What does a double question mark do in C#?
<blockquote>
  <p><strong>Possible Duplicates:</strong><br />
  <a href="http://stackoverflow.com/questions/770186/-null-coalescing-operator-what-does-coalescing-mean">?? Null Coalescing Operator --&gt; What does coalescing mean?</a><br />
  <a href="http://stackoverflow.com/questions/446835/what-do-two-question-marks-together-mean-in-c">What do two question marks together mean in C#?</a>  </p>
</blockquote>

<p>I couldn't find this question being asked here so I figured I would ask it. What does a double question mark do in C#?</p>

<p>Example:</p>

<pre><code>x = y ?? z;
</code></pre>
