title: Set form as Parent throw exception "Top-level control cannot be added to a control"
body: <p>I want to access variables of a form from another form.  On clicking a button inside my Main form, I want to set my Main form as Parent, then bring up another form (child form) wherein I will access variables of the Main form.  My click handler is as follow:</p>
  
  <pre><code>private void btnSystem_Click(object sender, EventArgs e)
  {
      Form_EnterPassword EP = new Form_EnterPassword();
      EP.Parent = this;        //error: Top-level control cannot be added to a control
      EP.ShowDialog();
  }
  </code></pre>
  
  <p>It compiles fine without any error.  However, when I run the Main form and click on the System button, it throws me an exception.  I do something similar in another code (not mine) with the same button click, and encounter no error (just with setting Main form as Parent).</p>
  
  <p>What am I doing wrong?  Is there something in my Main code that cause this?</p>