Dns或'Socket'不包含定义

Question

MSDN网页在以下链接中有一个示例异步服务器套接字： https://msdn.microsoft.com/en-us/library/fx6588te(v=vs.110).aspx

编写如下：

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];  
        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.ASCII.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("<EOF>") > -1) {  
                // All the data has been read from the   
                // client. Display it on the console.  
                Console.WriteLine("Read {0} bytes from socket. \n Data : {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 = Encoding.ASCII.GetBytes(data);  

        // Begin sending the data to the remote device.  
        handler.BeginSend(byteData, 0, byteData.Length, 0,  
            new AsyncCallback(SendCallback), handler);  
    }  

    private static void SendCallback(IAsyncResult ar) {  
        try {  
            // Retrieve the socket from the state object.  
            Socket handler = (Socket) ar.AsyncState;  

            // Complete sending the data to the remote device.  
            int bytesSent = handler.EndSend(ar);  
            Console.WriteLine("Sent {0} bytes to client.", bytesSent);  

            handler.Shutdown(SocketShutdown.Both);  
            handler.Close();  

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

    public static int Main(String[] args) {  
        StartListening();  
        return 0;  
    }  
}

但是，我发现即使只是复制文件，编译器也会遇到很多构建错误，所有这些似乎都是因为它找不到定义。我几乎肯定这与我如何设置我的库有关，尽管我不知道如何解决这个问题。

下面是错误消息的图片，非常感谢任何帮助！

The error message