title: How do I get all controls of a form in Windows Forms?
body: <p>I have a <code>Form</code> named <code>A</code>.</p>
  
  <p><code>A</code> contains lots of different controls, including a main <code>GroupBox</code>. This <code>GroupBox</code> contains lots of tables and others <code>GroupBox</code>es. I want to find a control which has e.g. tab index 9 in form <code>A</code>, but I don't know which <code>GroupBox</code> contains this control.</p>
  
  <p>How can I do this?</p>