Powershell parameter sets and optional parameters - powershell

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.