foo.ps1

[cmdletbinding()]
param (
[parameter(parametersetname='test1')]
[switch]$p1,

[parameter(parametersetname='test2')]
[switch]$p2
)

DynamicParam
{
    switch ($pscmdlet.parametersetname) {

    test1
    {
      $attributeCollection = New-Object -Type System.Collections.ObjectModel.Collection[System.Attribute]

      $attributes = New-Object -Type System.Management.Automation.ParameterAttribute
      $attributes.ParameterSetName = "test1"
      $attributes.Mandatory = $false
      $attributeCollection.Add($attributes)

      $attributes = New-Object -Type System.Management.Automation.ValidateRangeAttribute(1, 3)
      $attributeCollection.Add($attributes)

      $dynParam1 = New-Object -Type System.Management.Automation.RuntimeDefinedParameter("Count", [Int32], $attributeCollection)

      $paramDictionary = New-Object -Type System.Management.Automation.RuntimeDefinedParameterDictionary
      $paramDictionary.Add("Count", $dynParam1)
      return $paramDictionary
    }

    test2
    {
      $attributeCollection = New-Object -Type System.Collections.ObjectModel.Collection[System.Attribute]

      $attributes = New-Object -Type System.Management.Automation.ParameterAttribute
      $attributes.ParameterSetName = "test2"
      $attributes.Mandatory = $false
      $attributeCollection.Add($attributes)

      $attributes = New-Object -Type System.Management.Automation.ValidateRangeAttribute(1, 10)
      $attributeCollection.Add($attributes)

      $dynParam1 = New-Object -Type System.Management.Automation.RuntimeDefinedParameter("Count", [Int32], $attributeCollection)

      $paramDictionary = New-Object -Type System.Management.Automation.RuntimeDefinedParameterDictionary
      $paramDictionary.Add("Count", $dynParam1)
      return $paramDictionary
    }
    }
}


process {
write-output $psboundparameters.Count
}

In fact there may not even be a need to assign a valid to $attributes.ParameterSetName, since in Dave's cmdlet the parameter was available all the time. Even less code!

Thanks guys.

Definitely something to look into over the next few days.