https://stackoverflow.com/questions/2697247/how-to-determine-if-an-event-is-already-subscribed
he 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; }