Using TCP Services

The TcpClient class requests data from an Internet resource using TCP. The methods and properties of TcpClient abstract the details for creating a Socket for requesting and receiving data using TCP. Because the connection to the remote device is represented as a stream, data can be read and written with .NET Framework stream-handling techniques.

The TCP protocol establishes a connection with a remote endpoint and then uses that connection to send and receive data packets. TCP is responsible for ensuring that data packets are sent to the endpoint and assembled in the correct order when they arrive.

To establish a TCP connection, you must know the address of the network device hosting the service you need and you must know the TCP port that the service uses to communicate. The Internet Assigned Numbers Authority (Iana) defines port numbers for common services (see Service Name and Transport Protocol Port Number Registry). Services not on the Iana list can have port numbers in the range 1,024 to 65,535.

The following example demonstrates setting up a TcpClient to connect to a time server on TCP port 13:

Imports System.Net.Sockets
Imports System.Text

Public Class TcpTimeClient
    Private const portNum As Integer = 13
    Private const hostName As String = "host.contoso.com"

    ' Entry point  that delegates to C-style main Private Function.
    Public Overloads Shared Sub Main()
        System.Environment.ExitCode = _
            Main(System.Environment.GetCommandLineArgs())
    End Sub

    Overloads Public Shared Function Main(args As String()) As Integer
        Try
            Dim client As New TcpClient(hostName, portNum)

            Dim ns As NetworkStream = client.GetStream()

            Dim bytes(1024) As Byte
            Dim bytesRead As Integer = ns.Read(bytes, 0, bytes.Length)

            Console.WriteLine(Encoding.ASCII.GetString(bytes, 0, bytesRead))

        Catch e As Exception
            Console.WriteLine(e.ToString())
        End Try

        client.Close()

        Return 0
    End Function
End Class
using System;
using System.Net.Sockets;
using System.Text;

public class TcpTimeClient
{
    private const int portNum = 13;
    private const string hostName = "host.contoso.com";

    public static int Main(String[] args)
    {
        try
        {
            var client = new TcpClient(hostName, portNum);

            NetworkStream ns = client.GetStream();

            byte[] bytes = new byte[1024];
            int bytesRead = ns.Read(bytes, 0, bytes.Length);

            Console.WriteLine(Encoding.ASCII.GetString(bytes,0,bytesRead));

            client.Close();

        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }

        return 0;
    }
}

TcpListener is used to monitor a TCP port for incoming requests and then create either a Socket or a TcpClient that manages the connection to the client. The Start method enables listening, and the Stop method disables listening on the port. The AcceptTcpClient method accepts incoming connection requests and creates a TcpClient to handle the request, and the AcceptSocket method accepts incoming connection requests and creates a Socket to handle the request.

The following example demonstrates creating a network time server using a TcpListener to monitor TCP port 13. When an incoming connection request is accepted, the time server responds with the current date and time from the host server.

Imports System.Net
Imports System.Net.Sockets
Imports System.Text

Public Class TcpTimeServer

    Private const portNum As Integer = 13

    ' Entry point that delegates to C-style main Private Function.
    Public Overloads Shared Sub Main()
        System.Environment.ExitCode = _
            Main(System.Environment.GetCommandLineArgs())
    End Sub

    Overloads Public Shared Function Main(args As String()) As Integer
        Dim done As Boolean = False

        Dim listener As New TcpListener(IPAddress.Any, portNum)

        listener.Start()

        While Not done
            Console.Write("Waiting for connection...")
            Dim client As TcpClient = listener.AcceptTcpClient()

            Console.WriteLine("Connection accepted.")
            Dim ns As NetworkStream = client.GetStream()

            Dim byteTime As Byte() = _
                Encoding.ASCII.GetBytes(DateTime.Now.ToString())

            Try
                ns.Write(byteTime, 0, byteTime.Length)
                ns.Close()
                client.Close()
            Catch e As Exception
                Console.WriteLine(e.ToString())
            End Try
        End While

        listener.Stop()

        Return 0
    End Function
End Class
using System;
using System.Net;
using System.Net.Sockets;
using System.Text;

public class TcpTimeServer
{

    private const int portNum = 13;

    public static int Main(String[] args)
    {
        bool done = false;

        var listener = new TcpListener(IPAddress.Any, portNum);

        listener.Start();

        while (!done)
        {
            Console.Write("Waiting for connection...");
            TcpClient client = listener.AcceptTcpClient();

            Console.WriteLine("Connection accepted.");
            NetworkStream ns = client.GetStream();

            byte[] byteTime = Encoding.ASCII.GetBytes(DateTime.Now.ToString());

            try
            {
                ns.Write(byteTime, 0, byteTime.Length);
                ns.Close();
                client.Close();
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }
        }

        listener.Stop();

        return 0;
    }

}