Network Programming in C#

The .NET framework provides two namespaces, System.Net and System.Net.Sockets for network programming. The classes and methods of these namespaces help us to write programs, which can communicate across the network. The communication can be either connection oriented or connectionless. They can also be either stream oriented or data-gram based. The most widely used protocol TCP is used for stream-based communication and UDP is used for data-grams based applications.
The System.Net.Sockets.Socket is an important class from the System.Net.Sockets namespace. A Socket instance has a local and a remote end-point associated with it. The local end-point contains the connection information for the current socket instance.
There are some other helper classes like IPEndPoint, IPADdress, SocketException etc, which we can use for Network programming. The .NET framework supports both synchronous and asynchronous communication between the client and server. There are different methods supporting for these two types of communication.

A synchronous method is operating in blocking mode, in which the method waits until the operation is complete before it returns. But an asynchronous method is operating in non-blocking mode, where it returns immediately, possibly before the operation has completed.

Dns Class
The System.net namespace provides this class, which can be used to creates and send queries to obtain information about the host server from the Internet Domain Name Service (DNS). Remember that in order to access DNS, the machine executing the query must be connected to a network. If the query is executed on a machine, that does not have access to a domain name server, a System.Net.SocketException is thrown. All the members of this class are static in nature. The important methods of this class are given below.
public static IPHostEntry GetHostByAddress(string address)
Where address should be in a dotted-quad format like "202.87.40.193". This method returns an IPHostEntry instance containing the host information. If DNS server is not available, the method returns a SocketException.
public static string GetHostName()
This method returns the DNS host name of the local machine.
In my machine Dns.GetHostName() returns vrajesh which is the DNS name of my machine.
public static IPHostEntry Resolve(string hostname)
This method resolves a DNS host name or IP address to a IPHostEntry instance. The host name should be in a dotted-quad format like 127.0.01 or www.microsoft.com.
IPHostEntry Class
This is a container class for Internet host address information. This class makes no thread safety guarantees. The following are the important members of this class.
AddressList Property
Gives an IPAddress array containing IP addresses that resolve to the host name.
Aliases Property
Gives a string array containing DNS name that resolves to the IP addresses in AddressList property.
The following program shows the application of the above two classes.
using System;
using System.Net;
using System.Net.Sockets;
class MyClient
{
public static void Main()
{
IPHostEntry IPHost = Dns.Resolve("www.hotmail.com");
Console.WriteLine(IPHost.HostName);
string []aliases = IPHost.Aliases;
Console.WriteLine(aliases.Length);
IPAddress[] addr = IPHost.AddressList;
Console.WriteLine(addr.Length);
for(int i= 0; i < addr.Length ; i++)
{
Console.WriteLine(addr[i]);
}
}
}
IPEndPoint Class
This class is a concrete derived class of the abstract class EndPoint. The IPEndPoint class represents a network end point as an IP address and a port number. There is couple of useful constructors in this class:
IPEndPoint(long addresses, int port)
IPEndPoint (IPAddress addr, int port)
IPHostEntry IPHost = Dns.Resolve("www.c-sharpcorner.com");
Console.WriteLine(IPHost.HostName);
string []aliases = IPHost.Aliases;
IPAddress[] addr = IPHost.AddressList;
Console.WriteLine(addr[0]);
EndPoint ep = new IPEndPoint(addr[0],80);

Socket Programming: Synchronous Clients
The steps for creating a simple synchronous client are as follows.

1. Create a Socket instance.
2. Connect the above socket instance to an end-point.
3. Send or Receive information.
4. Shutdown the socket
5. Close the socket

The Socket class provides a constructor for creating a Socket instance.
public Socket (AddressFamily af, ProtocolType pt, SocketType st)
Where AddressFamily, ProtocolType and SocketTYpe are the enumeration types declared inside the Socket class.
The AddressFamily member specifies the addressing scheme that a socket instance must use to resolve an address. For example AddressFamily.InterNetwork indicates that an IP version 4 addresses is expected when a socket connects to an end point.
The SocketType parameter specifies the socket type of the current instance. For example SocketType.Stream indicates a connection-oriented stream and SocketType.Dgram indicates a connectionless stream.
The ProtocolType parameter specifies the ptotocol to be used for the communication. For example ProtocolType.Tcp indicates that the protocol used is TCP and ProtocolType.Udp indicates that the protocol using is UDP.
public Connect (EndPoint ep)
The Connect() method is used by the local end-point to connect to the remote end-point. This method is used only in the client side. Once the connection has been established the Send() and Receive() methods can be used for sending and receiving the data across the network.
The Connected property defined inside the class Socket can be used for checking the connection. We can use the Connected property of the Socket class to know whether the current Socket instance is connected or not. A property value of true indicates that the current Socket instance is connected.
IPHostEntry IPHost = Dns.Resolve("www.c-sharpcorner.com");
Console.WriteLine(IPHost.HostName);
string []aliases = IPHost.Aliases;
IPAddress[] addr = IPHost.AddressList;
Console.WriteLine(addr[0]);
EndPoint ep = new IPEndPoint(addr[0],80);
Socket sock = new
Socket(AddressFamily.InterNetwork,SocketType.Stream,ProtocolType.Tcp);
sock.Connect(ep);
if(sock.Connected)
Console.WriteLine("OK");
The Send() method of the socket class can be used to send data to a connected remote socket.
public int Send (byte[] buffer, int size, SocketFlags flags)
Where byte[] parameter storing the data to send to the socket, size parameter containing the number of bytes to send across the network. The SocketFlags parameter can be a bitwise combination of any one of the following values defined in the System.Net.Sockets.SocketFlags enumerator.

SocketFlags.None
SocketFlags.DontRoute
SocketFlags.OutOfBnd
The method Send() returns a System.Int32 containing the number of bytes send.Remember that there are other overloaded versions of Send() method as follows.
public int Send (byte[] buffer, SocketFlags flags)
public int Send (byte[] buffer)
public int Send (byte[] buffer,int offset, int size, SocketFlags flags)
The Receive() method can be used to receive data from a socket.
public int Receive(byte[] buffer, int size, SocketFlags flags)
Where byte[] parameter storing the data to send to the socket, size parameter containing the number of bytes to send across the network. The SocketFlags parameter can be a bitwise combination of any one of the following values defined in the System.Net.Sockets.SocketFlags enumerator explained above.
The overloaded versions of Receive() methods are shown below.
public int Receive (byte[] buffer, SocketFlags flags)
public int Receive (byte[] buffer)
public int Receive (byte[] buffer,int offset, int size, SocketFlags flags)
When the communication across the sockets is over, the connection between the sockets can be terminated by invoking the method ShutDown()
public void ShutDown(SocketShutdown how)
Where ‘how’ is one of the values defined in the SocketSHutdown enumeration. The value SoketShutdown.Send means that the socket on the other end of the connection is notified that the current instance would not send any more data.
The value SoketShutdown.Receive means that the socket on the other end of the connection is notified that the current instance will not receive any more data and the value SoketShutdown.Both means that both the action are not possible.
Remember that the ShutDown() method must be called before the Close(0 method to ensure that all pending data is sent or received.
A socket can be closed by invoking the method Close().
public void Close()
This method closes the current instance and releases all managed and un-managed resources allocated by the current instance. This method internally calls the Dispose() method with an argument of ‘true’ value, which frees both managed and un-managed resources used by the current instance.
protected virtual void Dispose(bool)
The above method closes the current instance and releases the un-managed resources allocated by the current instance and exceptionally release the managed resources also. An argument value of ‘true’ releases both managed and un-managed resources and a value of ‘false’ releases only un-managed resources.
The source code for a simple synchronous client by using the sockets is show below. The following program can send an HTTP request to a web server and can read the response from the web server.
using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
class MyClient
{
public static void Main()
{
IPHostEntry IPHost = Dns.Resolve("www.google.com");
Console.WriteLine(IPHost.HostName);
string []aliases = IPHost.Aliases;
IPAddress[] addr = IPHost.AddressList;
Console.WriteLine(addr[0]);
EndPoint ep = new IPEndPoint(addr[0],80);
Socket sock = new Socket(AddressFamily.InterNetwork,SocketType.Stream,ProtocolType.Tcp);
sock.Connect(ep);
if(sock.Connected)
Console.WriteLine("OK");
Encoding ASCII = Encoding.ASCII;
string Get = "GET / HTTP/1.1\r\nHost: " + "www. google.com" +
"\r\nConnection: Close\r\n\r\n";
Byte[] ByteGet = ASCII.GetBytes(Get);
Byte[] RecvBytes = new Byte[256];
sock.Send(ByteGet, ByteGet.Length, 0);
Int32 bytes = sock.Receive(RecvBytes, RecvBytes.Length, 0);
Console.WriteLine(bytes);
String strRetPage = null;
strRetPage = strRetPage + ASCII.GetString(RecvBytes, 0, bytes);
while (bytes > 0)
{
bytes = sock.Receive(RecvBytes, RecvBytes.Length, 0);
strRetPage = strRetPage + ASCII.GetString(RecvBytes, 0, bytes);
Console.WriteLine(strRetPage );
}
sock.ShutDown(SocketShutdown.Both);
sock.Close();
}
}