What is ?? in my Property?
<blockquote>
  <p><strong>Possible Duplicate:</strong><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>can any one explain this syntax.</p>

<pre><code>protected string CompanyProductSeriesId
{ 
   get 
   { 
       return Request.QueryString["CPGId"] 
              ?? (ViewState["CPSId"] == null 
                  ? "" 
                  : ViewState["CPGId"].ToString()); 
   } 
}
</code></pre>

<p>I want to under stand the ?? in this syntax.</p>
