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.Runtime.Remoting andSystem.Runtime.Remoting.Channels.Tcp namespaces.Note that this class, CoHello, derives from MarshalByRefObject.[6] This is the key to distributedcomputing in.NET because it gives this object a distributed identity, allowing the object to bereferenced across application domains, or even process and machine boundaries.A marshal-by-reference object requires a proxy to be set up on the client side and a stub to be set up on the serverside, but since both of these are automatically provided by the infrastructure, you don't have to do anyextra work.Your job is to derive from MarshalByRefObject to get all the support for distributedcomputing.[6]If you fail to do this, your object will not have a distributed identity since the default is marshal-by -value, which means that a copy ofthe remote object is created on the client side.using System;using System.Runtime.Remoting;using System.Runtime.Remoting.Channels;using System.Runtime.Remoting.Channels.Tcp;public class CoHello : MarshalByRefObject{public static void Main( ){TcpChannel channel = new TcpChannel(4000);ChannelServices.RegisterChannel(channel);RemotingConfiguration.RegisterWellKnownServiceType (typeof(CoHello), // Type name"HelloDotNet", // URI69 WellKnownObjectMode.Singleton // SingleCall or Singleton);System.Console.WriteLine("Hit to exit.");System.Console.ReadLine( );}public void SayHello( ){Console.WriteLine("Hello, Universe of.NET");}}The SayHello( ) method is public, meaning that any external client can call this method.As you cansee, this method is very simple, but the interesting thing is that a remote client application (which we'lldevelop shortly) can call it because the Main( ) function uses the TcpChannel class.Look carefully a tMain( ), and you'll see that it instantiates a TcpChannel, passing in a port number from which theserver will listen for incoming requests.[7][7]Believe it or not, all you really have to do is replace TcpChannel with HttpChannel to take advantage of HTPP and SOAP as theunderlying communication protocols.Once we have created a channel object, we then register the channel to the ChannelServices, whichsupports channel registration and object resolution.Having done this, you must then register yourobject with the RemotingConfiguration so that it can be activated you do this by calling theRegisterWellKnownServiceType( ) method of the RemotingConfiguration class.When you call thismethod, you must pass in the class name, a URI, and an object-activation mode.The URI is importantbecause it's a key element that the client application will use to refer specifically to this registeredobject.At the time of this writing, the object-activation mode can be either Singleton, which means thatthe same object will service many calls, or SingleCall, which means an object will service at most onecall.[8] Here's how to build this distributed application:[8]In this example, we've shown you the code to create a channel, register a channel with the channel services, and register your objectwith the remoting configuration, but you don't have to write all this code if you provide an application configuration file with all thisinformation.csc server.csOnce you've done this, you can start the server program, which will wait endlessly until you hit theEnter key.The server is now ready to service client requests.4.2.2 Remote Hello ClientNow that we have a server waiting, let's develop a client to invoke the remote SayHello( ) method.Instead of registering an object with the remoting configuration, we need to activate a remote object.So let's jump into the code now to see how this works.As you examine the following program, notethese items:" We're using types in the System.Runtime.Remoting andSystem.Runtime.Remoting.Channels.Tcp namespaces, since we want to use the TCPchannel." Our Client class doesn't need to derive from anything because it's not a server -side object thatneeds to have a distributed identity." Since we're developing a client application, we don't really need to specify a clien t port whenwe instantiate the TcpChannel.Other than these items, the key thing to note is object activation, shown in the second boldfacestatement in the following code.To invoke remote methods, you must first activate the remote objectand obtain an associated proxy on the client side.To activate the object and get a reference to the70 .NET Framework Essentialsassociated proxy, you call the GetObject( ) method of the Activator class.When you do this, you mustpass along the remote class name and its fully qualified location, including the complete URI.Onceyou've successfully done this, you can then invoke remote methods.using System;using System.Runtime.Remoting;using System.Runtime.Remoting.Channels;using System.Runtime.Remoting.Channels.Tcp;public class Client{public static void Main( ){try{TcpChannel channel = new TcpChannel( );ChannelServices.RegisterChannel(channel);CoHello h = (CoHello) Activator.GetObject(typeof(CoHello), // Remote type"tcp://127.1:4000/HelloDotNet" // Location);h.SayHello( );}catch(Exception e){Console.WriteLine(e.ToString( ));}}}To build this client application, you must include references to the server.exe assembly:csc /r:Server.exe Client.csIf you're familiar with DCOM, you must be relieved to find that it's relatively simple to write distributedapplications in.NET.[9][9]In fact, if you have a copy of Learning DCOM (O'Reilly, 1999) handy, compare these programs with their DCOM counterparts inAppendix D, Hello, Universe, and you will see what we mean.4.2.3 Distributed Garbage CollectorBecause the new distributed garbage collector is so cool, we must briefly cover this facility.Instead ofusing DCOM's delta pinging, which requires few network packets when compared to normal pinging(but still too many for a distributed protocol),.NET remoting u ses leases to manage object lifetimes.Ifyou've ever renewed the lease to an IP address on your Dynamic Host Configuration Protocol (DHCP)network, you've pretty much figured out this mechanism because it's based on similar concepts.In
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