How to get key and value pairs from converted JSON file

I converted in PowerShell a JSON file into a variable.

$JSON = Get-Content -Path "$LogFile" -Raw | ConvertFrom-Json

Sample (Object):

Id                  : Directory_825efe20-2c90-4b90-885f-589a38360cb9
Category            : ApplicationManagement
CorrelationId       : 825efe20-2c90-4b90-885f-589a38360cb9
Result              : success
ResultReason        : 
ActivityDisplayName : Update service principal
ActivityDateTime    : 20.09.2023 08:00:05
LoggedByService     : Core Directory
OperationType       : Update
InitiatedBy         : @{User=; App=}
TargetResources     : {@{Id=14fa3ae9-6dc9-4267-a681-368aa59ffbc8; DisplayName=ODS API; Type=ServicePrincipal; UserPrincipalName=; GroupType=; ModifiedProperties=System.Object[]}}
AdditionalDetails   : {@{Key=User-Agent; Value=Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/116.0.0.0 Safari/537.36 Edg/116.0.1938.62}, @{Key=AppId; Value=563e60f7-4012-491a-b10b-493afb3f2154}}

What is the best way (regarding performance) to extract a specific key/value pair for example the "User-Agent" from the AdditionalDetails property?

Thank you for your assistance!

Solution

^{Note: See the bottom section for sample JSON that the solutions can be tested with.}

A concise solution that performs reasonably well, using the intrinsic .Where() method:

$objectOfInterest = 
  $JSON.AdditionalDetails.Where({ $_.Key -eq 'User-Agent' }, 'First')[0]

Note the need for [0], due to .Where() always returning a collection of results (even though by definition there is at most one in this case).

The following is likely faster, but more verbose and less conceptually direct, using a foreach statement:

$objectOfInterest = 
  foreach ($obj in $JSON.AdditionalDetails) { 
    if ($obj.Key -eq 'User-Agent') { $obj; break }
  }

The slowest - but arguably most PowerShell-idiomatic - option is to use the Where-Object cmdlet:

$objectOfInterest = 
  $JSON.AdditionalDetails | Where-Object Key -eq User-Agent

^{Note the use of simplified syntax, i.e. Key -eq User-Agent in lieu of { $_.Key -eq 'User-Agent' }}

Note that this will invariably process all input objects, because Where-Object offers no way to stop filtering once a match has been found. To do so, additional work would be required.

This surprising omission - especially given that Where-Object's expression-mode counterpart, .Where() does have this feature, as shown above ('First') - is the subject of GitHub issue #13834.

As for why this is slowest:

The PowerShell pipeline (|), due to its object-by-object handoff between the commands involved, invariably introduces overhead.
- However, note that the pipeline is an elegant, core feature of PowerShell that is generally worth taking advantage of, and only worth avoiding if performance requires it.
This overhead alone is not dramatic, but exacerbated by the currently inefficient implementation of the Where-Object cmdlet (as well as ForEach-Object), as of PowerShell (Core) 7.3.x
- See GitHub issue #10982 for a discussion and suggested improvements.
Finally, as noted, the fact that Where-Object (and ForEach-Object) invariably processes all input, with no ability to (gracefully) stop the pipeline once a match has been found, introduces overhead.
- Introducing a -First switch, as discussed in the aforementioned GitHub issue #13834, would solve that specific problem.
- However, there's also a need for a general mechanism to gracefully stop a pipeline from user code, as discussed in GitHub issue #3821.

Sample JSON that the solutions above can be tested with (the basic structure matches the one implied by the for-display representation of the parsed-from-JSON object shown in your question):

# Note: Better to call this variable $fromJSON, for instance.
$JSON = @'
{
  "foo": "bar",
  "AdditionalDetails": [
    {
      "Key": "AppId",
      "Value": "563e60f7-4012-491a-b10b-493afb3f2154"
    },
    {
      "Key": "User-Agent",
      "Value": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/116.0.0.0 Safari/537.36 Edg/116.0.1938.62"
    }
  ]
}
'@ | ConvertFrom-Json