title: System.NullReferenceException When checking if != null
body: <p>I'm using an ASHX handler, i want the handler to check if Session != null.</p>
  
  <pre><code>if (context.Session["Username"] != null)
  </code></pre>
  
  <p>And i get this error pointing this line:</p>
  
  <blockquote>
    <p>System.NullReferenceException: Object reference not set to an instance of an object.</p>
  </blockquote>
  
  <p>What's the problem?</p>