Passing null to a mandatory parameter to a function

Question

Effectively my problem comes down to this:

I can\'t have a function with a mandatory parameter and pass $null to that parameter:

Function Foo
{
    P         


        

Answer 1

You should be able to use the [AllowEmptyString()] parameter attribute for [string] parameters and/or the [AllowNull()] parameter attribute for other types. I've added the [AllowEmptyString()] attribute to the function from your example:

Function Foo {
    Param
    (
        [Parameter(Mandatory = $true)]
        [AllowEmptyString()]  <#-- Add this #>
        [string] $Bar
    )

    Write-Host $Bar
}

Foo -Bar $null

For more info, check out the about_Functions_Advanced_Parameters help topic.

Be aware that PowerShell will coerce a $null value into an instance of some types during parameter binding for mandatory parameters, e.g., [string] $null becomes an empty string and [int] $null becomes 0. To get around that you have a few options:

• Remove the parameter type constraint. You can check for $null in your code and then cast into the type you want.
• Use System.Nullable (see the other answer for this). This will only work for value types.
• Rethink the function design so that you don't have mandatory parameters that should allow null.