Allow Integer Parameter to Accept Null in a PowerShell Function

April 6, 2019 powershell

One of the great things about PowerShell being based on .NET is that we get access to the huge number of types built into the framework.

A problem I came across today was that I needed to have a function that took a mandatory integer parameter, but that parameter needed to allow Null. In .NET, there is a generic type System.Nullable<T> that allows other types to take on a null value.

Here’s how I solved it:

# filepath: d:\source\GitHub\PlagueHO\plagueho.github.io\src\posts\2019\04\2019-04-06-allow-integer-parameter-to-accept-null-in-a-powershell-function.md
function Set-AdapterVlan {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory = $true)]
        $Adapter,

        [Parameter(Mandatory = $true)]
        [AllowNull()]
        [Nullable[System.Int32]]
        $VlanId
    )
    
    if ($null -eq $VlanId) {
        # If VlanId is null, clear the VLAN ID from the adapter
        $Adapter | Set-VMNetworkAdapterVlan -Untagged
    }
    else {
        # Otherwise, set the VLAN ID
        $Adapter | Set-VMNetworkAdapterVlan -VlanId $VlanId -Access
    }
}

This allows me to call the function above like this:

Set-AdapterVlan -Adapter $adapter -VlanId $null

Which will clear the VLAN ID from the virtual network adapter.

The magic is in the parameter definition:

[Parameter(Mandatory = $true)]
[AllowNull()]
[Nullable[System.Int32]]
$VlanId

The [AllowNull()] attribute allows the $VlanId parameter to accept a null even though it is mandatory, and the [Nullable[System.Int32]] allows $VlanId to be assigned a null value.

This isn’t something I use often, but I thought it was worth sharing!