WCF

WCF 提供了三种实例上下文模式：PreCall、PreSession 以及 Single。开发人员通过 ServiceBehavior.InstanceContextMode 就可以很容易地控制服务对象的实例管理模式。而当 WCF 释放服务对象时，会检查该对象是否实现了 IDisposable 接口，并调用其 Dispose 方法，以便及时释放相关资源，同时也便于我们观察对象释放行为。

1. PreCall

在 PreCall 模式下，即便使用同一个代理对象，也会为每次调用创建一个服务实例。调用结束后，服务实例被立即释放(非垃圾回收)。对于不支持 Session 的 Binding，如 BasicHttpBinding，其缺省行为就是 PreCall。

[ServiceContract]
public interface IMyService
{
    [OperationContract]
    void Test();
}

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
public class MyServie : IMyService, IDisposable
{
    public MyServie()
    {
        Console.WriteLine("Constructor:{0}", this.GetHashCode());
    }

    [OperationBehavior]
    public void Test()
    {
        Console.WriteLine("Test:{0}", OperationContext.Current.SessionId);
    }

    public void Dispose()
    {
        Console.WriteLine("Dispose");
    }
}

public class WcfTest
{
    public static void Test()
    {
        AppDomain.CreateDomain("Server").DoCallBack(delegate
        {
            ServiceHost host = new ServiceHost(typeof(MyServie), new Uri("http://localhost:8080/MyService"));
            host.AddServiceEndpoint(typeof(IMyService), new WSHttpBinding(), "");
            host.Open();
        });

        //-----------------------

        IMyService channel = ChannelFactory<IMyService>.CreateChannel(new WSHttpBinding(),
            new EndpointAddress("http://localhost:8080/MyService"));

        using (channel as IDisposable)
        {
            channel.Test();
            channel.Test();
        }
    }
}

输出：
Constructor:30136159
Test:urn:uuid:df549447-52ba-4c54-9432-31a7a533d9b4
Dispose
Constructor:41153804
Test:urn:uuid:df549447-52ba-4c54-9432-31a7a533d9b4
Dispose

2. PreSession

PreSession 模式需要绑定到支持 Session 的 Binding 对象。在客户端代理触发终止操作前，WCF 为每个客户端维持同一个服务对象，因此 PreSession 模式可用来保持调用状态。也正因为如此，PreSession 在大并发服务上使用时要非常小心，避免造成服务器过度负担。虽然支持 Session 的 Binding 对象缺省就会启用 PreSession 模式，但依然建议你强制指定 SessionMode.Required 和 InstanceContextMode.PerSession。

[ServiceContract(SessionMode = SessionMode.Required)]
public interface IMyService
{
    [OperationContract]
    void Test();
}

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)]
public class MyServie : IMyService, IDisposable
{
    public MyServie()
    {
        Console.WriteLine("Constructor:{0}", this.GetHashCode());
    }

    [OperationBehavior]
    public void Test()
    {
        Console.WriteLine("Test:{0}", OperationContext.Current.SessionId);
    }

    public void Dispose()
    {
        Console.WriteLine("Dispose");
    }
}

public class WcfTest
{
    public static void Test()
    {
        AppDomain.CreateDomain("Server").DoCallBack(delegate
        {
            ServiceHost host = new ServiceHost(typeof(MyServie), new Uri("http://localhost:8080/MyService"));
            host.AddServiceEndpoint(typeof(IMyService), new WSHttpBinding(), "");
            host.Open();
        });

        //-----------------------

        IMyService channel = ChannelFactory<IMyService>.CreateChannel(new WSHttpBinding(), 
            new EndpointAddress("http://localhost:8080/MyService"));
            
        using (channel as IDisposable)
        {
            channel.Test();
            channel.Test();
        }
    }
}

输出：
Constructor:30136159
Test:urn:uuid:2f01b61d-40c6-4f1b-a4d6-4f4bc3e8847a
Test:urn:uuid:2f01b61d-40c6-4f1b-a4d6-4f4bc3e8847a
Dispose

3. Single

一如其名，服务器会在启动时，创建一个唯一(Singleton)的服务对象。这个对象为所有的客户端服务，并不会随客户端终止而释放。

[ServiceContract]
public interface IMyService
{
    [OperationContract]
    void Test();
}

[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class MyServie : IMyService, IDisposable
{
    public MyServie()
    {
        Console.WriteLine("Constructor:{0}; {1}", DateTime.Now, this.GetHashCode());
    }

    [OperationBehavior]
    public void Test()
    {
        Console.WriteLine("Test:{0}; {1}", DateTime.Now, OperationContext.Current.SessionId);
    }

    public void Dispose()
    {
        Console.WriteLine("Dispose:{0}", DateTime.Now);
    }
}

public class WcfTest
{
    public static void Test()
    {
        AppDomain.CreateDomain("Server").DoCallBack(delegate
        {
            ServiceHost host = new ServiceHost(typeof(MyServie), new Uri("http://localhost:8080/MyService"));
            host.AddServiceEndpoint(typeof(IMyService), new BasicHttpBinding(), "");
            host.Open();
        });

        //-----------------------

        for (int i = 0; i < 2; i++)
        {
            IMyService channel = ChannelFactory<IMyService>.CreateChannel(new BasicHttpBinding(), 
                new EndpointAddress("http://localhost:8080/MyService"));
                
            using (channel as IDisposable)
            {
                channel.Test();
                channel.Test();
            }
        }
    }
}

输出：

Constructor:2007-4-17 17:31:01; 63238509
Test:2007-4-17 17:31:03;
Test:2007-4-17 17:31:03;
Test:2007-4-17 17:31:03;
Test:2007-4-17 17:31:03;

还有另外一种方式来启动 Single ServiceHost。

AppDomain.CreateDomain("Server").DoCallBack(delegate
{
    MyServie service = new MyServie();

    ServiceHost host = new ServiceHost(service, new Uri("http://localhost:8080/MyService"));
    host.AddServiceEndpoint(typeof(IMyService), new BasicHttpBinding(), "");
    host.Open();
});

此方式最大的好处是允许我们使用非默认构造，除此之外，和上面的例子并没有什么区别。

需要特别注意的是，缺省情况下，Single 会对服务方法进行并发控制。也就是说，多个客户端需要排队等待，直到排在前面的其他客户端调用完成后才能继续。看下面的例子。

[ServiceContract]
public interface IMyService
{
    [OperationContract]
    void Test();
}

[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single)]
public class MyServie : IMyService, IDisposable
{
    public MyServie()
    {
        Console.WriteLine("Constructor:{0}", this.GetHashCode());
    }

    [OperationBehavior]
    public void Test()
    {
        Console.WriteLine("Test:{0}", OperationContext.Current.SessionId);
        Thread.Sleep(2000);
        Console.WriteLine("Test End:{0}", OperationContext.Current.SessionId);
    }

    public void Dispose()
    {
        Console.WriteLine("Dispose");
    }
}

public class WcfTest
{
    public static void Test()
    {
        AppDomain.CreateDomain("Server").DoCallBack(delegate
        {
            ServiceHost host = new ServiceHost(typeof(MyServie), new Uri("http://localhost:8080/MyService"));
            host.AddServiceEndpoint(typeof(IMyService), new WSHttpBinding(), "");
            host.Open();
        });

        //-----------------------

        for (int i = 0; i < 2; i++)
        {
            new Thread(delegate()
            {
                IMyService channel = ChannelFactory<IMyService>.CreateChannel(new WSHttpBinding(), 
                    new EndpointAddress("http://localhost:8080/MyService"));
                    
                using (channel as IDisposable)
                {
                    while (true)
                    {
                        channel.Test();
                    }
                }
            }).Start();
        }
    }
}

输出：
Constructor:63238509
Test:urn:uuid:d613cfce-d454-40c9-9f4d-62b30a93ff27
Test End:urn:uuid:d613cfce-d454-40c9-9f4d-62b30a93ff27
Test:urn:uuid:313de31e-96b8-47c2-8cf3-7ae743236dfc
Test End:urn:uuid:313de31e-96b8-47c2-8cf3-7ae743236dfc
Test:urn:uuid:d613cfce-d454-40c9-9f4d-62b30a93ff27
Test End:urn:uuid:d613cfce-d454-40c9-9f4d-62b30a93ff27
Test:urn:uuid:313de31e-96b8-47c2-8cf3-7ae743236dfc
Test End:urn:uuid:313de31e-96b8-47c2-8cf3-7ae743236dfc
Test:urn:uuid:d613cfce-d454-40c9-9f4d-62b30a93ff27
Test End:urn:uuid:d613cfce-d454-40c9-9f4d-62b30a93ff27
Test:urn:uuid:313de31e-96b8-47c2-8cf3-7ae743236dfc
Test End:urn:uuid:313de31e-96b8-47c2-8cf3-7ae743236dfc
...

我们可以通过修改并发模式(ConcurrencyMode)来改变这种行为，但需要自己维护多线程安全。

[ServiceBehavior(InstanceContextMode=InstanceContextMode.Single, ConcurrencyMode=ConcurrencyMode.Multiple)]
public class MyServie : IMyService, IDisposable
{
    //...
}

输出：
Constructor:10261382
Test:urn:uuid:2bdac798-f774-40f2-b8f9-58de8d599d3c
Test:urn:uuid:9a328d0e-8df7-4885-94ed-e04ceedc5a09
Test End:urn:uuid:2bdac798-f774-40f2-b8f9-58de8d599d3c
Test:urn:uuid:2bdac798-f774-40f2-b8f9-58de8d599d3c
Test End:urn:uuid:9a328d0e-8df7-4885-94ed-e04ceedc5a09
Test:urn:uuid:9a328d0e-8df7-4885-94ed-e04ceedc5a09
Test End:urn:uuid:2bdac798-f774-40f2-b8f9-58de8d599d3c
Test:urn:uuid:2bdac798-f774-40f2-b8f9-58de8d599d3c
Test End:urn:uuid:9a328d0e-8df7-4885-94ed-e04ceedc5a09
Test:urn:uuid:9a328d0e-8df7-4885-94ed-e04ceedc5a09
Test End:urn:uuid:2bdac798-f774-40f2-b8f9-58de8d599d3c
Test:urn:uuid:2bdac798-f774-40f2-b8f9-58de8d599d3c
Test End:urn:uuid:9a328d0e-8df7-4885-94ed-e04ceedc5a09
Test:urn:uuid:9a328d0e-8df7-4885-94ed-e04ceedc5a09