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The main advantage of using the Socket class is that almost all operations can be performed asynchronously, which is useful for large-scale applications. The details of asynchronous I/O
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14. Networking
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are covered in 10, "Streams, Files, and I/O," but the Socket class also allows connections to be made and received in this manner. Table 14-15 details the methods that are available for asynchronous programming in the Socket class.
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Table 14-15. The Asynchronous Methods in the Socket Class
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Method BeginAccept() EndAccept() BeginConnect() EndConnect() BeginReceive() BeginReceiveFrom() EndReceive() EndReceiveFrom() BeginSend() BeginSendTo() EndSend() EndSendTo()
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Synchronous Method Accept() Connect() Receive() ReceiveFrom()
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Description Accepts a connection from a client Creates a connection to a server Reads data from a socket into a byte array
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Send() SendTo()
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Sends data to a connected socket
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Summary
The networking classes in the .NET Framework are broadly comparable to their Java equivalents. The TcpListener, TcpClient, and UdpClient classes are all easy to use but don't expose asynchronous features. The Socket class allows complete control over the connection but is difficult to use and requires a deeper knowledge of socket programming, often exposing the WinSock32 API in a confusing manner. Overall, the networking classes are well designed, and we feel that they are better thought out than the Java equivalents, but there is a gap where the partially abstracted functionality of the Java Socket class would be useful.
15. Remoting
15. Remoting
.NET Remoting is the functional equivalent of the Java Remote Method Invocation (RMI) framework. Both systems allow applications to communicate between processes and machines, enabling objects in one application to manipulate objects in another. RMI was added to Java after the original release of the platform, while the remoting system in .NET has clearly been factored into the platform design from the start. The deep integration into the .NET Framework makes the remoting system easier to use than RMI. Some key differences are:
Remoting doesn't rely on a registry to locate instances of remote classes. Services must be well known, implying that the client must know the location of the remote service at run time. Remoting doesn't require remote operations to be handled via interfaces. Remoting doesn't require stub classes to be compiled. The support for handling remote operations is intrinsic to the CLR.
First Steps
The best way of demonstrating remoting is by example. In this section, we'll build a simple remote service that adds a series of integer values to a running total. The client sends the server an array of integers and receives back a state object that contains the running total. There is little purpose in such contrived functionality, but the simplicity of this remote service will allow us to illustrate the features of the remoting system. All of the code files that we'll create in the following sections should be saved in a single directory. For these examples, the references for the application must include the System.Runtime.Remoting namespace included in the System.Runtime.Remoting.dll assembly. For more information about compiling with assemblies, see 3, "Creating Assemblies."
Creating the Server
First we define a server class to sum the array of integers sent from the client, accumulating a running total. The class is listed here and should be saved to a file named CountServer.cs.
using System; public class CountServer : MarshalByRefObject { private CountState o_state; public CountServer() { o_state = new CountState(); }
15. Remoting public CountState SumNumbers(params int[] p_values) { foreach (int x_value in p_values) { o_state.AddNumber(x_value); } return o_state; } public CountState State { get { return o_state; } }
The parts of this class that relate to remoting are marked in boldface. We indicate that the class can be made available remotely by deriving the class from System.MarshalByRefObject. MarshalByRefObject is discussed in the "Copies and References" section later in this chapter. We keep track of the running total with the CountState class. CountState provides methods to add new values to the total, clear the total, and return the current total. We have annotated CountState with the Serializable attribute, meaning that it will be passed by value across the network. For general information about this attribute, consult 10, "Streams, Files, and I/O"; see the "Copies and References" section in this chapter for details of the impact on remoting. The CountState class should be saved to a file named CountState.cs in the same directory as the CountServer.cs file.
using System; [Serializable] public class CountState { private int o_total; public void AddNumber(int p_value) { o_total += p_value; } public void Clear() { o_total = 0; } public int GetTotal() { return o_total; } }
The CountState and CountServer classes define the functionality for the remoting example, but we still need to define the remoting service. We have created the Start class, shown here, which makes the CountServer type available to remote clients. This class should be saved to a file named Start.cs in the same directory as the previous two files.
15. Remoting using using using using System; System.Runtime.Remoting; System.Runtime.Remoting.Channels; System.Runtime.Remoting.Channels.Http;
class Start { static void Main(string[] p_args) { // create and register the channel HttpChannel x_channel = new HttpChannel(20172); ChannelServices.RegisterChannel(x_channel); // register the count server for remoting RemotingConfiguration.RegisterWellKnownServiceType( typeof(CountServer), "CountServer", WellKnownObjectMode.Singleton); Console.WriteLine("Press return to exit."); Console.ReadLine(); } }
This example introduces the concept of channels. Channels are the means by which clients are able to access the remote service. By using the HttpChannel class and setting the constructor argument to 20172, we're stating that the service will accept client requests via the HTTP protocol on port 20172. We then register the new channel with the ChannelServices class to activate the network settings and make the channels available for use. Channels are discussed in detail in the "Channels" section later in this chapter. After creating the channels, we need to register the CountServer type; until the type is registered, the remoting system won't accept client requests. This is known as publishing and is achieved with the RegisterWellKnownServiceType method of the RemotingConfiguration class. For the details of publishing objects, see the section entitled "Publishing and Activation" later in this chapter.
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