using System;
using System.Threading;

namespace TestProject
{
    class Program
    {
        static void Main(string[] args)
        {
            CreateInstanceOfA();
            Console.ReadKey();
        }

        static void CreateInstanceOfA()
        {
            var a = new A();
        }
    }

    interface IA
    {
        void PrintHello();
    }

    class A : IA
    {
        private B _b;

        public A()
        {
            _b = new B(this);
        }

        ~A()
        {
            Console.WriteLine("Instance of class A had been garbage-collected.");
        }

        public void PrintHello()
        {
            Console.WriteLine("Hello from class A!");
        }
    }

    class B
    {
        private IA _a;
        private bool _working;

        public B(IA a)
        {
            _working = true;
            _a = a;

            var workingThread = new Thread(ThreadProc);
            workingThread.Start();
        }

        ~B()
        {
            _working = false;
            Console.WriteLine("Instance of class B had been garbage-collected.");
        }

        private void ThreadProc()
        {
            while (_working)
            {
                _a.PrintHello();
                Thread.Sleep(100);
            }
        }
    }
}