task.waitAll

 childref.Start();
            var t1 = Task.Run(() =>
            {
                Console.WriteLine("TaskRun启动的线程："+Thread.CurrentThread.ManagedThreadId);
            });
            Task.WaitAll(new Task[] { childref, t1 });
            Console.WriteLine("所有线程结束后："+ Thread.CurrentThread.ManagedThreadId);