Why does passing $null to a parameter with AllowNull() result in an error?

Question

Consider the following code:

function Test
{
    [CmdletBinding()]
    param
    (
        [parameter(Mandatory=$true)]
        [AllowNull()]
        [String         


        

Answer 1

You also need to add the [AllowEmptyString()] attribute if you plan on allowing nulls and empty strings.

function Test
{
    [CmdletBinding()]
    param
    (
        [parameter(Mandatory=$true)]
        [AllowNull()]
        [AllowEmptyString()]
        [String]
        $ComputerName
    ) 
    process{}
}

Test -ComputerName $null