Getting an error when checking if a variable is null
<p>I have the following code:</p>

<pre><code>var topicValue = Model.Topic;
var replaceResult = string.Empty;
if (topicValue != null &amp; topic.Contains(topicValue)) {

}
</code></pre>

<p>My intent is for the if not to be executed if topicValue is null. However I am getting an error message saying:</p>

<blockquote>
  <p>Object reference not set to an instance of an object.</p>
</blockquote>

<p>Can anyone explain how I can make this work?</p>
