即时通信常用手段
1.第三方平台 谷歌、腾讯 环信等多如牛毛，其中谷歌即时通信是免费的，但免费就是免费的并不好用。其他的一些第三方一般收费的，使用要则限流（1s/限制x条消息）要么则限制用户数。
但稳定性什么都还不错，又能将服务压力甩出
2.System.Net.Sockets.Socket，也能写一套较好的服务器端。在.NET 4.5之前用较多，使用起来麻烦。需要对数据包进行解析等操作（但貌似网上有对超长包的处理方法）
3.System.Net.WebSockets.WebSocket，这个，是.NET 4.5出来的东西，对服务器环境也有所要求，IIS8及以上。意味着Windows Server2008R2自带的IIS不支持，Windows8及Server2012以上自带的IIS可以。本文主要将这种方式的实例
完整流程
1).客户端请求连接
ws = new WebSocket(‘ws://‘ + window.location.hostname + ‘:‘ + window.location.port + ‘/Handler1.ashx?user=‘ + $("#user").val());2).服务端获取连接对象并存储到连接池中
CONNECT_POOL.Add(user, socket);
3).连接对象开始监听（每个客户端与服务器保存长链接）
WebSocketReceiveResult result = await socket.ReceiveAsync(buffer, CancellationToken.None);
4).客户端A发送消息给B
ws.send($("#to").val() + "|" + $(‘#content‘).val());5).服务端A的连接对象监听到来自A的消息
string userMsg = Encoding.UTF8.GetString(buffer.Array, 0, result.Count);
6).解析消息体（B|你好我是A）得到接收者ID，根据接收者ID到连接池中查找B的服务端连接对象，并通过B的连接对象将消息推送给B客户端
WebSocket destSocket = CONNECT_POOL[descUser];<br>...<br>...<br>await destSocket.SendAsync(buffer, WebSocketMessageType.Text, true, CancellationToken.None);
7).服务端A连接对象继续监听
WebSocketReceiveResult result = await socket.ReceiveAsync(buffer, CancellationToken.None);
8).B客户端接收到推送过来的消息
ws.onmessage = function (evt) { $(‘#msg‘).append(‘<p>‘ + evt.data + ‘</p>‘);}下面则是完整代码
客户端部分
客户端异常简单，正常情况直接用WebSocket，然后监听WebSocket的几个事件就ok。连接的时候可将当前连接者的ID传入（用户编号），发送消息的时候 采用 “接收者ID|我是消息内容” 这种方式，如“A|A你好，我是B！”
但如用移动端使用还是有一些常见的场景需要处理下的
1：手机关屏幕，IOS关掉屏幕的时候WebSocket会立即失去连接，Android则会等待一段时间才会失去连接。服务器端能检测到失去连接
2：网络不稳定，断网情况WebSocket也不会立即失去连接，服务器端不能知道。（可以服务端设计心跳机制，定时给连接池中的用户发送消息，来检测用户是否保持连接）
3：其他等等...（突然关机、后台结束应用）
无论哪种，客户端在发送消息（或者网络恢复连接、亮屏）的时候可以先判断ws的状态，如果不是连接状态则需要重连（new下即可）
<!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml"><head> ???<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/> ???<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0"/> ???<title></title> ???<script src="jquery-1.11.3.min.js"></script> ???<script> ???????var ws; ???????$().ready(function () { ???????????$(‘#conn‘).click(function () { ???????????????ws = new WebSocket(‘ws://‘ + window.location.hostname + ‘:‘ + window.location.port + ‘/Handler1.ashx?user=‘ + $("#user").val()); ???????????????$(‘#msg‘).append(‘<p>正在连接</p>‘); ???????????????ws.onopen = function () { ???????????????????$(‘#msg‘).append(‘<p>已经连接</p>‘); ???????????????} ???????????????ws.onmessage = function (evt) { ???????????????????$(‘#msg‘).append(‘<p>‘ + evt.data + ‘</p>‘); ???????????????} ???????????????ws.onerror = function (evt) { ???????????????????$(‘#msg‘).append(‘<p>‘ + JSON.stringify(evt) + ‘</p>‘); ???????????????} ???????????????ws.onclose = function () { ???????????????????$(‘#msg‘).append(‘<p>已经关闭</p>‘); ???????????????} ???????????}); ???????????$(‘#close‘).click(function () { ???????????????ws.close(); ???????????}); ???????????$(‘#send‘).click(function () { ???????????????if (ws.readyState == WebSocket.OPEN) { ???????????????????ws.send($("#to").val() + "|" + $(‘#content‘).val()); ???????????????} ???????????????else { ???????????????????$(‘#tips‘).text(‘连接已经关闭‘); ???????????????} ???????????}); ???????}); ???</script></head><body> ???<div> ???????<input id="user" type="text" /> ???????<input id="conn" type="button" value="连接" /> ???????<input id="close" type="button" ?value="关闭"/><br /> ???????<span id="tips"></span> ???????<input id="content" type="text" /> ???????<input id="send" type="button" ?value="发送"/><br /> ???????<input id="to" type="text" />目的用户 ???????<div id="msg"> ???????</div> ???</div></body></html>服务器端部分
服务器端使用Handler（也可用WebAPI）来做，主要用WebSocket的类来实现。代码中都有相对详细的注释，这边只说一些需要注意的问题
1：Dictionary<string,WebSocket> CONNECT_POOL：用户连接池。请求Handler的时候会将当前连接者的用户ID传入，服务器端维护着所有已连接的用户ID和当前用户的WebSocket连接对象
2：Dictionary<string,List<MessageInfo>> MESSAGE_POOL：离线消息池。如果A->B发送消息，B当前因为某种原因没在线（突然断网/黑屏等原因），会将这条消息先保存起来（2天），待B连接后立马将B的离线消息推送给他。（2：MessageInfo：离线Entity。记录当前离线消息的时间、内容）
using System;using System.Collections;using System.Collections.Generic;using System.Linq;using System.Net.WebSockets;using System.Text;using System.Threading;using System.Threading.Tasks;using System.Web;using System.Web.WebSockets;namespace WebApplication1{ ???/// <summary> ???/// 离线消息 ???/// </summary> ???public class MessageInfo ???{ ???????public MessageInfo(DateTime _MsgTime, ArraySegment<byte> _MsgContent) ???????{ ???????????MsgTime = _MsgTime; ???????????MsgContent = _MsgContent; ???????} ???????public DateTime MsgTime { get; set; } ???????public ArraySegment<byte> MsgContent { get; set; } ???} ???/// <summary> ???/// Handler1 的摘要说明 ???/// </summary> ???public class Handler1 : IHttpHandler ???{ ???????private static Dictionary<string, WebSocket> CONNECT_POOL = new Dictionary<string, WebSocket>();//用户连接池 ???????private static Dictionary<string, List<MessageInfo>> MESSAGE_POOL = new Dictionary<string, List<MessageInfo>>();//离线消息池 ???????public void ProcessRequest(HttpContext context) ???????{ ???????????if (context.IsWebSocketRequest) ???????????{ ???????????????context.AcceptWebSocketRequest(ProcessChat); ???????????} ????????} ???????private async Task ProcessChat(AspNetWebSocketContext context) ???????{ ???????????WebSocket socket = context.WebSocket; ???????????string user = context.QueryString["user"].ToString(); ???????????try ???????????{ ???????????????#region 用户添加连接池 ???????????????//第一次open时，添加到连接池中 ???????????????if (!CONNECT_POOL.ContainsKey(user)) ???????????????????CONNECT_POOL.Add(user, socket);//不存在，添加 ???????????????else ???????????????????if (socket != CONNECT_POOL[user])//当前对象不一致，更新 ???????????????????????CONNECT_POOL[user] = socket; ???????????????#endregion ???????????????#region 离线消息处理 ???????????????if (MESSAGE_POOL.ContainsKey(user)) ???????????????{ ???????????????????List<MessageInfo> msgs = MESSAGE_POOL[user]; ???????????????????foreach (MessageInfo item in msgs) ???????????????????{ ???????????????????????await socket.SendAsync(item.MsgContent, WebSocketMessageType.Text, true, CancellationToken.None); ???????????????????} ???????????????????MESSAGE_POOL.Remove(user);//移除离线消息 ???????????????} ???????????????#endregion ???????????????string descUser = string.Empty;//目的用户 ???????????????while (true) ???????????????{ ???????????????????if (socket.State == WebSocketState.Open) ???????????????????{ ???????????????????????ArraySegment<byte> buffer = new ArraySegment<byte>(new byte[2048]); ???????????????????????WebSocketReceiveResult result = await socket.ReceiveAsync(buffer, CancellationToken.None); ???????????????????????????????????????????????#region 消息处理（字符截取、消息转发） ???????????????????????try ???????????????????????{ ???????????????????????????#region 关闭Socket处理，删除连接池 ???????????????????????????if (socket.State != WebSocketState.Open)//连接关闭 ???????????????????????????{ ???????????????????????????????if (CONNECT_POOL.ContainsKey(user)) CONNECT_POOL.Remove(user);//删除连接池 ???????????????????????????????break; ???????????????????????????} ???????????????????????????#endregion ???????????????????????????string userMsg = Encoding.UTF8.GetString(buffer.Array, 0, result.Count);//发送过来的消息 ???????????????????????????string[] msgList = userMsg.Split(‘|‘); ???????????????????????????if (msgList.Length == 2) ???????????????????????????{ ???????????????????????????????if (msgList[0].Trim().Length > 0) ???????????????????????????????????descUser = msgList[0].Trim();//记录消息目的用户 ???????????????????????????????buffer = new ArraySegment<byte>(Encoding.UTF8.GetBytes(msgList[1])); ???????????????????????????} ???????????????????????????else ???????????????????????????????buffer = new ArraySegment<byte>(Encoding.UTF8.GetBytes(userMsg)); ???????????????????????????if (CONNECT_POOL.ContainsKey(descUser))//判断客户端是否在线 ???????????????????????????{ ???????????????????????????????WebSocket destSocket = CONNECT_POOL[descUser];//目的客户端 ???????????????????????????????if (destSocket != null && destSocket.State == WebSocketState.Open) ???????????????????????????????????await destSocket.SendAsync(buffer, WebSocketMessageType.Text, true, CancellationToken.None); ???????????????????????????} ???????????????????????????else ???????????????????????????{ ???????????????????????????????Task.Run(() => ???????????????????????????????{ ???????????????????????????????????if (!MESSAGE_POOL.ContainsKey(descUser))//将用户添加至离线消息池中 ???????????????????????????????????????MESSAGE_POOL.Add(descUser, new List<MessageInfo>()); ???????????????????????????????????MESSAGE_POOL[descUser].Add(new MessageInfo(DateTime.Now, buffer));//添加离线消息 ???????????????????????????????}); ???????????????????????????} ???????????????????????} ???????????????????????catch (Exception exs) ???????????????????????{ ???????????????????????????//消息转发异常处理，本次消息忽略 继续监听接下来的消息 ???????????????????????} ???????????????????????#endregion ???????????????????} ???????????????????else ???????????????????{ ???????????????????????break; ???????????????????} ???????????????}//while end ???????????} ???????????catch (Exception ex) ???????????{ ???????????????//整体异常处理 ???????????????if (CONNECT_POOL.ContainsKey(user)) CONNECT_POOL.Remove(user); ???????????} ???????} ???????public bool IsReusable ???????{ ???????????get ???????????{ ???????????????return false; ???????????} ???????} ???}}PS:对Dictionary的操作可以写两个方案Add和Remove，并用lock，这样子并发会稳定点
from:https://www.cnblogs.com/Tsong/p/6385585.html#undefined
.NET 即时通信，WebSocket服务端实例
原文地址：https://www.cnblogs.com/youmingkuang/p/9273669.html