How to return a SelectedItems value when listbox items are inside an "add_SelectedValueChanged" block - powershell

Question

After selecting an option from the drop down list ($RCVROption), when choosing an option from the list provided ($listBox), the $listBox.SelectedItems values are always Null. How do I get those values to assign to variable $x
This is the script I tried, my goal is to have the text from $listBox.SelectedItems to be used as a value that represents a file name, so that when you select an option from the list, and click OK, it runs that file.
Add-Type -AssemblyName System.Windows.Forms
Add-Type -AssemblyName System.Drawing
[System.Windows.Forms.Application]::EnableVisualStyles()
$form = New-Object System.Windows.Forms.Form
$form.Text = 'DMP Receiver Tail'
$form.Size = New-Object System.Drawing.Size(400,300)
$form.StartPosition = 'CenterScreen'
$OKButton = New-Object System.Windows.Forms.Button
$OKButton.Location = New-Object System.Drawing.Point(115,220)
$OKButton.Size = New-Object System.Drawing.Size(75,23)
$OKButton.Text = 'OK'
$OKButton.DialogResult = [System.Windows.Forms.DialogResult]::OK
$form.AcceptButton = $OKButton
$form.Controls.Add($OKButton)
$CancelButton = New-Object System.Windows.Forms.Button
$CancelButton.Location = New-Object System.Drawing.Point(190,220)
$CancelButton.Size = New-Object System.Drawing.Size(75,23)
$CancelButton.Text = 'Cancel'
$CancelButton.DialogResult = [System.Windows.Forms.DialogResult]::Cancel
$form.CancelButton = $CancelButton
$form.Controls.Add($CancelButton)
$label = New-Object System.Windows.Forms.Label
$label.Location = New-Object System.Drawing.Point(60,20)
$label.Size = New-Object System.Drawing.Size(280,20)
$label.Text = 'Which Receiver Would You Like To Tail?:'
$form.Controls.Add($label)
$RCVROption = new-object System.Windows.Forms.combobox
$RCVROption.Location = new-object System.Drawing.Size(20,40)
$RCVROption.Size = new-object System.Drawing.Size(335,30)
[void] $RCVROption.Items.Add('Receiver 560')
[void] $RCVROption.Items.Add('Receiver 2560')
$RCVROption.tabIndex = '0'
$RCVROption.DropDownStyle = [System.Windows.Forms.ComboBoxStyle]::DropDownList;
$RCVROption.add_SelectedValueChanged(
{
if($RCVROption.SelectedItem -eq 'Receiver 560')
{
$listBox = New-Object System.Windows.Forms.Listbox
$listBox.Location = New-Object System.Drawing.Point(40,100)
$listBox.Size = New-Object System.Drawing.Size(260,80)
$listBox.SelectionMode = 'MultiExtended'
[void] $listBox.Items.Add('DMP-560-Line1')
[void] $listBox.Items.Add('DMP-560-Line2')
[void] $listBox.Items.Add('DMP-560-Line3')
[void] $listBox.Items.Add('DMP-560-Line4')
[void] $listBox.Items.Add('DMP-560-Line5')
$form.Controls.Add($listBox)
}
ELSEIF($RCVROption.SelectedItem -eq 'Receiver 2560')
{
$listBox = New-Object System.Windows.Forms.Listbox
$listBox.Location = New-Object System.Drawing.Point(40,100)
$listBox.Size = New-Object System.Drawing.Size(260,80)
$listBox.SelectionMode = 'MultiExtended'
[void] $listBox.Items.Add('DMP-2560-Line1')
[void] $listBox.Items.Add('DMP-2560-Line2')
[void] $listBox.Items.Add('DMP-2560-Line3')
[void] $listBox.Items.Add('DMP-2560-Line4')
[void] $listBox.Items.Add('DMP-2560-Line5')
$form.Controls.Add($listBox)
}
}
)
$form.Controls.Add($RCVROption)
$form.Topmost = $true
$result = $form.ShowDialog()
if ($result -eq [System.Windows.Forms.DialogResult]::OK)
{
$x = $listBox.SelectedItems
$Fpath = 'D:\Toolbar\On Call\DMP-Tail\' + $x + '.exe'
Start-process -filepath $FPath
$Fpath
}

Answer 1

Based on your code, I'm guessing you come from a C# background. PowerShell does something weird where it can normally reach outside of script block's scope {code in scriptblock} and use variables that already exist at the time the scriptblock is executed. But it can't easily create or add new variables in the parent/outer scope.
Your code is creating $listBox inside the $RCVROption.add_SelectedValueChanged({code in scriptblock}) event scriptblock, and adding it to the controls for the form, so it continues to exist in the form, but not at the global/script scope of the your code.
I've reworked your code in a format that I've been learning over the last year. Everything you had should still be there, only in a more condensed form. You don't have to use this format, but I personally really like it. How PowerShell converts things like "Cancel" into [System.Windows.Forms.DialogResult]::Cancel is a bit of a mistery to me, but this type of conversion seems very reliable.
This version of the code creates the $listBox prior to firing of the $RCVROption.add_SelectedValueChanged event, clears the items in $listBox, and then populates the ListBox with the new values. The values are in an array, and piped into the [void] $listBox.Items.Add($_) command, executing it once per item in the array.
using namespace System.Windows.Forms
using namespace System.Drawing
Set-StrictMode -Version 3.0
Add-Type -AssemblyName System.Windows.Forms
Add-Type -AssemblyName System.Drawing
[Application]::EnableVisualStyles()
$form = [Form]#{
Text = 'DMP Receiver Tail'
Size = "400,300"
StartPosition = 'CenterScreen'
Topmost = $true
}
$OKButton = [Button]#{
DialogResult = "OK"
Location = "115,220"
Size = "75,23"
Text = 'OK'
}
$form.AcceptButton = $OKButton
$form.Controls.Add($OKButton)
$CancelButton = [Button]#{
DialogResult = "Cancel"
Location = "190,220"
Size = "75,23"
Text = 'Cancel'
}
$form.CancelButton = $CancelButton
$form.Controls.Add($CancelButton)
$label = [Label]#{
Location = "60,20"
Size = "280,20"
Text = 'Which Receiver Would You Like To Tail?:'
}
$form.Controls.Add($label)
$RCVROption = [ComboBox]#{
DropDownStyle = "DropDownList"
Location = "20,40"
Size = "335,30"
tabIndex = '0'
}
[void] $RCVROption.Items.Add('Receiver 560')
[void] $RCVROption.Items.Add('Receiver 2560')
$listBox = [Listbox]#{
Location = "40,100"
SelectionMode = 'MultiExtended'
Size = "260,80"
}
$form.Controls.Add($listBox)
$RCVROption.add_SelectedValueChanged({
if($RCVROption.SelectedItem -eq 'Receiver 560') {
$listBox.Items.Clear()
'DMP-560-Line1', 'DMP-560-Line2', 'DMP-560-Line3', 'DMP-560-Line4', 'DMP-560-Line5' | ForEach-Object {
[void] $listBox.Items.Add($_)
}
}
elseif($RCVROption.SelectedItem -eq 'Receiver 2560') {
$listBox.Items.Clear()
'DMP-2560-Line1', 'DMP-2560-Line2', 'DMP-2560-Line3', 'DMP-2560-Line4', 'DMP-2560-Line5' | ForEach-Object {
[void] $listBox.Items.Add($_)
}
}
})
$form.Controls.Add($RCVROption)
$result = $form.ShowDialog()
if ($result -eq [System.Windows.Forms.DialogResult]::OK) {
$x = $listBox.SelectedItems
$Fpath = 'D:\Toolbar\On Call\DMP-Tail\' + $x + '.exe'
Start-process -filepath $FPath
$Fpath
}