Powershell function Params getting unexpected values when CSV fields are not present - powershell

Question

I am writing a PowerShell function which can take pipeline input. Specifically I am testing it with Import-CSV. Many of the params are not mandatory, which means sometimes the CSV will not have those columns. For boolean values this is working as expected, but with string values, a missing CSV field yields a copy of the row object in the string field.
Here is an example problem parameter:
[Parameter(Mandatory=$False,
ValueFromPipeline=$True,
ValueFromPipelinebyPropertyName=$True,
HelpMessage="TCP Port of the remote server")]
[Alias('Port', 'Remote Server Port')]
[string]$RemoteServerPort = "5500",
Now, if the field is missing, I would expect the value to be "5500" as specified, but instead I get:
$RemoteServerPort = #{Name=KG; IP=10.1.1.1; Username=Admin; Password=}
I've done some looking around, but frankly I'm not even sure what to search for.

Answer 1

This is because you specified ValueFromPipeline=$True so that PoSh coerces the piped object to a string if it cannot bind the parameter by property name. You could solve that by removing ValueFromPipeline=$True from this parameter and introduce another one to be bound to the piped object, i.e. something like this
function MyTestFunc() {
param(
[Parameter(ValueFromPipeline=$True)]
[object]$PipedObj,
[Parameter(Mandatory=$False,
ValueFromPipelinebyPropertyName=$True,
HelpMessage="TCP Port of the remote server")]
[Alias('Port', 'Remote Server Port')]
[string]$RemoteServerPort = "5500"
)
Write-Host "Port: $RemoteServerPort / Obj: $PipedObj"
}
$o1 = [pscustomobject]#{"Server" = "123"; "Port" = "12345"}
$o2 = [pscustomobject]#{"Server" = "1234"; "OS" = "Win3.1"}
$o1 | MyTestFunc
$o2 | MyTestFunc
Will result in
Port: 12345 / Obj: #{Server=123; Port=12345}
Port: 5500 / Obj: #{Server=1234; OS=Win3.1}
A way to see in detail what is actually happening behind the scenes is to use Trace-Command like so
Trace-Command ParameterBinding {$o2 | MyTestFunc} -PSHost