C#使用委托时可能导致内存泄漏

public class DoA2EventArgs : EventArgs
        {
        }


        public class ClassA
        {
            public delegate void DoAEventHandler(object sender, EventArgs e);
            public event EventHandler<DoA2EventArgs> DoA2EventHandler;  
            public DoAEventHandler DoAEvent;

            protected void OnDoA2Event(DoA2EventArgs args)
            {
                EventHandler<DoA2EventArgs> handler = DoA2EventHandler;

                if (handler != null)
                {
                    handler(this, args);
                }
            }

            public void DoA()
            {
                if (DoAEvent != null)
                {
                    DoAEvent(this, new EventArgs());
                }
            }

            public void DoA2()
            {
                OnDoA2Event(new DoA2EventArgs());
            }

            ~ClassA()
            {
                Console.WriteLine("Class A Finished!");
            }
        }

        public class ClassB
        {
            ClassA a;
            
            public ClassB()
            {
                a = new ClassA();
                a.DoAEvent = DoA;
            }

            public void DoA(object sender, EventArgs e)
            {
            }

            ~ClassB()
            {
                Console.WriteLine("Class B Finished!");
            }
        }

            ClassB b = new ClassB();
            ClassA a = new ClassA();
            a.DoAEvent = b.DoA;
            a.DoA2EventHandler += new EventHandler<DoA2EventArgs>(b.DoA2);
            b = null;

            GC.Collect();

a = null;

b = null;