
    $exception = [System.ArgumentException]::new("argexception")
    $err = [System.Management.Automation.ErrorRecord]::new( `
        $exception,`
        'error',`
        [System.Management.Automation.ErrorCategory]::NotSpecified,`
        $null)
    $cmdlet = [Microsoft.PowerShell.Commands.GetDateCommand]::new()
    $cmdlet.ThrowTerminatingError($err)
}

function ExceptionError
{
    $exception = [System.ArgumentException]::new("argexception")
    throw $exception
}

[joeyaiello]

Okay, now I really don't know what's going on. With your two functions, TerminatingError;2+2 shows that it's not really terminating, but ExceptionError;2+2 is terminating.

[joeyaiello]

Unless you have some more insight, @dantraMSFT, I'll probably open another issue and figure out what the intended behavior is supposed to be.

[SteveL-MSFT]

cc @BrucePay

Behavior with TerminatingError and Exception is inconsistent, not sure if intended.

[joeyaiello]

Alright, I'm closing it, I'll follow up offline or in another issue.

[joeyaiello]

For posterity, my understanding of terminating errors was wrong. A terminating error cancels execution of the current pipeline or expression, not of an entire script block.

A better way to check for terminating errors is to just use try/catch:

C:\Program Files\PowerShell\6.0.0-alpha.18> try {"fdasf" | convertfrom-json} catch {1}
1
C:\Program Files\PowerShell\6.0.0-alpha.18> try {ls foo} catch {1}
ls : Cannot find path 'C:\Program Files\PowerShell\6.0.0-alpha.18\foo' because it does not exist.
At line:1 char:6
+ try {ls foo} catch {1}
+      ~~~~~~
    + CategoryInfo          : ObjectNotFound: (C:\Program File....0-alpha.18\foo:String) [Get-ChildItem], ItemNotFound
   Exception
    + FullyQualifiedErrorId : PathNotFound,Microsoft.PowerShell.Commands.GetChildItemCommand

[mklement0]

@joeyaiello:

I had the same misconception, and I think the source is that the current help topics conflate _statement_-terminating errors with _script_-terminating errors - please see Our Error Handling, Ourselves - time to fully understand and properly document PowerShell's error handling.