Reply To: Advanced Function Optional Parameter Problem

The problem is probably when you pass a null / empty value to the New-VM cmdlet, based on what I can see so far. You'd need to construct your $params hashtable so that only the defined parameters are in it when you call New-VM. Something like this:

$params = @{
    Name               = $name
    MemoryStartupBytes = $MemoryStartupBytes
    Generation         = 2
    NewVHDPath         = "$VHDFolder\$name\$name.vhdx"
    NewVHDSizeBytes    = $VHDSizeBytes
    BootDevice         = $BootDevice
    ComputerName       = $ComputerName
    Path               = $VMFolder
}
 
if ($SwitchName) { $params['SwitchName'] = $SwitchName }
 
New-VM @params

Also, you'll want to get away from using ValueFromPipeline in combination with String parameters. Any object can be represented as a string, which will give you some crazy results when you start trying to pipe objects to this function. Based on what I see so far, ValueFromPipelineByPropertyName should be all you need.