A curious C# syntax with a question mark
<pre><code>private enum E_Week
{
   Mon = 0,
   Tue,
   . . .
}
</code></pre>

<p>What does the following code mean?</p>

<pre><code>E_Week? week= null;
</code></pre>

<p>Is it equal to the following code? What is the function of the '?' sign here?</p>

<pre><code>E_Week week= null;
</code></pre>
