How can a function tell if a parameter was passed in as an alias, or an object in the pipeline's property was matched as an alias? How can it get the original name?
Suppose my Powershell cmdlet accepts pipeline input and I want to use ValueFromPipelineByPropertyName. I have an alias set up because I might be getting a few different types of objects, and I want to be able to do something slightly different depending on what I receive.
function Test-DogOrCitizenOrComputer
{
[CmdletBinding()]
Param
(
# Way Overloaded Example
[Parameter(Mandatory=$true,
ValueFromPipeline=$true,
ValueFromPipelineByPropertyName=$true,
Position=0)]
[Alias("Country", "Manufacturer")]
[string]$DogBreed,
[Parameter(Mandatory=$true,
ValueFromPipelineByPropertyName=$true,
Position=1)]
[string]$Name
)
# For debugging purposes, since the debugger clobbers stuff
$foo = $MyInvocation
$bar = $PSBoundParameters
# This always matches.
if ($MyInvocation.BoundParameters.ContainsKey('DogBreed')) {
"Greetings, $Name, you are a good dog, you cute little $DogBreed"
}
# These never do.
if ($MyInvocation.BoundParameters.ContainsKey('Country')) {
"Greetings, $Name, proud citizen of $Country"
}
if ($MyInvocation.BoundParameters.ContainsKey('Manufacturer')) {
"Greetings, $Name, future ruler of earth, created by $Manufacturer"
}
}
At first, it seems to work:
PS> Test-DogOrCitizenOrComputer -Name Keith -DogBreed Basset
Greetings, Keith, you are a good dog, you cute little Basset
The problem is apparent when we try an Alias:
PS> Test-DogOrCitizenOrComputer -Name Calculon -Manufacturer HP
Greetings, Calculon, you are a good dog, you cute little HP
Bonus fail, doesn't work via pipeline:
PS> New-Object PSObject -Property @{'Name'='Fred'; 'Country'='USA'} | Test-DogOrCitizenOrComputer
Greetings, Fred, you are a good dog, you cute little USA
PS> New-Object PSObject -Property @{'Name'='HAL'; 'Manufacturer'='IBM'} | Test-DogOrCitizenOrComputer
Greetings, HAL, you are a good dog, you cute little IBM
Both $MyInvocation.BoundParameters and $PSBoundParameters contain the defined parameter names, not any aliases that were matched. I don't see a way to get the real names of arguments matched via alias.
It seems PowerShell is not only being 'helpful' to the user by silently massaging arguments to the right parameters via aliases, but it's also being 'helpful' to the programmer by folding all aliased inputs into the main parameter name. That's fine, but I can't figure out how to determine the actual original parameter passed to the Cmdlet (or the object property passed in via pipeline)
How can a function tell if a parameter was passed in as an alias, or an object in the pipeline's property was matched as an alias? How can it get the original name?
I don't think there is any way for a Function to know if an Alias has been used, but the point is it shouldn't matter. Inside the function you should always refer to the parameter as if its used by it's primary name.
If you need the parameter to act different depending on whether it's used an Alias that is not what an Alias is for and you should instead use different parameters, or a second parameter that acts as a switch.
By the way, if you're doing this because you want to use multiple parameters as ValueFromPipelineByPropertyName, you already can with individual parameters and you don't need to use Aliases to achieve this.
Accepting value from the pipeline by Value does need to be unique, for each different input type (e.g only one string can be by value, one int by value etc.). But accepting pipeline by Name can be enabled for every parameter (because each parameter name is unique).