Q:

Adding a 'select all' function to drop down GUI

I have a drop down menu GUI and can't figure out how to add a "select all months" option. 
The problem is quite complex as the '$group' output is being used for a number of different things such as a get-childitem filter, file path with a variable, and a filtering and organising system using individual file names and ordering them by month.  So is there a way to add an option like.  Numbers 01-12.  Or is there a more sophisticated GUI which allow for easier multi selection of numbers? 

[array]$DropDownArrayItems = "","01","02","03","04","05","06","07","08","09","10","11","12"
[array]$DropDownArray = $DropDownArrayItems | sort

# This Function Returns the Selected Value and Closes the Form

function Return-DropDown {
    if ($DropDown. SelectedItem -eq $null){
        $DropDown. SelectedItem = $DropDown. Items[0]
        $script:Choice = $DropDown. SelectedItem. ToString()
        $Form. Close()
    }
    else{
        $script:Choice = $DropDown. SelectedItem. ToString()
        $Form. Close()
    }
}

function SelectGroup{
    [void] [System. Reflection. Assembly]::LoadWithPartialName("System. Windows. Forms")
    [void] [System. Reflection. Assembly]::LoadWithPartialName("System. Drawing")

    $Form = New-Object System. Windows. Forms. Form

    $Form.width = 300
    $Form.height = 150
    $Form. Text = ”Select Filter Month”

    $DropDown = new-object System. Windows. Forms. ComboBox
    $DropDown. Location = new-object System. Drawing. Size(100,10)
    $DropDown. Size = new-object System. Drawing. Size(130,30)

    ForEach ($Item in $DropDownArray) {
     [void] $DropDown. Items. Add($Item)
    }

    $Form. Controls. Add($DropDown)

    $DropDownLabel = new-object System. Windows. Forms. Label
    $DropDownLabel. Location = new-object System. Drawing. Size(10,10) 
    $DropDownLabel.size = new-object System. Drawing. Size(100,40) 
    $DropDownLabel. Text = "Select Month:"
    $Form. Controls. Add($DropDownLabel)

    $Button = new-object System. Windows. Forms. Button
    $Button. Location = new-object System. Drawing. Size(100,50)
    $Button. Size = new-object System. Drawing. Size(100,20)
    $Button. Text = "OK"
    $Button. Add_Click({Return-DropDown})
    $form. Controls. Add($Button)
    $form. ControlBox = $false

    $Form. Add_Shown({$Form. Activate()})
    [void] $Form. ShowDialog()

    return $script:choice
}

$Group = $null
$Group = SelectGroup
while ($Group -like ""){
    $Group = SelectGroup
}
write-host $Group

A:

Adding an option is easy:
[array]$DropDownArrayItems = "", "01", "02", "03",
"04", "05", "06", "07", "08", "09", "10", 
"11", "12", 'Select all months'

To have an array of integers as output one can do:
if ($Group -eq 'Select all months') {
    1..12 | ForEach-Object { '{0:00}' -f $_ }
}

Thiw will output:
01
02
03
04
05
06
07
08
09
10
11
12

