这个项目是一个控制台应用程序：

服务器端：

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
// State object for reading client data asynchronously     
public class StateObject
{
    // Client socket.     
    public Socket workSocket = null;
    // Size of receive buffer.     
    public const int BufferSize = 1024;
    // Receive buffer.     
    public byte[] buffer = new byte[BufferSize];
    // Received data string.     
    public StringBuilder sb = new StringBuilder();
}
public class AsynchronousSocketListener
{
    // Thread signal.     
    public static ManualResetEvent allDone = new ManualResetEvent(false);
    public AsynchronousSocketListener()
    {
    }
    public static void StartListening()
    {
        // Data buffer for incoming data.     
        byte[] bytes = new Byte[1024];
        // Establish the local endpoint for the socket.     
        // The DNS name of the computer     
        // running the listener is "host.contoso.com".     
        //IPHostEntry ipHostInfo = Dns.Resolve(Dns.GetHostName());
        //IPAddress ipAddress = ipHostInfo.AddressList[0];
        IPAddress ipAddress = IPAddress.Parse("127.0.0.1");
        IPEndPoint localEndPoint = new IPEndPoint(ipAddress, 11000);
        // Create a TCP/IP socket.     
        Socket listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        // Bind the socket to the local     
        //endpoint and listen for incoming connections.     
        try
        {
            listener.Bind(localEndPoint);
            listener.Listen(100);
            while (true)
            {
                // Set the event to nonsignaled state.     
                allDone.Reset();
                // Start an asynchronous socket to listen for connections.     
                Console.WriteLine("Waiting for a connection...");
                listener.BeginAccept(new AsyncCallback(AcceptCallback), listener);
                // Wait until a connection is made before continuing.     
                allDone.WaitOne();
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
        Console.WriteLine("\nPress ENTER to continue...");
        Console.Read();
    }
    public static void AcceptCallback(IAsyncResult ar)
    {
        // Signal the main thread to continue.     
        allDone.Set();
        // Get the socket that handles the client request.     
        Socket listener = (Socket)ar.AsyncState;
        Socket handler = listener.EndAccept(ar);
        // Create the state object.     
        StateObject state = new StateObject();
        state.workSocket = handler;
        handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);
    }
    public static void ReadCallback(IAsyncResult ar)
    {
        String content = String.Empty;
        // Retrieve the state object and the handler socket     
        // from the asynchronous state object.     
        StateObject state = (StateObject)ar.AsyncState;
        Socket handler = state.workSocket;
        // Read data from the client socket.     
        int bytesRead = handler.EndReceive(ar);
        if (bytesRead > 0)
        {
            // There might be more data, so store the data received so far.     
            state.sb.Append(Encoding.UTF8.GetString(state.buffer, 0, bytesRead));
            // Check for end-of-file tag. If it is not there, read     
            // more data.     
            content = state.sb.ToString();
            if (content.IndexOf("<结束>") > -1)
            {
                // All the data has been read from the     
                // client. Display it on the console.     
                Console.WriteLine("读取 {0} bytes 从客户端。 \n 数据： {1}", content.Length, content);
                // Echo the data back to the client.     
                Send(handler, content);
            }
            else
            {
                // Not all data received. Get more.     
                handler.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReadCallback), state);
            }
        }
    }
    private static void Send(Socket handler, String data)
    {
        // Convert the string data to byte data using ASCII encoding.     
        byte[] byteData = http://www.mamicode.com/Encoding.UTF8.GetBytes(data);"发送 {0} bytes 到客户机。", bytesSent);
            handler.Shutdown(SocketShutdown.Both);
            handler.Close();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }
    public static int Main(String[] args)
    {
        StartListening();
        return 0;
    }
}

客户端代码：

using System;
using System.Net;
using System.Net.Sockets;
using System.Threading;
using System.Text;
// State object for receiving data from remote device.     
public class StateObject
{
    // Client socket.     
    public Socket workSocket = null;
    // Size of receive buffer.     
    public const int BufferSize = 256;
    // Receive buffer.     
    public byte[] buffer = new byte[BufferSize];
    // Received data string.     
    public StringBuilder sb = new StringBuilder();
}
public class AsynchronousClient
{
    // The port number for the remote device.     
    private const int port = 11000;
    // ManualResetEvent instances signal completion.     
    private static ManualResetEvent connectDone = new ManualResetEvent(false);
    private static ManualResetEvent sendDone = new ManualResetEvent(false);
    private static ManualResetEvent receiveDone = new ManualResetEvent(false);
    // The response from the remote device.     
    private static String response = String.Empty;
    private static void StartClient()
    {
        // Connect to a remote device.     
        try
        {
            // Establish the remote endpoint for the socket.     
            // The name of the     
            // remote device is "host.contoso.com".     
            //IPHostEntry ipHostInfo = Dns.Resolve("user");
            //IPAddress ipAddress = ipHostInfo.AddressList[0];
            IPAddress ipAddress = IPAddress.Parse("127.0.0.1");
            IPEndPoint remoteEP = new IPEndPoint(ipAddress, port);
            // Create a TCP/IP socket.     
            Socket client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            // Connect to the remote endpoint.     
            client.BeginConnect(remoteEP, new AsyncCallback(ConnectCallback), client);
            connectDone.WaitOne();
            // Send test data to the remote device.     
            Send(client, "这是一个测试字符串<结束>");
            sendDone.WaitOne();
            // Receive the response from the remote device.     
            Receive(client);
            receiveDone.WaitOne();
            // Write the response to the console.     
            Console.WriteLine("服务器返回： {0}", response);
            // Release the socket.     
            client.Shutdown(SocketShutdown.Both);
            client.Close();
            Console.ReadLine();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }
    private static void ConnectCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the socket from the state object.     
            Socket client = (Socket)ar.AsyncState;
            // Complete the connection.     
            client.EndConnect(ar);
            Console.WriteLine("Socket连接到 {0}", client.RemoteEndPoint.ToString());
            // Signal that the connection has been made.     
            connectDone.Set();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }
    private static void Receive(Socket client)
    {
        try
        {
            // Create the state object.     
            StateObject state = new StateObject();
            state.workSocket = client;
            // Begin receiving the data from the remote device.     
            client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReceiveCallback), state);
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }
    private static void ReceiveCallback(IAsyncResult ar)
    {
        try
        {
            // Retrieve the state object and the client socket     
            // from the asynchronous state object.     
            StateObject state = (StateObject)ar.AsyncState;
            Socket client = state.workSocket;
            // Read data from the remote device.     
            int bytesRead = client.EndReceive(ar);
            if (bytesRead > 0)
            {
                // There might be more data, so store the data received so far.     

                state.sb.Append(Encoding.UTF8.GetString(state.buffer, 0, bytesRead));
                // Get the rest of the data.     
                client.BeginReceive(state.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(ReceiveCallback), state);
            }
            else
            {
                // All the data has arrived; put it in response.     
                if (state.sb.Length > 1)
                {
                    response = state.sb.ToString();
                }
                // Signal that all bytes have been received.     
                receiveDone.Set();
            }
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }
    private static void Send(Socket client, String data)
    {
        // Convert the string data to byte data using ASCII encoding.     
        byte[] byteData = http://www.mamicode.com/Encoding.UTF8.GetBytes(data);"发送 {0} bytes 到服务器。", bytesSent);
            // Signal that all bytes have been sent.     
            sendDone.Set();
        }
        catch (Exception e)
        {
            Console.WriteLine(e.ToString());
        }
    }
    public static int Main(String[] args)
    {
        StartClient();
        return 0;
    }
}

分别在bin目录下找到编译好的执行程序运行，可以测试结果

Socket 异步通信示例