Is it not possible to show a FolderBrowserDialog from a non-UI thread?

Question

I'm facing problem in showing FolderBrowserDialog instance created and called from a non-UI thread. It doesn't get renders properly.

Being more specific, it doesn't shows the folder tree but displays only the Make New Folder OK and Cancel

Answer 1

All the shell dialogs, including FolderBrowserDialog, require the COM apartment for the thread to be set to STA. You are probably missing the Thread.SetApartmentState() call:

    private void button1_Click(object sender, EventArgs e) {
        var t = new Thread(() => new FolderBrowserDialog().ShowDialog());
        t.IsBackground = true;
        t.SetApartmentState(ApartmentState.STA);
        t.Start();
    }

Beware that you cannot set the owner of the dialog, it easily gets lost behind a window of another application. Which makes showing forms or dialogs on a worker thread less than a good idea.