PowerShell Parameter Set Requires Named Parameter - powershell

Question

I have the following snippet of a functions parameters and their sets
function Test {
[CmdletBinding(DefaultParameterSetName='StringConsole')]
param (
[Parameter(Mandatory,
ValueFromPipelineByPropertyName,
ParameterSetName = 'ObjectFile')]
[Parameter(Mandatory,
ValueFromPipelineByPropertyName,
ParameterSetName = 'StringFile')]
[Alias("PSPath")]
[ValidateNotNullOrEmpty()]
[string]
$Path,
[Parameter(Mandatory,
ValueFromPipeline,
ParameterSetName='StringFile',
Position = 0)]
[Parameter(Mandatory,
ValueFromPipeline,
ParameterSetName='StringConsole',
Position = 0)]
[ValidateNotNullOrEmpty()]
[string]
$Message,
[Parameter(Mandatory,
ValueFromPipeline,
ParameterSetName='ObjectFile',
Position = 0)]
[Parameter(Mandatory,
ValueFromPipeline,
ParameterSetName='ObjectConsole',
Position = 0)]
[ValidateNotNullOrEmpty()]
[object]
$Object,
[Parameter(ParameterSetName='StringFile')]
[Parameter(ParameterSetName='StringConsole')]
[ValidateSet('Information', 'Verbose', 'Warning', 'Error', 'Object')]
[string]
$Severity = 'Information',
[Parameter(ParameterSetName='StringFile')]
[Parameter(ParameterSetName='StringConsole')]
[switch]
$NoPreamble,
[Parameter(ParameterSetName = 'StringConsole')]
[Parameter(ParameterSetName = 'ObjectConsole')]
[switch]
$Console
)
}
If I call the function using
Test 'Hello, World'
it properly uses the StringConsole default parameter set from CmdletBinding
If I call the function using
Test -Message 'Hello, World' -Path C:\SomeFile.txt
It properly uses the StringFile parameter set
But if I call the function using
Test 'Hello, World' -Path C:\SomeFile.txt
I get this error and the function doesn't execute:
Parameter set cannot be resolved using the specified named parameters
The error specifically states it couldn't resolve the parameter set using the NAMED parameters. If a parameter gets bound by position does it not also satisfy the "named" parameter? Or do you have to specifically bind the parameter using the name?
Is there anyway I could design the parameter sets to make my last example work and not throw an error?

Answer 1

The logic used for your parameter sets looks perfectly fine but the issue is that you have 2 parameters with Position = 0 (-Message and -Object), normally this wouldn't be a problem but one of them is of the type System.Object and since all objects inherit from this class, no matter what you pass as argument in position 0 it will match this parameter. Since the other parameter on Position = 0 is of type System.String then 'Hello, World' (a string but also an object) matches both parameter sets and the binder has no idea which one did you mean to use.
A very easy way of seeing this, without changing your current code and just adding $PSCmdlet.ParameterSetName to the function's body, would be to pass an integer as positional parameter and everything works as expected:
function Test {
[CmdletBinding(DefaultParameterSetName='StringConsole')]
param(
# same param block here
)
'Using: ' + $PSCmdlet.ParameterSetName
}
Test 0 -Path C:\SomeFile.txt # => Using: ObjectFile