using System;
using System.Text;
using System.Threading;
using System.Net;
using System.Net.Sockets;

namespace TCPServer
{
    class Server
    {
        TcpListener tcpListener;
        Thread listenThread;

        public void Start()
        {
            tcpListener = new TcpListener(IPAddress.Any, 2000);
            listenThread = new Thread(new ThreadStart(ListenForClients));
            listenThread.Start();
        }

        private void ListenForClients()
        {
            tcpListener.Start();

            while (true)
            {
                TcpClient client = tcpListener.AcceptTcpClient();
                Thread clientThread = new Thread(new ParameterizedThreadStart(HandleClientComm));
                clientThread.Start(client);
            }
        }

        private void HandleClientComm(object client)
        {
            while (true)
            {
                TcpClient tcpClient = (TcpClient)client;

                if (tcpClient.Connected)
                {
                    NetworkStream clientStream = tcpClient.GetStream();

                    byte[] message = new byte[tcpClient.ReceiveBufferSize];
                    int bytesRead = 0;

                    do
                    {
                        try
                        {
                            bytesRead = clientStream.Read(message, 0, tcpClient.ReceiveBufferSize);
                        }
                        catch (Exception)
                        {
                            break;
                        }

                        if (bytesRead == 0) break;

                        String result = ASCIIEncoding.ASCII.GetString(message, 0, bytesRead);
                        String ip = tcpClient.Client.RemoteEndPoint.ToString();

                        Console.WriteLine(ip + " " + result);

                        Thread sendThread = new Thread(new ParameterizedThreadStart(SendMessageBack));
                        sendThread.Start(tcpClient);

                        Thread importThread = new Thread(new ParameterizedThreadStart(ImportInDB));
                        importThread.Start(result);

                    } while (clientStream.DataAvailable);

                }
            }
        }

        private void SendMessageBack(object client)
        {
            try
            {
                TcpClient tcpClient = (TcpClient)client;
                NetworkStream clientStream = tcpClient.GetStream();

                byte[] message = ASCIIEncoding.ASCII.GetBytes("OK");
                clientStream.Write(message, 0, message.Length);
                clientStream.Flush();
            }
            catch (Exception)
            {
                Console.WriteLine("Client ist verschwunden.");
            }
        }

        private void ImportInDB(object data)
        { 
            //kommt noch
        }
    }
}

if (tcpClient.Connected)
{
       //....
}
else 
{
       break;
}

if (bytesRead == 0) break;

if (bytesRead == 0) return;