title: How to determine if an event is already subscribed
body: <p>In my .NET application I am subscribing to events from another class. The subscription is conditional. I am subscribing to events when the control is visible and de-subscribing it when it become invisible. However, in some conditions I do not want to de-subscribe the event even if the control is not visible as I want the result of an operation which is happening on a background thread.</p>
  
  <p>Is there a way through which I can determine if a class has already subscribed to that event?</p>
  
  <p>I know we can do it in the class which will raise that event by checking the event for <code>null</code>, but how do I do it in a class which will subscribe to that event?</p>