Implementing UDP: Let’s get quizzical

The C# Column - Yashawant Kanetkar

Unlike the TCP protocol, the UDP protocol is connectionless. Here there is no setting up of connections needed, instead the data is sent in packets. The .NET framework provides the UdpClient class for connectionless transmission. Like TcpListener and TcpClient, UdpClient also wraps the Socket class. The UdpClient’s Send( ) and Receive( ) methods invoke the SendTo( ) and ReceiveFrom( ) methods of the Socket( ) class respectively.

To demonstrate the usage of the connectionless service we have created two applications—Quizmaster and Player. The idea is basically to conduct an online quiz, where the quizmaster would ask questions to two players on two different machines. The player who responds with a correct answer first wins. Create a Windows application named Quizmaster and design the form as shown below.

The quizmaster asks a question by typing it in the ‘Post a Question’ text box named que. He is also supposed to provide an answer to crosscheck the answers supplied by the players in the ‘Correct Answer’ text box named ans. Then he should specify the names of the computers on which the ‘Player’ application is executing. We have allowed two players to play at one time. These names should be supplied in the ‘Computer 1’ and ‘Computer 2’ text boxes named pcname1 and pcname2. On receiving the answers, they would get displayed in the messages list box.

Let us first take a look at the Quizmaster application. After typing the question-answer along with the computer names, the quizmaster clicks the ‘Send’ button. This invokes the following handler:

private void sendbut_Click(object sender, System.EventArgs e)
{
   string question;
   byte[] senddata;
   question = que.Text + “@” + Dns.GetHostName();
   answer = ans.Text ;
   senddata = Encoding.ASCII.GetBytes(question);
   try
   {
	   client.Send(senddata, senddata.Length, pcname1.Text, 201);
   }
   catch
   {
	   MessageBox.Show (“Cannot send to “ + pcname1.Text);
   }
   try
   {
	   client.Send(senddata, senddata.Length, pcname2.Text, 201);
   }
   catch
   {
	   MessageBox.Show(“Cannot send to “ + pcname2.Text);
   }
}

In this handler we have first collected the question as well as the answer in the strings question and answer respectively. We have made answer a data member of the class because we will need it in another method to crosscheck the answers provided by the players. Next we have appended the question with the computer name separating them with an ‘@’. The GetHostName( ) method of the Dns class returns a string containing the host name of the machine. Then we have encoded the data in an array of bytes called senddata. The data is sent using the Send( ) method of the UdpClient class. For this we have added a data member called client of the type UdpClient to the Form1 class. The Send( ) method takes four parameters-an array of bytes to be sent, the length of the array of bytes, the computer name where we wish to send the data and the port number to which the data would be sent. We chose the port number to be 201 (you can choose any other). Now the Quizmaster application should be ready to accept answers from the players. The player who first sends the correct answer wins. This calls for waiting for a packet to arrive at the specified port. For this we have started a different thread from the constructor of the Form1 class. This thread would start waiting for a packet as soon as the application starts. The following code written inside the constructor launches a separate thread.

Thread svthread = new Thread(new ThreadStart(startserver));
   svthread.IsBackground = true;
   svthread.Start();

The startserver() method is given below

public void startserver()
{
   // 1-Create an IPEndPoint to receive messages
   IPEndPoint recvpt = new IPEndPoint ( IPAddress.Any, 0 ) ;
   byte[] data; 
   string str, from;
   int index;
   while(true)
   {
		// 2-Receive data
		data = server.Receive(ref recvpt);
		str = Encoding.ASCII.GetString (data);
		index = str.LastIndexOf(“@”);
		from = str.Substring(index + 1);
		messages.Items.Add(str);
		str = str.Remove(index, str.Length - index);
		// 3-Check the answer 
		string result;
		if(str.CompareTo(answer)==0)
		{
			result = “Correct! your rank is “ + rank + “@” + HostName();
			rank += 1;
		}
		else
		result = “Your answer is wrong!” + “@” + Dns.GetHostName();
		if(rank == 3)
		rank = 1;
		
		// 4-Send the result
		byte[] senddata;
		senddata=Encoding.ASCII.GetBytes(result);
		try
		{
			client.Send ( senddata, senddata.Length, from, 201 ) ;
		}
		catch
		{
			MessageBox.Show ( “Cannot send result” ) ;
		}
   }
} 

Here, we have firstly created an object of the IPEndPoint class. We need it to receive messages from the client. To the constructor of the IPEndPoint class we have passed a default IPAddress using the Any Filed of the IPAddress class. The Any field specifies that the server should listen for client activity on all network interfaces. We have also passed a port number 0 to indicate that it should pick any available port.

The player sends his answer to the quizmaster in packets. To receive the packet we have called the Receive( ) method of the UdpClient class. We have added server as a data member of the Form1 class as shown below.

UdpClient server=new UdpClient(200);

The Receive( ) method returns an array of bytes containing the answer, which we have after converting into string displayed in the list box. The player would send his answer along with the computer name separated with ‘@’. To check whether the answer is correct or not we must first separate the answer and computer name.

The correctness of answer is checked using the CompareTo( ) method of the String class.

If the answer is correct we have constructed a string result containing a rank. We have added rank as an integer data member of the Form1 class and initialised it to 1. If the answer does not match then we have stored a string saying that the answer is wrong. If the answer is right, the rank is also incremented by 1 so that whoever replies next will be ranked 2nd. If the rank exceeds 2 we have reset it to 1 because there are only two players.

Next we have sent the result string using the Send( ) method of the UdpClient class.

Separating the string, checking the answer and sending the result should be carried out every time a packet is received. So, we have done them in a while loop.

Now let us take a look at the ‘Player’ application. The ‘Player’ is a very simple application. Its form contains a label named question. We have changed the font size of the text that would get displayed in the label to 11 and changed its style to bold. All the player needs to do is type in the answer and click the ‘Send’ button.

We chose 201 as the port number where the player would receive packets and 200 as the port number for sending the packets. In the player application too we have started a new thread in the constructor of the Form1 class that waits for a packet to arrive, as shown below.

Thread svthread=new Thread(new ThreadStart(startplayer));
svthread.IsBackground=true;
svthread.Start(); 

The startplayer() method is given below

public void startplayer()
{
    IPEndPoint recvpt = new IPEndPoint(IPAddress.Any, 201);
    byte[] data;
    string str;
    int index;
    while (true)
    {
       data=server.Receive(ref recvpt);
       str=Encoding.ASCII.GetString(data); 
       index=str.LastIndexOf(“@”);
       from=str.Substring(index+1);
       str=str.Remove(index, str.Length - index);
       question.Text=str ;
    }
}

Here also we have created an object of the IPEndPoint class and collected its reference in recvpt. After receiving the question we have separated the question and the computer name in str and from respectively. server (a reference of UdpClient instance) and from (a reference to a string) have been added to the Form1 class as data members. Then we have displayed the question on the label named question.

As soon as the player receives a question he has to type in the correct answer and click the ‘Send’ button. This would invoke the following handler.

private void sendbut_Click ( object sender, System.EventArgs e )
{
  byte[ ] senddata ;
  string answer ;
  answer = ans.Text + “@” + Dns.GetHostName( ) ;
  senddata = Encoding.ASCII.GetBytes ( answer ) ;
  try
  {
    client.Send ( senddata, senddata.Length, from, 200 ) ;
  }
  catch
  {
    MessageBox.Show ( “Error sending data” ) ;
  }
}

Here we have collected the answer provided by the player in a string called answer and appended it with an ‘@’ followed by the host name of the player machine. After encoding it in a byte array we have again used the Send( ) method of the UdpClient class to send the data. client has been added as the data member of the Form1 class.

Now that both the applications are ready, we can enjoy quizzing!

Yashavant Kanetkar, one of the first Express Computer columnists, is an established software expert, speaker and author with several best-sellers to his credit, including titles like “Let Us C” and the “Fundas” series. Contact him at kanet@nagpur.dot.net.in