Build reusable pipeline commands

The idea

An advanced function can participate in the pipeline through a parameter marked ValueFromPipeline. Begin initializes state once, process handles each input object, and end emits a final summary or completes aggregation. Keep per-record work in process; placing it only in end does not provide the intended record-by-record behavior. Parameter validation states input constraints before the main calculation proceeds.

For functions that change external state, SupportsShouldProcess exposes preview and confirmation behavior, but the function must actually call $PSCmdlet.ShouldProcess and honor its result before making the change. The attribute alone is not protection. Return structured results with enough information for callers to inspect the outcome, and keep diagnostics separate. Test empty input, multiple records, invalid arguments, and the preview path as distinct contracts.

function Measure-Positive {
  [CmdletBinding()]
  param([Parameter(ValueFromPipeline)][int]$Value)
  begin { $count = 0 }
  process { if ($Value -gt 0) { $count++ } }
  end { [pscustomobject]@{ PositiveCount = $count } }
}
(@(-1, 2, 0, 3) | Measure-Positive).PositiveCount

2

Begin initializes one counter, process sees all four input objects, and end emits one structured summary. Only the two positive values contribute.