C#

class Program
    {
        readonly static object obj1 = new object();
        readonly static object obj2 = new object();

        static void F1()
        {
            lock (obj1)
                lock (obj2)
                {
                    Console.Write("1");
                }
        }
        static void F2()
        {
            lock (obj2)
                lock (obj1)
                {
                    Console.Write("2");
                }
        }

        static void Main(string[] args)
        {
            Task[] tasks = new[]
            {
                Task.Factory.StartNew(() => { for (int i = 0; i < 100; i++) F1(); }),
                Task.Factory.StartNew(() => { for (int i = 0; i < 100; i++) F2(); }),
            };
            Task.WaitAll(tasks);
            Console.WriteLine();
            Console.WriteLine("all done");
        }
    }

OnBtnAdd()
{
    do_operation();
    //do the operation, and succeeded
    //then I want to destroy this child form.
    //here How to destroy the child form?
}