C# syntax ?? in control statement
<p>Hi I have some code here which I dont understand</p>

<pre><code>  public ObservableCollection&lt;Packet&gt; Items
    {
        get
        {
            this.items = this.items ?? this.LoadItems();
            return this.items;
        }
    }
</code></pre>

<p>What does the ?? means?</p>
