How to use a default value with parameter sets in PowerShell?

Question

I have this function

Function Do-ThisOrThat
{
[cmdletbinding()]
param (
    [Parameter(Mandatory = $false, ParameterSetName="First")]
    [string]$FirstParam = "MyFirstParam",

    [Parameter(Mandatory = $false, ParameterSetName="Second")]
    [string]$SecondParam
    )
    Write-Output "Firstparam: $FirstParam. SecondParam $SecondParam"
}

If I call the function like this Do-ThisOrThat I want it to detect that the $FirstParam has a default value and use that. Is it possible? Running it as is only works if I specify a parameter.

e.g. this works: Do-ThisOrThat -FirstParam "Hello"

Answer 1

You need to tell PowerShell which of your parameter sets is the default one:

[cmdletbinding(DefaultParameterSetName="First")]

This will allow you to invoke Do-ThisOrThat without any parameters as well as with a -SecondParameter value, and $FirstParam will have its default value in both cases.

Note, however, that based on how your parameter sets are defined, if you do specify an argument, you can't do so positionally - you must use the parameter name (-FirstParam or -SecondParam).