Powershell parameter sets and optional parameters

Question

I'm writing a function for which two parameters should be exclusive and optional.

Here are valid inputs:

new-event -Title sometitle -Text sometext -TimestampHappened 1234567 -SomeOtherOptionalParam somestring

new-event -Title sometitle -Text sometext -DateHappened (get-date) -SomeOtherOptionalParam somestring

new-event -Title sometitle -Text sometext -SomeOtherOptionalParam somestring

new-event -Title sometitle -Text sometext

Here is an invalid input:

new-event -Title sometitle -Text sometext -DateHappened (get-date) -TimestampHappened 1234567 -SomeOtherOptionalParam somestring

Here is my code so far:

[CmdletBinding()]

# Most parameters belong to Default, New-Event:ByDate and New-Event:ByTimestamp parameter sets
param (
     [Parameter(
        Position=0,
        Mandatory=$True,
        ParameterSetName="Default"
    )]
    [Parameter(
        Position=0,
        Mandatory=$True,
        ParameterSetName="New-Event:ByDate"
    )]
    [Parameter(
        Position=0,
        Mandatory=$True,
        ParameterSetName="New-Event:ByTimestamp"
    )]
    [ValidateNotNullOrEmpty()]
    [String]$Title,

[Parameter(
        Position=1,
        Mandatory=$True,
        ParameterSetName="Default"
    )]
    [Parameter(
        Position=1,
        Mandatory=$True,
        ParameterSetName="New-Event:ByDate"
    )]
    [Parameter(
        Position=1,
        Mandatory=$True,
        ParameterSetName="New-Event:ByTimestamp"
    )]
    [ValidateNotNullOrEmpty()]
    [String]$Text,

 [Parameter(
        Position=2,
        Mandatory=$False,
        ParameterSetName="New-Event:ByDate"
    )]
    [ValidateNotNullOrEmpty()]
    [datetime]$DateHappened,

[Parameter(
    Position=2,
    Mandatory=$False,
    ParameterSetName="New-Event:ByTimestamp"
    )]
    [ValidateNotNullOrEmpty()]
    [Double]$TimestampHappened,


   [Parameter(
        Position=3,
        Mandatory=$False,
        ParameterSetName="Default"
    )]
   [Parameter(
        Position=3,
        Mandatory=$False,
        ParameterSetName="New-Event:ByDate"
    )]
    [Parameter(
        Position=3,
        Mandatory=$False,
        ParameterSetName="New-Event:ByTimestamp"
    )]
    [String]$SomeOtherParam,
    ...

Here is what I get when I call Get-Help:

PS> get-help New-Event

NAME
    New-Event

SYNOPSIS
    Post an event to the stream.


SYNTAX
    New-Event [-Title] <String> [-Text] <String> [[-TimestampHappened] <Double>] [[-Priority] <String>] [[-Hostname] <String>] [[-Tags] <String[]>] [[-AlertType] <String>] [<CommonParameters>]

    New-Event [-Title] <String> [-Text] <String> [[-DateHappened] <DateTime>] [[-Priority] <String>] [[-Hostname] <String>] [[-Tags] <String[]>] [[-AlertType] <String>]  <String>] [<CommonParameters>]

    New-Event [-Title] <String> [-Text] <String> [[-Priority] <String>] [[-Hostname] <String>] [[-Tags] <String[]>] [[-AlertType] <String>] [<CommonParameters>]

However here is the error I get when I try to call the function with only the two mandatory parameters:

New-Event -Title test -Text text
New-Event : Parameter set cannot be resolved using the specified named parameters.
At line:1 char:1
+ New-Event -Title test -Text text
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidArgument: (:) [New-Event], ParameterBindingException
    + FullyQualifiedErrorId : AmbiguousParameterSet,New-Event

I'm missing something here, but I can't figure out what...

How can I get two parameters that are mutually exclusive and optional?

Answer 1

This makes perfect sense. You have 3 parameter sets, and the 2 mandatory parameters are included on every set. How could PowerShell determine which set you meant to use?

Luckily the [CmdletBinding()] attribute can take a parameter that helps with this exact case: DefaultParameterSetName. Setting this allows PowerShell to use this set in the case of (certain) ambiguities. Use it like so:

[CmdletBinding(DefaultParameterSetName='Default')]

Note that in this case, you named it default; it could have been named anything.