code 128 barcode asp.net Using TCP/IP Sockets in Font

Printing Data Matrix in Font Using TCP/IP Sockets

TCP/IP sockets provide a low level of control over what crosses over a network. A TCP/IP socket is a logical connection between two computers through which either computer can send or receive data at any time. This connection remains open until it is explicitly closed by either of the computers involved. This provides a high degree of flexibility but raises various issues that you ll examine in this chapter, so unless you really need a very high degree of control, you re better off using the more abstract network protocols you ll look at later in this chapter. The classes you need in order to work with TCP/IP sockets are contained in the namespace System.Net, as summarized in Table 10-1.

This class is used by the server to listen for incoming requests. This class is used by both the client and the server to control how data is sent over a network. This class can be used to both send and receive data over a network. It sends bytes over a network, so it is typically wrapped in another stream type to send text. This class can be used to wrap the NetworkStream class in order to read text from it. The StreamReader provides the methods ReadLine and ReadToEnd, which both return a string of the data contained in the stream. Various different text encodings can be used by supplying an instance of the System.Text.Encoding class when the StreamWriter is created. This class can be used to wrap the NetworkStream class in order to write text to it. The StreamWriter provides the methods Write and WriteLine, which both take a string of the data to be written to the stream. Various different text encodings can be used by supplying an instance of the System.Text.Encoding class when the StreamWriter is created.

In this chapter s first example, you ll build a chat application, consisting of a chat server (shown in Listing 10-1) and a client (shown in Listing 10-2). It is the chat server s job to wait and listen for clients that connect. Once a client connects, it must ask the client to provide a username, and then it must constantly listen for incoming messages from all clients. Once it receives an incoming message, it must push that message out to all clients. It is the job of the client to connect to the server and provide an interface to allow the user to read the messages received and to write messages to send to the other users. The TCP/IP connection works well for this type of application because the connection is always available, and this allows the server to push any incoming messages directly to the client without polling from the client. Listing 10-1. A Chat Server #light open System open System.IO open System.Net open System.Net.Sockets open System.Threading open System.Collections.Generic type ClientTable() = class let clients = new Dictionary<string,StreamWriter>()

/// Add a client and its stream writer member t.Add(name,sw:StreamWriter) = lock clients (fun () -> if clients.ContainsKey(name) then sw.WriteLine("ERROR - Name in use already!") sw.Close() else clients.Add(name,sw)) /// Remove a client and close it, if no one else has done that first member t.Remove(name) = lock clients (fun () -> clients.Remove(name) |> ignore) /// Grab a copy of the current list of clients member t.Current = lock clients (fun () -> clients.Values |> Seq.to_array) /// Check whether a client exists member t.ClientExists(name) = lock clients (fun () -> clients.ContainsKey(name) |> ignore)

end type Server() = class let clients = new ClientTable() let sendMessage name message = let combinedMessage = Printf.sprintf "%s: %s" name message for sw in clients.Current do try lock sw (fun () -> sw.WriteLine(combinedMessage) sw.Flush()) with | _ -> () // Some clients may fail let emptyString s = (s = null || s = "") let handleClient (connection : TcpClient) = let stream = connection.GetStream() let sr = new StreamReader(stream) let sw = new StreamWriter(stream) let rec requestAndReadName() = sw.WriteLine("What is your name "); sw.Flush()