Select-Object -First affects prior cmdlet in the pipeline

Question

The PowerShell `Strongly Encouraged Development Guidelines` that cmdlets should Implement for the Middle of a Pipeline but I suspect that isn't doable for a parameter as `-Last` for the `Select-Object`. Simply because you can't determine the last entry upfront. In other words: you will need to wait for the input stream to finish until you define the last entry. To prove this, I wrote a little script: ``` $Data = 1..5 | ForEach-Object {[pscustomobject]@{Index = "$_"}} $Data | ForEach-Object { Write-Host 'Before' $_.Index; $_ } | Select-Object -Last 5 | ForEach-Object { Write-Host 'After' $_.Index } ``` and compared this to `Select-Object *`: ``` $Data | ForEach-Object { Write-Host 'Before' $_.Index; $_ } | Select-Object * | ForEach-Object { Write-Host 'After' $_.Index } ``` With results (right: `Select-Object -Last 5`, left: `Select-Object *`): ``` -Last 5 * ------- - Before 1 Before 1 Before 2 After 1 Before 3 Before 2 Before 4 After 2 Before 5 Before 3 After 1 After 3 After 2 Before…

Accepted Answer

`Select-Object` affects the upstream commands by cheating

That might sound like a joke, but it's not.

To optimize pipeline streaming performance, `Select-Object` uses a trick not available to a regular user developing a `Cmdlet` - it throws a `StopUpstreamCommandsException`.

Once caught, the runtime (indirectly) calls `StopProcessing()` on all the preceding commands, but does not treat it as a terminating error event, allowing the downstream cmdlets to continue executing.

This is extremely useful when you have slow or computationally heavy command early in a pipeline:

```
# this will only take ~3 seconds to return with the StopUpstreamCommand behavior
# but would have incurred 8 extra seconds of "waiting to discard" otherwise
Measure-Command {
  1..5 |ForEach-Object { Start-Sleep -Seconds 1; $_ } |Select-Object -First 3
}
```

Select-Object -First affects prior cmdlet in the pipeline

Solution

Problem