Unable to modify function argument in Powershell - function

Question

I have a function that is similar to this:
function A-Function{
[CmdletBinding(SupportsShouldProcess=$True)]
param (
[Parameter(Position=0, HelpMessage="A Test string", Mandatory=$true)]
[string]$Path,
[Parameter(Position=1, HelpMessage="The list of file names to download.", ValueFromPipeline=$True)]
[string[]]$testVar,
[Parameter(HelpMessage= "The username")]
[string]$User,
[Parameter(HelpMessage= "The password")]
[string]$Password,
[Parameter(HelpMessage= "The credentials used.")]
[Net.NetworkCredential]$Credential = (New-Object Net.NetworkCredential("Anonymous", ""))
)
Begin {
$Path = "TEST_" + $Path
if ($User) {
if ($Password) {
$Credential = New-Object Net.NetworkCredential($User, $Password)
}
}
}
Process {
$Path
$Credential
}
End {
}
}
If I run "A-Function test -User test -Password -test", I get the output :
TEST_test
UserName Password SecurePassword Domain
-------- -------- -------------- ------
test test System.Security.SecureString
This is what I expected the output to be. However, if I run this command instead:
"Test" | A-Function test -User test -Password -test
I get this output instead:
TEST_test
UserName Password SecurePassword Domain
-------- -------- -------------- ------
Anonymous System.Security.SecureString
In other words, in the second scenario, it hasn't changed the value of the Credential argument in the Begin section, but in the first, it has. I don't understand why this is, can someone explain it?
Regards

Answer 1

If you do a
trace-command parameterbinding {"Test" | A-Function test -User test -Password test} -pshost
you will see that $credential, since it is not being passed as argument to the function, it is bound with the default value each time during process. Path, on the other hand, is bound only once since you pass it to the function and hence the change that you do in begin is available in process.
This is definitely a bug / something not really optimal, as this is not reproduced in Powershell v3. In v3, you get the desired output of credential being test rather than anonymous.
You can fix this for now, of course, by using a local variable in begin or scoping the existing ones with $script:Path etc:
function A-Function{
[CmdletBinding(SupportsShouldProcess=$True)]
param (
[Parameter(Position=0, HelpMessage="A Test string", Mandatory=$true)]
[string]$Path,
[Parameter(Position=1, HelpMessage="The list of file names to download.", ValueFromPipeline=$True)]
[string[]]$testVar,
[Parameter(HelpMessage= "The username")]
[string]$User,
[Parameter(HelpMessage= "The password")]
[string]$Password,
[Parameter(HelpMessage= "The credentials used.")]
[Net.NetworkCredential]$Credential = (New-Object Net.NetworkCredential("Anonymous", ""))
)
Begin {
$script:Path = "TEST_" + $Path
if ($User) {
if ($Password) {
$script:Credential = New-Object Net.NetworkCredential($User, $Password)
}
}
}
Process {
$script:Path
$script:Credential
}
End {
}
}
"Test" | A-Function test -User test -Password test

Answer 2

Yeah, I've been bitten by this. The function parameters get reset every iteration of the Process block. Just set a local variable in your Begin block and then use that local variable in the Process block instead of using the parameter variable.
Edit:
I have to confess that I didn't look very hard at your sample code as I immediately recognized a problem from the combination of the title of your question and the fact that you were setting $Path in the Begin block. Looking at your function a little harder, makes me wonder why you have a Process block when you don't process any pipeline input.
My recollection is that changes you make to the function parameters in the Begin block will survive the first iteration of the Process block. They will only get reset on subsequent iterations (or, perhaps, at the end of the first iteration).