FolderBrowserDialog not opening in fsharp script - f#

Question

I am having a simple script to select a folder,
the process is not responding on call to ShowDialog(),
below is script file, runtime config file and cli command to run,
I think I’m missing something, or my understanding is wrong,
test.fsx
#I #"C:\Program Files\dotnet\packs\Microsoft.WindowsDesktop.App.Ref\6.0.10\ref\net6.0"
#r "System.Windows.Forms"
open System
open System.Windows.Forms
let folderPath = new Label()
let button = new Button(Text = "Select Folder")
let ini_dir = #"C:\Users\hayeskev\source\Workspaces"
let openFolderHandler =
EventHandler(fun _ _ ->
let folderDlg = new FolderBrowserDialog()
folderDlg.InitialDirectory <- ini_dir
folderDlg.ShowNewFolderButton <- true
let dlgResult = folderDlg.ShowDialog()
if dlgResult.Equals(DialogResult.OK) then
folderPath.Text <- folderDlg.SelectedPath
else
folderPath.Text <- "Error")
button.Click.AddHandler(openFolderHandler)
let layout = new FlowLayoutPanel(Dock=DockStyle.Fill)
layout.Controls.Add(button)
layout.Controls.Add(folderPath)
let form = new Form()
form.Controls.Add(layout)
form.ShowDialog()
C:\Program Files\dotnet\sdk\6.0.402\FSharp\fsi.runtimeconfig.json
{
"runtimeOptions": {
"tfm": "net6.0",
"frameworks": [
{
"name": "Microsoft.NETCore.App",
"version": "6.0.10"
},
{
"name": "Microsoft.WindowsDesktop.App",
"version": "6.0.10"
}
],
"configProperties": {
"System.Reflection.Metadata.MetadataUpdater.IsSupported": false
}
}
}
running using
dotnet fsi test.fsx
also tried it with sta thread attribute
[<STAThread>]
do
Application.EnableVisualStyles()
Application.SetCompatibleTextRenderingDefault(false)
use form = new MainGuiForm()
Application.Run(form)

Answer 1

I reproduced this using a regular F# project and got the same behavior. To fix it, I had to call AddHandler on the STA thread. So my working code is:
open System
open System.Windows.Forms
let folderPath = new Label()
let button = new Button(Text = "Select Folder")
let ini_dir = #"C:\Users\hayeskev\source\Workspaces"
let openFolderHandler =
EventHandler(fun _ _ ->
let folderDlg = new FolderBrowserDialog()
folderDlg.InitialDirectory <- ini_dir
folderDlg.ShowNewFolderButton <- true
let dlgResult = folderDlg.ShowDialog()
if dlgResult.Equals(DialogResult.OK) then
folderPath.Text <- folderDlg.SelectedPath
else
folderPath.Text <- "Error")
[<STAThread>]
button.Click.AddHandler(openFolderHandler) // IMPORTANT: Must follow <STAThread>
let layout = new FlowLayoutPanel(Dock=DockStyle.Fill)
layout.Controls.Add(button)
layout.Controls.Add(folderPath)
let form = new Form()
form.Controls.Add(layout)
form.ShowDialog() |> ignore
Result looks like: