title: How to count all open forms of 1 specific class?
body: <p>I have a few Form-classes that derive from <code>Form</code>. And I want to count all open form instances of 1 special class like Form2. It's a simple WinForms application (no Mdi).</p>
  
  <ul>
  <li>Form1 (main)</li>
  <li><strong>Form2</strong> (can be many)</li>
  <li>Form3 (other forms)</li>
  </ul>
  
  <p>The app can't be started multiple times. So it's just about counting the windows in this one application.</p>
  
  <p>My ideas:</p>
  
  <ul>
  <li>Can <code>Application</code> give me a list of open windows ?</li>
  <li>WinApi, enumerate windows of 1 application ?</li>
  <li>put every new Form2 window into a list (<em>I wish to avoid that</em>)</li>
  </ul>