arrays

PS C:\> Function Test-Array {

Param (
    [parameter(Mandatory=$true)]
    [ValidateSet('a','b')]
    [System.Collections.ArrayList]$Letters = @()
)
    $letters
}

Test-Array -Letters a

Test-Array : Cannot process argument transformation on parameter 'Letters'. Cannot convert the "a" value of type
"System.String" to type "System.Collections.ArrayList".
At line:11 char:21
+ Test-Array -Letters a
+                     ~
    + CategoryInfo          : InvalidData: (:) [Test-Array], ParameterBindingArgumentTransformationException
    + FullyQualifiedErrorId : ParameterArgumentTransformationError,Test-Array