use-custom-class-as-validator.ps1

# Custom validator for one element
class ValidateElementAttribute : System.Management.Automation.ValidateArgumentsAttribute
{
    [void]  Validate([object]$arguments, [System.Management.Automation.EngineIntrinsics]$engineIntrinsics)
    {

        # Validation happens here
        if ($arguments -ne "foo") {
            throw "you should have chosen foo"
        }
    }
}

# Custom validator for multiple elements
class ValidateElementsAttribute : System.Management.Automation.ValidateEnumeratedArgumentsAttribute
{
    [void]  ValidateElement([object]$arguments)
    {
        
        # Check each element here
        if ($arguments -notmatch "^f") {
            throw "nope... not f*. maybe try 'foo' or 'flute' or 'fsck'"
        }
               
    }
}

function Do-Something
{
    [cmdletbinding()]
    param(
        [ValidateElement()]
        $A,
        [ValidateElements()]
        $B
    )

    if ($A) {
        write-host $A
    }

    if ($B) {
        write-host $B
    }

}

# Throws an exception
Do-Something -A poo

# Does not throw exception
Do-Something -A foo

# Throws exception
Do-Something -B @("foo","food","pooo")

# Does not throw exception
Do-Something -B @("foo","food","flute")