networkComms.net2.3.1开源版本，基于gpl V3协议。因为不能公开3.x版本的源码，所以基于此版本进行学习。3.X版本进行了诸多改进和Bug修复，使用方法上两者相差不大。

namespace NetworkCommsDotNet
{

    /// <summary>
    /// Connection对象  这个类是TcpConnection和 UDPConnnection连接类的父类
    /// Connection由以下五个文件组成 大家注意到每个类前面都有个 partial关键字
    /// ConnectionCreate.cs <1>
    /// ConnectionDelegatesHandlers.cs <2>
    /// ConnectionIncomingData.cs <3>
    /// ConnectionSendClose.cs <4>
    /// ConnectionStatic.cs  <5>
    /// </summary>

    public abstract partial class Connection
    {
        static ManualResetEvent workedThreadSignal = new ManualResetEvent(false);
        static volatile bool shutdownWorkerThreads = false;
        static object staticConnectionLocker = new object();
        static Thread connectionKeepAliveWorker;

        /// <summary>
        /// 一些默认的设置
        /// </summary>
        static Connection()
        {
            ConnectionKeepAlivePollIntervalSecs = 30;
            MaxNumSendTimes = 100;
            MinNumSendsBeforeConnectionSpecificSendTimeout = 4;
            MinSendTimeoutMS = 2000;
            MinimumMSPerKBSendTimeout = 20;
            DefaultMSPerKBSendTimeout = 1000;
            NumberOfStDeviationsForWriteTimeout = 3;
        }

        /// <summary>
        /// 每KB数据发送超时的最小时间 默认为10.
        /// </summary>
        public static double MinimumMSPerKBSendTimeout { get; set; }

        /// <summary>
        /// 写入间隔的最大时间  默认100
        /// </summary>
        public static int MaxNumSendTimes { get; set; }

        /// <summary>
        /// The minimum number of writes before the connection specific write timeouts will be used. Default is 3.
        /// </summary>
        public static int MinNumSendsBeforeConnectionSpecificSendTimeout { get; set; }

        /// <summary>
        /// The default milliseconds per KB write timeout before connection specific values become available. Default is 1000. See <see cref="MinNumSendsBeforeConnectionSpecificSendTimeout"/>.
        /// </summary>
        public static int DefaultMSPerKBSendTimeout { get; set; }

        /// <summary>
        /// The minimum timeout for any sized send in milliseconds. Prevents timeouts when sending less than 1KB. Default is 500.
        /// </summary>
        public static int MinSendTimeoutMS { get; set; }

        /// <summary>
        /// The interval between keep alive polls of all connections. Set to int.MaxValue to disable keep alive poll
        /// </summary>
        public static int ConnectionKeepAlivePollIntervalSecs { get; set; }

        /// <summary>
        /// The number of standard deviations from the mean to use for write timeouts. Default is 4.0.
        /// </summary>
        public static double NumberOfStDeviationsForWriteTimeout { get; set; }

        /// 如果 connectionKeepAliveWorker线程没有启动，则启动之 这个线程主要用来进行心跳检测

        protected static void TriggerConnectionKeepAliveThread()
        {
            lock (staticConnectionLocker)
            {
                if (!shutdownWorkerThreads && (connectionKeepAliveWorker == null || connectionKeepAliveWorker.ThreadState == ThreadState.Stopped))
                {
                    connectionKeepAliveWorker = new Thread(ConnectionKeepAliveWorker);
                    connectionKeepAliveWorker.Name = "ConnectionKeepAliveWorker";
                    connectionKeepAliveWorker.Start();
                }
            }
        }

        /// 一个单独的静态工作者线程，用来保持连接是活动的

        private static void ConnectionKeepAliveWorker()
        {
            if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Debug("Connection keep alive polling thread has started.");
            DateTime lastPollCheck = DateTime.Now;

            while (!shutdownWorkerThreads)
            {
                try
                {
                    //We have a short sleep here so that we can exit the thread fairly quickly if we need too
                    if (ConnectionKeepAlivePollIntervalSecs == int.MaxValue)
                        workedThreadSignal.WaitOne(5000);
                    else
                        workedThreadSignal.WaitOne(100);

                    //Check for shutdown here
                    if (shutdownWorkerThreads) break;

                    //Any connections which we have not seen in the last poll interval get tested using a null packet
                    if (ConnectionKeepAlivePollIntervalSecs < int.MaxValue && (DateTime.Now - lastPollCheck).TotalSeconds > (double)ConnectionKeepAlivePollIntervalSecs)
                    {
                        AllConnectionsSendNullPacketKeepAlive();
                        lastPollCheck = DateTime.Now;
                    }
                }
                catch (Exception ex)
                {
                    NetworkComms.LogError(ex, "ConnectionKeepAlivePollError");
                }
            }
        }

        /// <summary>
        /// Polls all existing connections based on ConnectionKeepAlivePollIntervalSecs value. Serverside connections are polled slightly earlier than client side to help reduce potential congestion.
        /// </summary>
        /// <param name="returnImmediately"></param>
        private static void AllConnectionsSendNullPacketKeepAlive(bool returnImmediately = false)
        {
            if (NetworkComms.LoggingEnabled) NetworkComms.Logger.Trace("Starting AllConnectionsSendNullPacketKeepAlive");

            //Loop through all connections and test the alive state
            List<Connection> allConnections = NetworkComms.GetExistingConnection();
            int remainingConnectionCount = allConnections.Count;

#if WINDOWS_PHONE
            QueueItemPriority nullSendPriority = QueueItemPriority.High;
#else
            QueueItemPriority nullSendPriority = QueueItemPriority.AboveNormal;
#endif

            ManualResetEvent allConnectionsComplete = new ManualResetEvent(false);
            for (int i = 0; i < allConnections.Count; i++)
            {
                //We don‘t send null packets to unconnected udp connections
                UDPConnection asUDP = allConnections[i] as UDPConnection;
                if (asUDP != null && asUDP.UDPOptions == UDPOptions.None)
                {
                    if (Interlocked.Decrement(ref remainingConnectionCount) == 0)
                        allConnectionsComplete.Set();

                    continue;
                }
                else
                {
                    int innerIndex = i;
                    NetworkComms.CommsThreadPool.EnqueueItem(nullSendPriority, new WaitCallback((obj) =>
                    {
                        try
                        {
                            //If the connection is server side we poll preferentially
                            if (allConnections[innerIndex] != null)
                            {
                                if (allConnections[innerIndex].ConnectionInfo.ServerSide)
                                {
                                    //We check the last incoming traffic time
                                    //In scenarios where the client is sending us lots of data there is no need to poll
                                    if ((DateTime.Now - allConnections[innerIndex].ConnectionInfo.LastTrafficTime).TotalSeconds > ConnectionKeepAlivePollIntervalSecs)
                                        allConnections[innerIndex].SendNullPacket();
                                }
                                else
                                {
                                    //If we are client side we wait upto an additional 3 seconds to do the poll
                                    //This means the server will probably beat us
                                    if ((DateTime.Now - allConnections[innerIndex].ConnectionInfo.LastTrafficTime).TotalSeconds > ConnectionKeepAlivePollIntervalSecs + 1.0 + (NetworkComms.randomGen.NextDouble() * 2.0))
                                        allConnections[innerIndex].SendNullPacket();
                                }
                            }
                        }
                        catch (Exception) { }
                        finally
                        {
                            if (Interlocked.Decrement(ref remainingConnectionCount) == 0)
                                allConnectionsComplete.Set();
                        }
                    }), null);
                }
            }

            //Max wait is 1 seconds per connection
            if (!returnImmediately && allConnections.Count > 0)
            {
                if (!allConnectionsComplete.WaitOne(allConnections.Count * 2500))
                    //This timeout should not really happen so we are going to log an error if it does
                    NetworkComms.LogError(new TimeoutException("Timeout after " + allConnections.Count.ToString() + " seconds waiting for null packet sends to finish. " + remainingConnectionCount.ToString() + " connection waits remain. This error indicates very high send load or a possible send deadlock."), "NullPacketKeepAliveTimeoutError");
            }
        }

        /// <summary>
        /// Shutdown any static connection components
        /// </summary>
        /// <param name="threadShutdownTimeoutMS"></param>
        internal static void ShutdownBase(int threadShutdownTimeoutMS = 1000)
        {
            try
            {
                shutdownWorkerThreads = true;

                if (connectionKeepAliveWorker != null && !connectionKeepAliveWorker.Join(threadShutdownTimeoutMS))
                    connectionKeepAliveWorker.Abort();
            }
            catch (Exception ex)
            {
                NetworkComms.LogError(ex, "CommsShutdownError");
            }
            finally
            {
                shutdownWorkerThreads = false;
                workedThreadSignal.Reset();
            }
        }
    }
}

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