In network programming, you normally have a server, and any number of clients. The clients can connect to the server because they know its IP address and port. The IP address is a number identifying the machine (such as 192.168.5.185), and the port is a number used to connect to a particular server program (e.g. HTTP servers use port 80; SSH servers use port 22).

In the code above, we are simply starting a server and setting it to listen for connections on port 18000. The IP address is not important since it's the same as the machine running the program - so we set it to IPAddress.Any. A TCPListener is an actual server object: it allows us to accept connections from other machines and work with them. The TCPListener is started and then waits for a client to connect to it. When this happens, we obtain a TCPClient object. We can then talk to this client by obtaining its NetworkStream:

What we do here is wait for a line of text to arrive from the client that connected earlier, and store it in the line variable. After showing what we received, we use the StreamWriter to send back the same line of text.

If you press F5 now, all you get is a blank window: the program isn't doing anything while waiting for a connection.

Start a new console application for the client. Again, make sure you are using the correct libraries:

When the user types something and presses ENTER, it is stored in the input variable. We then use writer.WriteLine(input) to send the input to the server.

The writer.Flush() is very important. If you leave it out, the program will be stuck and send nothing to the server. Like most input/output (I/O), network streams are buffered. That means they usually wait to have a certain amount of data before actually sending it out. The Flush() call forces the data to be sent.

Always remember: streams and toilets must always be flushed.

When the server sends back its response, we store it in the response variable, and show it in the console window. If you run this program now, you get the following exception:

Well duh, that's because the server is not running. So go back to your first (server) program and leave it running. Then, run the second (client) program:

Amazing! :D You have just manage to make two programs talk to each other. If you haven't already, try putting the server on one machine and the client on another (don't forget to change the IP address in the client).

What we have done here is an example of a simple protocol. A protocol consists of the rules by which computers talk to each other. In this case:

Client connects to server.

Client sends a line of text to server.

Server sends back that same line of text.

This is called an echo protocol, because the server echoes what the client says. Something of this sort is actually a standard echo protocol (RFC862) intended mostly for debugging.

Naturally, what we did here is very simple. Many standard protocols, such as IMAP (used for email), can get very complicated. Also, you'll notice that a new server must be run in order to handle each new client. We'll deal with this another time. Finally, if you have been following the above code carefully, you'll notice that I didn't Flush() the stream in the server program, even though I was writing data to it. That's because the stream is automatically closed because of the using statement. When that happens, any data in the stream is flushed, so we don't need to do that in code.

As you can see, it is very easy to write network programs in C# (not so much in other languages, such as C). Stick around, because there is much more to learn about network programming, and I will be writing several other articles on the topic that go into more detail and show you how to do certain things (e.g. download email or webpages).