How to determine if an event is already subscribed

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.

Is there a way through which I can determine if a class has already subscribed to that event?

I know we can do it in the class which will raise that event by checking the event for null, but how do I do it in a class which will subscribe to that event?

The event keyword was explicitly invented to prevent you from doing what you want to do. It restricts access to the underlying delegate object so nobody can directly mess with the events handler subscriptions that it stores. Events are accessors for a delegate, just like a property is an accessor for a field. A property only permits get and set, an event only permits add and remove.

This keeps your code safe, other code can only remove an event handler if it knows the event handler method and the target object. The C# language puts an extra layer of security in place by not allowing you to name the target object.

And WinForms puts an extra layer of security in place so it becomes difficult even if you use Reflection. It stores delegate instances in an EventHandlerList with a secret "cookie" as the key, you'd have to know the cookie to dig the object out of the list.

Well, don't go there. It is trivial to solve your problem with a bit of code on your end:

private bool mSubscribed;

private void Subscribe(bool enabled)
{
    if (!enabled) textBox1.VisibleChanged -= textBox1_VisibleChanged;
    else if (!mSubscribed) textBox1.VisibleChanged += textBox1_VisibleChanged;

    mSubscribed = enabled;
}

Assuming that you have no access to the innards of the class declaring the event, you have no way to do it directly. Events only expose operators += and -=, nothing else. You will need a flag or some other mechanism in your subscribing class to know whether you are already subscribed or not.

  /// <summary>
  /// Determine if a control has the event visible subscribed to
  /// </summary>
  /// <param name="controlObject">The control to look for the VisibleChanged event</param>
  /// <returns>True if the control is subscribed to a VisibleChanged event, False otherwise</returns>
  private bool IsSubscribed(Control controlObject)
  {
     FieldInfo event_visible_field_info = typeof(Control).GetField("EventVisible",
        BindingFlags.Static | BindingFlags.NonPublic);
     object object_value = event_visible_field_info.GetValue(controlObject);
     PropertyInfo events_property_info = controlObject.GetType().GetProperty("Events",
        BindingFlags.NonPublic | BindingFlags.Instance);
     EventHandlerList event_list = (EventHandlerList)events_property_info.GetValue(controlObject, null);
     return (event_list[object_value] != null);
  }

Simply check whether the control is visible or not whenever the event handler is triggered.