Use listbox items in function C#

I have listed all directories in a folder in a listbox.

    private void Form1_Load_1(object sender, EventArgs e)
    {
          listBox3.Items.Clear();
          string[] dirs = Directory.GetDirectories("C:\\Users\\foo\\Desktop\\test");

          foreach (string dir in dirs)
          {
            listBox3.Items.Add(dir);
          }
    }

How can I list the subdirectories as well? As if it were the command "tree" of the windows command prompt. thanks

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Link to original article：https://stackoverflow.com/questions/45244389/use-listbox-items-in-function-c-sharp