当ServiceContract非常多的情况下，比如要self-host hundred of service的时候，or更多的时候，每次hosting都是建立一个tcp listen.这样，host的init工作会占用非常多的时间和资源。对于一些对start有较快需求的case,这明显会影响用户体验。本文意在寻求一种技术手段解决这个问题，让service只有在运行时才host。我们姑且称之为host just in time吧

先说一下思路。

client如果能发送消息给service,并且消息可达，先决条件就是消息到达service之前，service已经被host,注意，是说消息到达之前，而并不是在客户端发送消息之后，这就给我们提供了一个可以做手脚的时期，即消息从客户端发送出去到消息在被服务端接收之前这段时间，如果我们能Inspect它，并根据其self description，便能在消息发送到真正的服务之前，host那个service,而做这样一个事情，rounter明显是最在行的了。下面是实现host just in time的原理图。

按照上面的思路，做了一个简单的demo，过程如下

首先创建Contracts项目Jillzhang.Wcf.HostJIT.Contracts，包含两个ServiceContract:ICalculator和IActivity，代码分别为：

ICalculator：

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.ServiceModel;namespace Jillzhang.Wcf.HostJIT.Contracts{[ServiceContract]public interface  ICalculator{[OperationContract]int Add(int a, int b);}}

IActivity：

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.ServiceModel;namespace Jillzhang.Wcf.HostJIT.Contracts{[ServiceContract]public interface IActivity{[OperationContract]void Excute();}}

demo只模拟对这两个ServiceContract的host just in time

建立好契约项目之后，新建服务项目:Jillzhang.Wcf.HostJIT.Services,添加两个类：Caculator和Activity，分别实现ICalculator和IActivity，其代码如下：

Caculator：

using System;using System.Collections.Generic;using System.Linq;using System.Text;using Jillzhang.Wcf.HostJIT.Contracts;namespace Jillzhang.Wcf.HostJIT.Services{public class Calculator : ICalculator{public int Add(int a, int b){return a + b;}}}

Activity：

using System;using System.Collections.Generic;using System.Linq;using System.Text;using Jillzhang.Wcf.HostJIT.Contracts;namespace Jillzhang.Wcf.HostJIT.Services{public class Activity:IActivity{public void Excute(){Console.WriteLine("activity is running!");}}}

接着，我们需要简历本实例中最重要的项目：Jillzhang.Wcf.HostJIT.Dispatcher，这个项目的release自身就是一个对wcf service的self-host的hosting app.为了实现路由，截获以及调度的功能，实现了路由接口IRounter和类Dispatcher，下面是他们的code:

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.ServiceModel;using System.ServiceModel.Channels;using System.Collections;using Jillzhang.Wcf.HostJIT.Services;namespace Jillzhang.Wcf.HostJIT.Dispatcher{[ServiceContract]public interface IRounter{[OperationContract(Action = "*", ReplyAction = "*")]Message ProcessMessage(Message msg);}[ServiceBehavior(AddressFilterMode = AddressFilterMode.Any)]public class Dispatcher : IRounter{public static readonly Hashtable hostTable = new Hashtable();public Message ProcessMessage(Message msg){string to = msg.Headers.To.ToString();string serviceName = "";//todo:if no host,host firstif (to.IndexOf("8031") > -1){if (!hostTable.ContainsKey("Calculator")){ServiceHost host = new ServiceHost(typeof(Calculator));host.Opened += new EventHandler(delegate(object sender, EventArgs arg){Console.WriteLine("Calculator service is opened!");});host.Open();hostTable.Add("Calculator", host);}serviceName = "Calculator";}else{if (!hostTable.ContainsKey("Activity")){ServiceHost host = new ServiceHost(typeof(Activity));host.Opened += new EventHandler(delegate(object sender, EventArgs arg){Console.WriteLine("activity service is opened!");});host.Open();hostTable.Add("Activity", host);}serviceName = "Activity";}//todo:rounter the messageusing (ChannelFactory
    
      channelFactory = new ChannelFactory
     
      (serviceName)){var chanenl = channelFactory.CreateChannel();using (chanenl as IDisposable){return chanenl.ProcessMessage(msg);}}}}}
     
    

通过代码可以看出，首先Dispatcher是一个有路由功能的服务，它会根据message中Header中的To判断要调用的Service是哪一个，然后判断是否已经承载了那个最终目的的Service，如果没有，host frist and add it to hostTable ,host之后，就可以将消息调度给真正的服务了。

app.config的代码如下：

而在这个项目中，我们开始只需要host IRounter契约。host代码为：

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.ServiceModel;namespace Jillzhang.Wcf.HostJIT.Dispatcher{class Program{static void Main(string[] args){using (ServiceHost host = new ServiceHost(typeof(Dispatcher))){host.Opened += new EventHandler(delegate(object sender, EventArgs arg){Console.WriteLine("dispatcher service is opened!");});host.Open();Console.Read();}}}}

建立客户端程序Jillzhang.Wcf.HostJIT.Client，并且将app.config更改为：

Programe.cs为：

using System;using System.Collections.Generic;using System.Linq;using System.Text;using System.ServiceModel;using Jillzhang.Wcf.HostJIT.Contracts;namespace Jillzhang.Wcf.HostJIT.Client{class Program{static void Main(string[] args){Console.WriteLine("press any key when dispatcher service is ok!");Console.Read();CallCalculator();CallActivity();CallCalculator();CallActivity();Console.ReadKey(true);}static void CallCalculator(){using (ChannelFactory
    
      channelFactory = new ChannelFactory
     
      ("Calculator")){var channel = channelFactory.CreateChannel();using (channel as IDisposable){Console.WriteLine(channel.Add(1, 2).ToString());}}}static void CallActivity(){using (ChannelFactory
      
        channelFactory2 = new ChannelFactory
       
        ("Activity")){var channel = channelFactory2.CreateChannel();using (channel as IDisposable){channel.Excute();}}}}}
       
      
     
    

服务端：

最终验证效果和原来想象中一致。OK!

demo项目：