title: what does symbol '?' after type name mean
body: <p>I am using <a href="https://github.com/picoe/Eto" rel="nofollow">Eto gui framework</a>. I saw some magic grammar in their source code;
  for example:</p>
  
  <pre><code>int x;
  int? x;
  void func(int param);
  void func(int? param);
  </code></pre>
  
  <p>What's different? I am confused.
  and the symbol <code>?</code> is hard to google.</p>