Pipelining get-content to own function - function

Question

I'm trying to write a powershell function that receives a list of files from the get-content commandlet by piplining and processes them.
The pipeline looks like this:
get-content D:\filelist.txt | test-pipeline
For simplicity sake the function below should just show each line of the textfile.
function test-pipeline
{
<#
.Synopsis
#>
[CmdletBinding( SupportsShouldProcess=$true)]
Param([Parameter(Mandatory = $true,
ValueFromPipeLine = $true)]
[array]$filelist
)
foreach ($item in $filelist)
{
$item
}
}
My filelist is a normal .txt file and looks like this.
line 1
line 2
line 3
line 4
line 5
No matter what type of parameter I pipe to the function, it never works and shows only the last line of the text file in the $filelist variable. Can anybody help? Powershell Version is v2
Thanks in advance

Answer 1

The reason you see only your last line requires digging a bit into the nature of pipeline-able PowerShell functions. In the absence of explicit begin/process/end blocks that Swonkie alluded to, all code in the function operates as if it is in the end block, i.e. it is as if you wrote:
function test-pipeline
{
[CmdletBinding()]
Param(
[Parameter(ValueFromPipeLine = $true)][array]$filelist
)
END {
$filelist
}
}
But $filelist, being a pipeline variable, has only the current value in it when fed pipeline data; the end block runs when the pipeline input is exhausted, thus $filelist contains just the last value. Simply changing that end block to a process block--where it runs for each value in the pipeline, will give you the desired output:
function test-pipeline
{
[CmdletBinding()]
Param(
[Parameter(ValueFromPipeLine = $true)][array]$filelist
)
PROCESS {
$filelist
}
}
And notice that you do not need any kind of loop there--the pipeline is already providing the "loop".
That is just one of several ways to process pipeline data. Here's a close variant, which is a bit shorter: use filter instead of function, because filter with no explicit blocks operates as if all code is in the process block.
filter test-pipeline
{
[CmdletBinding()]
Param(
[Parameter(ValueFromPipeLine = $true)][array]$filelist
)
$filelist
}
To delve further into the fascinating and arcane world of writing functions for pipelining, take a look at the in-depth analysis I wrote, Down the Rabbit Hole: A Study in PowerShell Pipelines, Functions, and Parameters, published on Simple-Talk.com. Enjoy your PowerShell adventures!

Answer 2

Instead of function try filter:
filter Test-Pipeline {
...
}
This is basically a function with a processing block (which you need to process pipeline objects). Alternatively you can write a function with this block and optional begin and end blocks:
function Test-Pipeline {
begin {
}
process {
...
}
end {
}
}
More info: http://technet.microsoft.com/en-us/magazine/hh413265.aspx