The Network. Multithreading. This tutorial can be found on -

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1 This tutorial can be found on - Instant messaging is sweeping the world, and is rapidly replacing as the preferred electronic communications means. Behind this phenomenon, however, is a rather simple technology, which Chris Payne discusses (with step-by-step building instructions). Instant messaging is sweeping the world it is rapidly replacing as the preferred electronic communications means. Behind this phenomenon, however, is a rather simple technology, especially with.net's new programming paradigm. This article will give some background on the technologies needed to build an instant messenger, as well as build a simple one step by step. The Network The first thing that comes to mind when thinking about an instant messaging application (IM) is the Internet; after all, to send an instant message, you need to deal with a network. The System.Netnamespace in the.net Framework contains all the classes you'll ever need to handle network requests of any type (and for IM, you don't need many)..net introduces pluggable protocols, which is a way to create an application that can handle any type of Web request using only one class..net automatically determines the type of network traffic that is going on, and adjusts appropriately taking a lot of work away from you, the developer. In this article, we'll look at a few classes in the System.Net.Sockets namespace, which provides a more sophisticated interface for dealing with network traffic. You'll find, though, that even these are very simple. For an IM application, there are two classes we're interested in. System.Net.Sockets.TcpClientand System.Net.Sockets.TcpListener provide methods for you to send and receive data, respectively, using the TCP protocol. All you need to know is the IP address of the person you want to communicate with, and you're set to go. But wait a moment. An application cannot do two things at once. That is, your application cannot both send messages and receive them at the same time, which is a key feature of instant messaging. Somehow, you have to devise a way to accomplish two things at once we'll discuss this in the next section. Multithreading Everyone's heard of multithreading, but not many people know exactly what it means, much less how to do it properly. Every Windows application,.net or otherwise, runs in a thread, the basic component of processing. The operating system doles out resources to threads, and all execution takes place within a thread. All applications use threads, and many use only a single thread (for example, Notepad). As its name implies, a thread is linear. Only one thing can be going on at a time in a thread because a CPU can process only one thing at a time (though it does so very quickly). This is why most applications can only do one thing at a time why you can't send and receive IMs at the same time. With Windows, there's no rule that says an application can have only one thread. An application will always have a main thread (which the operating system creates when the application starts and then destroys when the application quits), but generally, the program is free to create additional threads as it needs them

2 (assuming, of course, that there are available resources). With multiple threads, you can do multiple things at once in a single application. (Note, however, that the CPU still technically only processes one thread at a time, but at least your application thinks there are two things going on at once.) Threading in.net is handled by the classes in the System.Threading namespace, of which we are interested in only one: Thread. When you create an instance of the Thread class, you are creating a new thread for the operating system to handle. This new thread can go off by itself, doing the things it needs to do, without bothering about the main thread. However, because the main thread is the one controlling the application, you need a way for the new thread to report back. Otherwise, the new thread would go on forever and your application would lose control of it a rogue thread. This process is done through callbacks. When the new thread needs to get in touch with the main thread, it raises an event, just like any other.net Windows Forms control, which the main thread catches and deals with appropriately. Now, imagine the IM application again. Your main thread is running the application, allowing you to type messages and send them. Meanwhile, in the background, you have another thread going that sits and listens for incoming messages. When it receives one, it notifies the main thread, which takes the message and displays it properly. The background thread then continues waiting for more messages. Simple as that! (Remember that the sending and receiving of messages is done with thetcpclient and TcpListener classes.) A Note About IM Applications Most IM applications that are around nowadays rely on a server/client approach. That is, you log into a server, and messages you send are sent to the server, which in turn routes them to their intended destinations. The server acts as the middleman. There are some advantages to doing it this way, although there are some disadvantages as well. The application we will build here is completely peer-to-peer (P2P), so it doesn't rely at all on a server. Rather, you connect directly to another user, and messages are not sent anywhere other than where they're supposed to go. This provides some added security, and much flexibility (but may not be so great for corporations that want to build up a registered database of IM users; that is, AOL or MSN).

3 Let's Build It! First, we need the basic framework a template for a standard Windows Forms application. This is shown in Listing 1. Listing 1 The Framework using System; using System.Windows.Forms; using System.Drawing; using System.Net.Sockets; using System.Threading; using System.IO; using System.ComponentModel; namespace MyNamespace { public class Chat : Form { private Button btsend = new Button(); private RichTextBox rtbmessage = new RichTextBox(); private RichTextBox rtbtype = new RichTextBox(); public Chat() { rtbmessage.dock = DockStyle.Top; rtbmessage.size = new Size(300,200); rtbtype.location = new Point(0,205); rtbtype.size = new Size(240,65); btsend.text = "Send"; btsend.click += new EventHandler(this.SendText); btsend.size = new Size(50,50); btsend.location = new Point(240,205); this.text = ".NET IM"; this.size = new Size(300,300); this.closing += new CancelEventHandler(this.CloseMe); this.controls.add(btsend); this.controls.add(rtbmessage); this.controls.add(rtbtype); private void SendText(Object Sender, EventArgs e) { private void CloseMe(Object Sender, CancelEventArgs e) { public static void Main() { Application.Run(new Chat());

4 This very simple code produces this screen Briefly, you first have the namespaces you'll use, taking into account the threading and net classes you'll need. Next, you have the namespace, class, and several Windows Forms controls to provide the user interface. These are all initialized in the constructor, Chat, and a few event handlers are added specifically, to handle the "Send" button click, and to perform some processing when the form closes. Those empty event handlers are shown next, followed by the Main method, which starts the application. Your basic IM window First thing's first. You need to create a mechanism to send messages. That's simple enough: declare a new TcpClient object somewhere between lines 11 and 14: private TcpClient objclient; The only time you actually need to use this object is when the user sends a message that is, when they click the "Send" button. So let's modify the SendText method: private void SendText(Object Sender, EventArgs e) { rtbmessage.text += strme + ": " + rtbtype.text + "\n"; objclient = new TcpClient(" ", 1000); StreamWriter w = new StreamWriter(objClient.GetStream()); w.write(rtbtype.text + "\n"); w.flush(); objclient.close(); rtbtype.text = ""; The second line displays the entered text in the RichTextBox. This allows you to keep a record of the conversation. The next line instantiates the TcpClient object. The first parameter of the constructor is the IP address of the person you're going to talk to. In this case, we're going to talk to ourselves (and before you ask, yes, I do have friends), so use the standard IP address that points to your local computer. You could

5 also use "localhost" here, or use any other DNS name that works for you. The second parameter is the port number you want to send to. This number is relatively arbitrary just choose something in the thousands. Next, you need to actually send the message. If you're familiar with the.net method of network messaging, you'll know that this is done by retrieving a Stream on the network that you can write to. Essentially, the GetStream method grabs the ear of the receiver, and prepares to yak into it. We use a StreamWriter object to do the actual writing. The Write method writes the contents from therichtextbox (followed by a new line character), and Flush ensures that the data is sent immediately (rather than waiting for any network buffering). Finally, you close the TcpClient. (This is very important; otherwise, you'd maintain a connection to the receiver, and that's not necessary.) Erase the contents of the "rtbtype" box, so the user can start anew. So now your little application can send messages to itself, but no one's listening yet. To do this, you need another thread and a TcpListener object. Add the following code before line 14: private Thread thdlistener; private TcpListener objlistener; Somewhere in the constructor, add the following code: thdlistener = new Thread(new ThreadStart(this.Listen)); thdlistener.start(); The first line instantiates your new thread. ThreadStart is actually a delegate (like EventHandler) that points the application to an event-handling method the Listen method in this case. Essentially, this first line tells the application that when the new thread starts, the first thing it should do is execute the Listen method. The second line actually starts the new thread off and running. Next, we need the Listen method: private void Listen() { string strtemp = ""; objlistener = new TcpListener(1000); objlistener.start(); do { TcpClient objclient = objlistener.accepttcpclient(); StreamReader objreader = new StreamReader(objClient.GetStream()); while (objreader.peek()!= -1) { strtemp += Convert.ToChar(objReader.Read()).ToString(); object[] objparams = new object[] {strtemp; strtemp = ""; this.invoke(new Invoker(this.ShowMessage), objparams); while(true!= false);

6 This method starts off by instantiating the TcpListener object; the third line tells it to listen on port 1000 (this should probably be the same as the port you're sending on; otherwise, you won't receive the communications). The Start method, naturally, starts the listener listening. Recall that the listener needs to alert the main thread when it hears something, and then goes back to waiting. Thus, we introduce the infinite do loop (because true will never equal false, this loop executes indefinitely). Inside this loop is where the receiving and notifying will be done. Now that the TcpListener is listening, you need to tell it what to listen for. The AcceptTcpClientmethod causes the TcpListener to sit there until a TcpClient (actually, any application that communicates via TCP) contacts it. If no one ever sends you an instant message, the execution will never get past the AcceptTcpClient line (which, if this weren't on a different thread, would cause your application to freeze). When you do finally receive some communication, you use thegetstream method to grab the raw contents of the message, wrapping it in a StreamReader to return the contents in a human readable format. Next, you go into a loop to read the message, character by character. Note that the Read method of the StreamReader returns everything in numbers. Thus, you need to use the Convert.ToChar method to change those numbers back into the text characters they represent. Peek tells you when you reach the end of the message. NOTE You can't use the StreamReader's ReadToEnd method here because, given the network communication, the StreamReader can't determine the end. It simply waits and waits and waits for more information, and your IM application isn't very useful. Let's skip down to the third-to-last line for a moment. Communication between threads is often a risky procedure if not done properly. You need a way to alert the main thread safely, and theinvoke method is your savior. Invoke takes two parameters: a delegate that points to a method that the main thread should execute once it receives the alert, and any parameters that should be sent to that method, in an array. The Invoker delegate here is a custom one. To create it, simply add the following code somewhere before line 14: public delegate void Invoker(String t); This delegate will pass a string (the received message) to the event handler. This string is the one you built from the StreamReader, but you need to put it in an array to properly send it usinginvoke; so this is done a few lines above. In a nutshell, here's the plan of execution: First, the new thread starts and executes the Listenmethod; then it instantiates a TcpListener, and waits for someone to contact it. When a submission is received, it retrieves the message into a string using a StreamReader; next, it alerts the main thread, telling it to execute the ShowMessage function, passing in the newly built string; then it goes back to waiting for a message. Rinse and repeat.

7 Let's look at the ShowMessage function. It's very simple: private void ShowMessage(String t) { rtbmessage.text += t + "\n"; This method simply takes the string passed into it, and displays it in the RichTextBox. There's only one thing left to do: When your application quits, you need to break the infinite loop in the Listen method and stop the TcpListener from listening. If Listen had been executed on the main thread, it would stop when your application closed. However, because it's in another thread, it will continue on, regardless of what the main thread does. So, in the event handler for the form'sclosing event, you stop the TcpListener: private void CloseMe(Object Sender, CancelEventArgs e) { objlistener.stop(); And you're done! As you can see, it doesn't take much to build a chat application. You can compile this code from the command line using the command (assuming, of course, that you saved this code as Chat.cs): csc /t:winexe /r:system.dll,system.drawing.dll,system.windows.forms.dll Chat.cs Listing 2 shows the code in its entirety, with some minor modifications to make it more user-friendly. This screen shows the output after some nice chatting with myself.

8 Listing 2 A complete Instant Messaging Application using System; using System.Windows.Forms; using System.Drawing; using System.Net.Sockets; using System.Threading; using System.IO; using System.ComponentModel; namespace MyNamespace { public class Chat : Form { public delegate void Invoker(String t); private Thread thdlistener; private TcpListener objlistener; private TcpClient objclient; private Button btsend = new Button(); private RichTextBox rtbmessage = new RichTextBox(); private RichTextBox rtbtype = new RichTextBox(); private string strfriend; private string strme; public Chat() { strfriend = " "; strme = "Chris"; thdlistener = new Thread(new ThreadStart(this.Listen)); thdlistener.start(); rtbmessage.dock = DockStyle.Top; rtbmessage.size = new Size(300,200); rtbtype.location = new Point(0,205); rtbtype.size = new Size(240,65); btsend.text = "Send"; btsend.click += new EventHandler(this.SendText); btsend.size = new Size(50,50); btsend.location = new Point(240,205); this.text = ".NET IM"; this.size = new Size(300,300); this.closing += new CancelEventHandler(this.CloseMe); this.controls.add(btsend); this.controls.add(rtbmessage); this.controls.add(rtbtype); private void SendText(Object Sender, EventArgs e) { rtbmessage.text += strme + ": " + rtbtype.text + "\n"; objclient = new TcpClient(strFriend, 1000); StreamWriter w = new StreamWriter(objClient.GetStream()); w.write(rtbtype.text + "\n"); w.flush(); objclient.close(); rtbtype.text = ""; private void Listen() { string strtemp = ""; objlistener = new TcpListener(1000); objlistener.start(); do { TcpClient objclient = objlistener.accepttcpclient(); StreamReader objreader = new StreamReader(objClient.GetStream()); while (objreader.peek()!= -1) { strtemp += Convert.ToChar(objReader.Read()).ToString(); object[] objparams = new object[] {strtemp;

9 strtemp = ""; this.invoke(new Invoker(this.ShowMessage), objparams); while(true!= false); private void ShowMessage(String t) { rtbmessage.text += strfriend + ": " + t + "\n"; private void CloseMe(Object Sender, CancelEventArgs e) { objlistener.stop(); public static void Main() { Application.Run(new Chat());