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Programming WCF Services, 3rd Edition by Juval Lowy

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Working with Channels

You can use channels directly to invoke operations on a service without ever resorting to using a proxy class. The ChannelFactory<T> class (and its supporting types), shown in Example 1-21, allows you to create a proxy on the fly.

Example 1-21. The ChannelFactory<T> class

public class ContractDescription
{
   public Type ContractType
   {get;set;}
   //More members
}

public class ServiceEndpoint
{
   public ServiceEndpoint(ContractDescription contract,Binding binding,
                          EndpointAddress address);
   public EndpointAddress Address
   {get;set;}
   public Binding Binding
   {get;set;}
   public ContractDescription Contract
   {get;}
   //More members
}

public abstract class ChannelFactory : ...
{
   public ServiceEndpoint Endpoint
   {get;}
   //More members
}
public class ChannelFactory<T> : ChannelFactory,...
{
   public ChannelFactory(ServiceEndpoint endpoint);
   public ChannelFactory(string configurationName);
   public ChannelFactory(Binding binding,EndpointAddress endpointAddress);

   public static T CreateChannel(Binding binding,EndpointAddress endpointAddress);
   public T CreateChannel();

   //More members
}

You need to provide the constructor of ChannelFactory<T> with the endpoint. This can be the endpoint name from the client config file, the binding and address objects, or a ServiceEndpoint object. Next, use the CreateChannel() method to obtain a reference to the proxy and use its methods. Finally, close the proxy by either casting it to IDisposable and calling the Dispose() method or casting it to ICommunicationObject and calling the Close() method:

ChannelFactory<IMyContract> factory = new ChannelFactory<IMyContract>();

IMyContract proxy1 = factory.CreateChannel();
using(proxy1 as IDisposable)
{
   proxy1.MyMethod();
}

IMyContract proxy2 = factory.CreateChannel();
proxy2.MyMethod();
ICommunicationObject channel = proxy2 as ICommunicationObject;
Debug.Assert(channel != null);
channel.Close();

You can also use the shorthand static CreateChannel() method to create a proxy given a binding and an address without directly constructing an instance of ChannelFactory<T>:

Binding binding = new NetTcpBinding();
EndpointAddress address = new EndpointAddress("net.tcp://localhost:8000");

IMyContract proxy = ChannelFactory<IMyContract>.CreateChannel(binding,address);
using(proxy as IDisposable)
{
   proxy.MyMethod();
}

The InProcFactory Class

To demonstrate the power of ChannelFactory<T>, consider my static helper class InProcFactory, defined as:

public static class InProcFactory
{
   public static I CreateInstance<S,I>() where I : class
                                         where S : I;
   public static void CloseProxy<I>(I instance) where I : class;
   //More members
}

InProcFactory is designed to streamline and automate in-proc hosting. The CreateInstance() method takes two generic type parameters: the type of the service S and the type of the supported contract I. CreateInstance() constrains S to derive from I. Using InProcFactory is straightforward:

IMyContract proxy = InProcFactory.CreateInstance<MyService,IMyContract>();

proxy.MyMethod();

InProcFactory.CloseProxy(proxy);

It literally takes a service class and hoists it up as a WCF service. This is very similar to the C# new operator, as these two lines are equivalent in their coupling to the service type:

IMyContract proxy = InProcFactory.CreateInstance<MyService,IMyContract>();
IMyContract obj = new MyService();

In the case of C#, the compiler verifies that the type supports the requested interface and then, in effect, casts the interface into the variable. In the absence of compiler support, InProcFactory requires the interface type so it will know which interface type to return.

Implementing InProcFactory<T>

All in-proc calls should use named pipes and should flow all transactions. You can use programmatic configuration to automate the configurations of both the client and the service, and use ChannelFactory<T> to avoid the need for a proxy. Example 1-22 shows the implementation of InProcFactory with some of the code removed for brevity.

Example 1-22. The InProcFactory class

public static class InProcFactory
{
   static readonly string BaseAddress = "net.pipe://localhost/" + Guid.NewGuid();
   static readonly Binding Binding;
   static Dictionary<Type,Tuple<ServiceHost,EndpointAddress>> m_Hosts =
                        new Dictionary<Type,Tuple<ServiceHost,EndpointAddress>>();
   static InProcFactory()
   {
      NetNamedPipeBinding binding = new NetNamedPipeBinding();
      binding.TransactionFlow = true;
      Binding = binding;
      AppDomain.CurrentDomain.ProcessExit += delegate
                                             {
                                        foreach(Tuple<ServiceHost,EndpointAddress>
                                                         record in m_Hosts.Values)
                                                   {
                                                      record.Item1.Close();
                                                   }
                                             };
   }
   public static I CreateInstance<S,I>() where I : class
                                         where S : I
   {
      EndpointAddress address = GetAddress<S,I>();
      return ChannelFactory<I>.CreateChannel(Binding,address);
   }
   static EndpointAddress GetAddress<S,I>() where I : class
                                            where S : class,I
   {
      Tuple<ServiceHost,EndpointAddress> record;

      if(m_Hosts.ContainsKey(typeof(S)))
      {
         hostRecord = m_Hosts[typeof(S)];
      }
      else
      {
         ServiceHost host = new ServiceHost(typeof(S));
         string address = BaseAddress + Guid.NewGuid();
         record = new Tuple<ServiceHost,EndpointAddress>(
                                               host,new EndpointAddress(address));
         m_Hosts[typeof(S)] = record;
         host.AddServiceEndpoint(typeof(I),Binding,address);
         host.Open();
      }
      return hostRecord;
   }
   public static void CloseProxy<I>(I instance) where I : class
   {
      ICommunicationObject proxy = instance as ICommunicationObject;
      Debug.Assert(proxy != null);
      proxy.Close();
   }
}

InProcFactory’s static constructor is called once per app domain, allocating in each case a new unique base address using a GUID. This allows you to use InProcFactory multiple times on the same machine, across app domains and processes.

The main challenge facing InProcFactory is that CreateInstance() can be called to instantiate services of every type. For every service type, there should be a single matching host (an instance of ServiceHost). Allocating a host instance for each call is not a good idea. The problem is what CreateInstance() should do when it is asked to instantiate a second object of the same type, like so:

IMyContract proxy1 = InProcFactory.CreateInstance<MyService,IMyContract>();
IMyContract proxy2 = InProcFactory.CreateInstance<MyService,IMyContract>();

The solution is for InProcFactory to internally manage a dictionary that maps service types to a particular host instance and the endpoint address using a tuple. When CreateInstance() is called to create an instance of a particular type, it looks in the dictionary using a helper method called GetAddress(). If the dictionary does not already contain the service type, this helper method creates a host instance for it. If it needs to create a host, GetAddress() programmatically adds an endpoint to that host, using a new GUID as the unique pipe name. GetAddress() stores the new host and its address in the dictionary. CreateInstance() then uses ChannelFactory<T> to create the proxy. In its static constructor, which is called upon the first use of the class, InProcFactory subscribes to the process exit event using an anonymous method to close all hosts when the process shuts down. Finally, to help the clients close the proxy, InProcFactory provides the CloseProxy() method, which queries the proxy to ICommunicationObject and closes it.

The WcfWrapper

If you wish to completely approximate the C# programming model you can wrap the in-proc factory (and thus, all of WCF) with my helper base class WcfWrapper, shown in Example 1-23:

Example 1-23. The WcfWrapper class

public abstract class WcfWrapper<S,I> : IDisposable,ICommunicationObject
                                                         where I : class
                                                         where S : class,I
{
   protected I Proxy
   {get;private set;}

   protected WcfWrapper()
   {
      Proxy = InProcFactory.CreateInstance<S,I>();
   }

   public void Dispose()
   {
      Close();
   }

   public void Close()
   {
      InProcFactory.CloseProxy(Proxy);
   }

   void ICommunicationObject.Close()
   {
      (Proxy as ICommunicationObject).Close();
   }
   //Rest of ICommunicationObject
}

Using WcfWrapper<S,I> is simple—derive from it and the contract and implement the operations on the contract by delegating to the Proxy property. For example, for this service definition:

[ServiceContract]
interface IMyContract
{
   [OperationContract]
   string MyMethod();
}

class MyService : IMyContract
{
   public string MyMethod()
   {...}
}

This is the matching wrapper class:

class MyClass : WcfWrapper<MyService,IMyContract>,IMyContract
{
   public string MyMethod()
   {
      return Proxy.MyMethod();
   }
}

Using the wrapper class is now indistinguishable from regular C# code and yet all the calls are actually WCF calls:

MyClass obj = new MyClass();
string text = obj.MyMethod();
obj.Close();

Appendix A further discusses the profound implications of this programming model.

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