Meaning of the ? operator in C# for Properties
<blockquote>
  <p><strong>Possible Duplicate:</strong><br>
  <a href="http://stackoverflow.com/questions/3183818/nullable-operator-in-c">? (nullable) operator in C#</a>  </p>
</blockquote>



<p>In System.Windows.Media.Animation I see the code as follows:</p>

<pre><code>    public double? By { get; set; }
</code></pre>

<p>What does the ? operator do here? Does anyone know?</p>

<p>I've tried to google this but it's hard to search for the operator if you don't know what its called by name. I've checked the page on Operators (<a href="http://msdn.microsoft.com/en-us/library/6a71f45d%28v=vs.80%29.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/6a71f45d(v=vs.80).aspx</a>) but the ? operator is not listed there.</p>

<p>Thanks!</p>
