title: How to ensure an event is only subscribed to once
body: <p>I would like to ensure that I only subscribe once in a particular class for an event on an instance.</p>
  
  <p>For example I would like to be able to do the following:</p>
  
  <pre><code>if (*not already subscribed*)
  {
      member.Event += new MemeberClass.Delegate(handler);
  }
  </code></pre>
  
  <p>How would I go about implementing such a guard?</p>