System.Net 是 .NET Framework 和 .NET Core 中提供网络通信的基础接口和类库之一，它包含了许多不同的类和接口，可以提供丰富的网络操作功能。

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System.Net 是什么库？

System.Net 是 .NET Framework 和 .NET Core 中提供网络通信的基础接口和类库之一，它包含了许多不同的类和接口，可以提供丰富的网络操作功能。

System.Net 中有哪些类和接口？

System.Net 中的类和接口可以分为高层次类和较低级别类两类。

高层次类包括 WebClient、HttpWebRequest、FtpWebRequest、SmtpClient 等。

WebClient：用于 HTTP/HTTPS 协议的下载与上传，如从远程服务器获取 Web 页面或文件。示例代码：

WebClient client = new WebClient();string downloadString = client.DownloadString("https://www.example.com");Console.WriteLine(downloadString);

HttpWebRequest：提供了更细粒度的 HTTP 操作控制，可以设置请求头、请求方法、代理等属性。示例代码：

CodeHttpWebRequest request = (HttpWebRequest)WebRequest.Create("https://www.example.com/index.html");request.Method = "GET";request.ContentType = "text/html";WebResponse response = request.GetResponse();Stream stream = response.GetResponseStream();StreamReader reader = new StreamReader(stream);string content = reader.ReadToEnd();Console.WriteLine(content);

FtpWebRequest：用于 FTP 协议的上传与下载。示例代码：

CodeFtpWebRequest request = (FtpWebRequest)WebRequest.Create("ftp://ftp.example.com/test.txt");request.Method = WebRequestMethods.Ftp.UploadFile;request.Credentials = new NetworkCredential("username", "password");byte[] fileContents = Encoding.UTF8.GetBytes("This is a test file.");request.ContentLength = fileContents.Length;Stream requestStream = request.GetRequestStream();requestStream.Write(fileContents, 0, fileContents.Length);requestStream.Close();FtpWebResponse response = (FtpWebResponse)request.GetResponse();Console.WriteLine("Upload File Complete, status {0}", response.StatusDescription);response.Close();

SmtpClient：用于 SMTP 协议的发送邮件。示例代码：

CodeMailMessage mail = new MailMessage();mail.From = new MailAddress("sender@example.com");mail.To.Add("recipient@example.com");mail.Subject = "Test Mail";mail.Body = "This is a test message.";SmtpClient client = new SmtpClient("smtp.example.com", 25);client.Credentials = new NetworkCredential("username", "password");client.EnableSsl = true;client.Send(mail);Console.WriteLine("Mail sent successfully.");

较低级别类包括 Socket、TcpClient、UdpClient、WebRequest 等。

Socket：提供了访问 TCP、UDP、RAW、IP 等协议的网络通信功能。示例代码：

byte[] buffer = new byte[1024];Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);socket.Connect("www.example.com", 80);string request = "GET /index.html HTTP/1.1\r\nHost: www.example.com\r\n\r\n";socket.Send(Encoding.UTF8.GetBytes(request));int count = socket.Receive(buffer);string response = Encoding.UTF8.GetString(buffer, 0, count);Console.WriteLine(response);

TcpClient：用于访问 TCP 协议的连接。示例代码：

TcpClient client = new TcpClient("www.example.com", 80);Stream stream = client.GetStream();string request = "GET /index.html HTTP/1.1\r\nHost: www.example.com\r\n\r\n";byte[] buffer = Encoding.UTF8.GetBytes(request);stream.Write(buffer, 0, buffer.Length);buffer = new byte[1024];int count = stream.Read(buffer, 0, buffer.Length);string response = Encoding.UTF8.GetString(buffer, 0, count);Console.WriteLine(response);

UdpClient：用于访问 UDP 协议的连接。示例代码：

UdpClient client = new UdpClient();client.Connect(new IPEndPoint(IPAddress.Parse("192.168.1.100"), 1234));string message = "Hello World";byte[] data = Encoding.UTF8.GetBytes(message);client.Send(data, data.Length);IPEndPoint remoteEP = null;data = client.Receive(ref remoteEP);string response = Encoding.UTF8.GetString(data);Console.WriteLine(response);

WebRequest：提供了一个通用的请求/响应机制，可以通过不同的协议（HTTP、FTP、FILE 等）下载或上传数据。示例代码：

WebRequest request = WebRequest.Create("https://www.example.com/index.html");WebResponse response = request.GetResponse();Stream stream = response.GetResponseStream();StreamReader reader = new StreamReader(stream);string content = reader.ReadToEnd();Console.WriteLine(content);

System.Net 中有哪些高级用法？

System.Net 中还有一些高级用法，包括 HttpListener、WebSocket、HttpClient、IPAddress、Dns、NetworkStream 等，这些类和接口可以更加灵活地实现网络通信。

HttpListener：可以实现一个基于 HTTP 协议的服务器，并监听来自客户端的请求。示例代码：

HttpListener listener = new HttpListener();listener.Prefixes.Add("http://localhost:8080/");listener.Start();Console.WriteLine("Listening...");HttpListenerContext context = listener.GetContext();HttpListenerRequest request = context.Request;HttpListenerResponse response = context.Response;string responseString = "Hello World!";byte[] buffer = Encoding.UTF8.GetBytes(responseString);response.ContentLength64 = buffer.Length;Stream output = response.OutputStream;output.Write(buffer, 0, buffer.Length);output.Close();listener.Stop();

WebSocket：提供了 WebSocket 协议的客户端和服务器实现，使得 web 应用程序可以轻松地进行双向通信。示例代码：

using System.Net.WebSockets;using System.Threading.Tasks;ClientWebSocket webSocket = new ClientWebSocket();await webSocket.ConnectAsync(new Uri("wss://echo.websocket.org/"), CancellationToken.None);await Task.WhenAll(Receive(webSocket), Send(webSocket));await webSocket.CloseAsync(WebSocketCloseStatus.NormalClosure, "", CancellationToken.None);async Task Receive(ClientWebSocket webSocket){    byte[] buffer = new byte[1024];    while (webSocket.State == WebSocketState.Open)    {        WebSocketReceiveResult result = await webSocket.ReceiveAsync(new ArraySegment(buffer), CancellationToken.None);        Console.WriteLine(Encoding.UTF8.GetString(buffer, 0, result.Count));    }}async Task Send(ClientWebSocket webSocket){    string message = "Hello World";    byte[] buffer = Encoding.UTF8.GetBytes(message);    await webSocket.SendAsync(new ArraySegment(buffer), WebSocketMessageType.Text, true, CancellationToken.None);}

HttpClient：提供了 HTTP 协议的客户端实现，支持 GET、POST 等请求方法。示例代码：

using System.Net.Http.Headers;HttpClient client = new HttpClient();client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));HttpResponseMessage response = await client.GetAsync("https://api.example.com/users");if (response.IsSuccessStatusCode){    string content = await response.Content.ReadAsStringAsync();    Console.WriteLine(content);}

IPAddress：表示 IP 地址，可以用于 IP 地址的解析和比较。示例代码：

IPAddress ipAddress1 = IPAddress.Parse("192.168.1.1");IPAddress ipAddress2 = IPAddress.Parse("192.168.1.100");Console.WriteLine(ipAddress1 < ipAddress2); // True

Dns：提供了域名系统（DNS）解析功能，可以将域名转换为 IP 地址。示例代码：

IPHostEntry hostEntry = Dns.GetHostEntry("www.example.com");foreach (var ipAddress in hostEntry.AddressList){    Console.WriteLine(ipAddress.ToString());}

NetworkStream：表示用于网络通信的数据流，可以用于处理 TCP 或 UDP 连接。示例代码：

TcpClient client = new TcpClient("www.example.com", 80);NetworkStream stream = client.GetStream();StreamWriter writer = new StreamWriter(stream);writer.Write("GET /index.html HTTP/1.1\r\nHost: www.example.com\r\n\r\n");writer.Flush();StreamReader reader = new StreamReader(stream);string response = reader.ReadToEnd();Console.WriteLine(response);