Powershell - [AllowEmptyString()] not working -

i have parameter follows:

[parameter(mandatory = false, valuefrompipelinebypropertyname = true)] [allowemptystring()] [validateset("1", "2", "3", "4", "5", "6", "critical", "important", "high", "medium", "low", "lowest")] public string priority { get; set; } 

when run command -priority "", not work

i know can skip argument, real problem me when execute command using pipe import-csv

if csv not have value, import-csv updates parameter value empty , following error:

cannot validate argument on parameter 'priority'. argument "" not belong set "1,2,3,4,5,6,critical,important,high,med ium,low,lowest" specified validateset attribute. supply argument in set , try command again. 

if extend validation set include "", the command if invoked individually runs successfully, still not work if pipe import-csv

i have tried allownull attribute, still same error

update: after discussing garath, appears that

1. allowemptystring not work, instead use "" in validateset

   the question still remains unanswered - why doesnt [allowemptystring()] work?

2. csv files must have trialing commas if there no values

   here seems if trailing commas not there import-csv passes null command , fails validation given validateset

   when trailing commas present, seems empty passed parameter acceptable set includes ""

i simulate problem in simple ps1 script, , working (saved fun2.ps1):

param( [parameter(mandatory=$false, valuefrompipelinebypropertyname=$true)] [string] [validateset("","1", "2", "3", "4", "5", "6")] $a )  write-host $a 

the csv below (doc.txt):

"colname" "2" "" "3" 

and powershell command is:

import-csv .\doc.txt | %{.\fun2.ps1 $_.colname} 

the outuput above commmand is:

ps c:\ps> import-csv .\doc.txt | %{.\fun2.ps1 $_.colname} 2  3 

so remove [allowemptystring()] , change validate set to:

[validateset("", "1", "2", "3", "4", "5", "6", "critical", "important", "high", "medium", "low", "lowest")]