C#线程池

 class Program
    {
        // 使用BeginXXX/EndXXX和IAsyncResult对象的方式被称为异步编程模型（APM模式）
        delegate string RunOnThreadPool(out int threadId);

        static void Main(string[] args)
        {
            int threadId = 0;
            // 给委托变量赋值
            RunOnThreadPool poolDelegate = Test;

            var t = new Thread(() => { Test(out threadId); });
            t.Start();
            t.Join();
            Console.WriteLine($"1.返回的线程Id：{threadId}");

            // 通过调用委托变量的BeginInvoke方法来运行委托（执行Test方法）
            IAsyncResult ar = poolDelegate.BeginInvoke(out threadId, Callback, "异步委托调用");
            ar.AsyncWaitHandle.WaitOne();
            // 调用委托的EndInvoke会等待异步操作（Test方法）完成
            // 异步操作执行完成之后，开始执行回掉函数；异步操作和回掉函数很可能会被线程池中同一个工作线程执行
            string result = poolDelegate.EndInvoke(out threadId, ar);
            Console.WriteLine($"2.返回的线程Id：{threadId}");
            Console.WriteLine($"返回值：{result}");

            Console.ReadKey();
        }

        static void Callback(IAsyncResult ar)
        {
            Console.WriteLine("开始执行回掉函数...");
            Console.WriteLine($"异步状态：{ar.AsyncState}");
            Console.WriteLine($"是否为线程池中的线程：{Thread.CurrentThread.IsThreadPoolThread}");
            Console.WriteLine($"线程池工作线程Id：{Thread.CurrentThread.ManagedThreadId}");
        }

        static string Test(out int threadId)
        {
            Console.WriteLine("开始测试方法...");
            Console.WriteLine($"是否为线程池中的线程：{Thread.CurrentThread.IsThreadPoolThread}");
            Thread.Sleep(TimeSpan.FromSeconds(2));
            threadId = Thread.CurrentThread.ManagedThreadId;
            return $"线程Id：{threadId}";
        }
    }

　　向线程池中放入异步操作

 class Program
    {
        static void Main(string[] args)
        {
            ThreadPool.QueueUserWorkItem(AsyncOperation);
            Thread.Sleep(TimeSpan.FromSeconds(2));
            ThreadPool.QueueUserWorkItem(AsyncOperation, "async state");
            Thread.Sleep(TimeSpan.FromSeconds(2));

            ThreadPool.QueueUserWorkItem(state =>
            {
                Console.WriteLine($"操作状态：{state}");
                Console.WriteLine($"工作线程Id：{Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(TimeSpan.FromSeconds(2));
            }, "lambda state");

            const int x = 1;
            const int y = 2;
            const string lambdaState = "lambda state 2";
            // 使用闭包机制，无需传递lambda表达式的状态参数
            ThreadPool.QueueUserWorkItem(_ =>
            {
                Console.WriteLine($"操作状态：{x + y},{lambdaState}");
                Console.WriteLine($"工作线程Id：{Thread.CurrentThread.ManagedThreadId}");
                Thread.Sleep(TimeSpan.FromSeconds(2));
            });

            Console.ReadKey();
        }

        static void AsyncOperation(object state)
        {
            Console.WriteLine($"操作状态：{state ?? "(null)"}");
            Console.WriteLine($"工作线程Id：{Thread.CurrentThread.ManagedThreadId}");
            Thread.Sleep(TimeSpan.FromSeconds(2));
        }
    }

　　线程池与并行度：