Question

public class A
{
    public delegate void D();
    public event D E;

    ...
}

class B
{
   void Test()
   {
       A a = new A();
       a.E += () => { ... };

       a = null;
   }
}

Can a be garbage collected when Test() is over or because of event subscription there is still reference to it somewhere?

Answer 1

Yes, a can be GC’d in your example. It won’t cause any problems either. Think about it.

Class1 a = new Class1();
Class2 b = new Class2();

a.SomeEvent += b.SomeMethod;

In this example, a holds a reference to b, but not the other way round. The a.SomeEvent member is like a List<delegate> (or close to it). It holds references to all the subscribers, so they can be called when needed.

So when a needs to be GC’d, it can be. It will be destroyed, along with the list of subscribers. b will remain happily alive without any problems.

The other way won’t work though – b cannot be collected, because a is still holding a reference to it. If it were collected, then a would get a reference to somewhere nonexistant, and the next attempt to raise the event would explode.