using System;
using System.Text;
using System.Windows.Forms;
using System.Net.Sockets;
using System.IO;
using System.Net;
using System.Threading;
using System.Security.Principal;

namespace Client
{
  public partial class Client : Form
  {
    public Client()
    {
      InitializeComponent();
    }

    TcpClient tcpClient;
    Stream tcpStream;
    Properties.Settings Props = Properties.Settings.Default;
    const int maxBuffer = 100;

    /// <summary> C L I E N T </summary>
    private void Form1_Load(object sender, EventArgs e)
    {
      Thread.Sleep(1000); // zu Demo-Zwecken: Zeit für Server-Start
      txtÜbertragungstext.Text = WindowsIdentity.GetCurrent().Name;
      IPAddress ipAdresse = null;

      try
      {
        this.Show(); Application.DoEvents();
        GetIpAdresse(ref ipAdresse, Dns.GetHostName());
        Melde("Der Client ist gestartet auf " + ipAdresse.ToString() + ":" + Props.Port + "\r\n");
        tcpClient = new TcpClient();
        Melde("Verbinde mit Server .....");

        tcpClient.Connect(ipAdresse, Props.Port);

        Melde("Mit Server verbunden!");
        Melde("Bitte den Text eingeben, " +
          "der übertragen werden soll und Enter drücken.");
      }
      catch (Exception exp)
      {
        Melde(exp.Message);
        return;
      }
    }

    private IPAddress GetIpAdresse(ref IPAddress ipAdresse, string hostName)
    {
      IPAddress[] ipAdressen = Dns.GetHostEntry(hostName).AddressList;
      foreach (IPAddress ip in ipAdressen)
        if (ip.AddressFamily == AddressFamily.InterNetwork)
        {
          ipAdresse = ip; break;
        }
      if (ipAdresse == null)
        throw new Exception("Keine IPV4-IP auflösbar.");
      return ipAdresse;
    }

    private void Melde(string msg)
    {
      txtMeldung.AppendText(msg + "\r\n");
    }

    private void btnOK_Click(object sender, EventArgs e)
    {
      try
      {
        tcpStream = tcpClient.GetStream();

        UTF8Encoding encoding = new UTF8Encoding(); 
        //Arbeiten Sie hier später über Decoder! Thema Fragmentierung von Doppelbytes.
        byte[] ba = encoding.GetBytes(txtÜbertragungstext.Text);
        Melde("\r\nÜbertrage zum Server : \"" + txtÜbertragungstext.Text + "\"");
        tcpStream.Write(ba, 0, ba.Length);
        byte[] buffer = new byte[maxBuffer];
        int gelesen = tcpStream.Read(buffer, 0, maxBuffer);
        string empfangen = encoding.GetString(buffer,0,gelesen);
        Melde("Empfangen: \"" + empfangen + "\"");
      }
      catch (Exception exp) { Melde(exp.Message); return; }
    }

    private void Form1_FormClosing(object sender, FormClosingEventArgs e)
    {
      if (tcpStream != null) tcpStream.Close();
      if (tcpClient != null) tcpClient.Close();
    }
  }
}
Server.cs: 

using System;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Net.Sockets;

namespace Server
{
  public partial class Server : Form
  {
    public Server()
    {
      InitializeComponent();
    }

    Properties.Settings Props = Properties.Settings.Default;
    Socket socket;
    TcpListener tcpListener;
    const int maxBuffer = 100;
    
    /// <summary> S E R V E R </summary>
    private void Form1_Load(object sender, EventArgs e)
    {
      IPAddress ipAdresse=null;

      try
      {
        this.Show(); Application.DoEvents();
        GetIpAdresse(ref ipAdresse, Dns.GetHostName());
        tcpListener = new TcpListener(ipAdresse, Props.Port);
        tcpListener.Start();
      }
      catch (Exception exp)
      {
        Melde("Beim Versuch der Auflösung der Addresse: " +
          ipAdresse.ToString() + " enstand folgender Fehler:\r\n" + exp.Message);
        return;
      }

      try
      {
        Melde("Der Server ist gestartet auf Port : " + Props.Port);
        Melde("Der lokale Endpunkt ist :" + tcpListener.LocalEndpoint);
        Melde("Warte auf Verbindung eines Clients ...");
        socket = tcpListener.AcceptSocket();
        Melde("Client-Verbindung akzeptiert von: " + socket.RemoteEndPoint);

        int received;
        byte[] buffer;
        string empfangen;

        do
        {
          buffer = new byte[maxBuffer];
          received = socket.Receive(buffer);
          if (received == 0)
          {
            Server_FormClosing(this, new FormClosingEventArgs(CloseReason.None, false));
            this.Close(); return; 
            // wenn der Client "" sendet, soll der Server beenden. (nur als Beispiel)
          }
          UTF8Encoding encoding = new UTF8Encoding();
          empfangen = encoding.GetString(buffer, 0, received);
          Melde("Vom Client empfangen : \"" + empfangen + "\"");
          string toSend = "ACK";
          socket.Send(encoding.GetBytes(toSend));
          Melde("\r\nSende \"" + toSend + "\" - Meldung");
        }
        while (received != 0);
      }
      catch (Exception exp)
      {
        Melde(exp.Message);
        return;
      }
    }

    private void Melde(string msg)
    {
      txtMeldung.AppendText(msg + "\r\n");
    }

    private IPAddress GetIpAdresse(ref IPAddress ipAdresse, string hostName)
    {
      IPAddress[] ipAdressen = Dns.GetHostEntry(hostName).AddressList;
      foreach (IPAddress ip in ipAdressen)
        if (ip.AddressFamily == AddressFamily.InterNetwork)
        { ipAdresse = ip; break; 
        }
      if (ipAdresse == null)
        throw new Exception("Keine IPV4-IP auflösbar.");
      return ipAdresse;
    }

    private void Server_FormClosing(object sender, FormClosingEventArgs e)
    {
      if (socket != null) socket.Close();
      if (tcpListener != null) tcpListener.Stop();
    }
  }
}