title: What does plus equals(+=) operator means here?
body: <p>I was working out sample code of Windows phone and often I see statements with <code>+=</code> operator.</p>
  
  <p>I know about <em>add assignment operator</em> which does the below operation</p>
  
  <pre><code>+= means a = a + b;  // used for both adding number and string concatenation
  </code></pre>
  
  <p>But this is new to me</p>
  
  <pre><code>phNumChoseTask = new PhoneNumberChooserTask();
  phNumChoseTask.Completed += new EventHandler&lt;PhoneNumberResult&gt;(phoneNumberChooserTask_Completed);
  </code></pre>
  
  <p>Here how does <code>+=</code> works?</p>