System.Net 是什麼庫？
System.Net 是 .NET Framework 和 .NET Core 中提供網絡通信的基礎接口和類庫之一，它包含了許多不同的類和接口，可以提供豐富的網絡操作功能。
System.Net 中有哪些類和接口？
System.Net 中的類和接口可以分為高層次類和較低級别類兩類。
高層次類包括 WebClient、HttpWebRequest、FtpWebRequest、SmtpClient 等；
WebClient：用于 HTTP/HTTPS 協定的下載下傳與上傳，如從遠端伺服器擷取 Web 頁面或檔案。示例代碼：
WebClient client = new WebClient();
string downloadString = client.DownloadString("https://www.example.com");
Console.WriteLine(downloadString); HttpWebRequest：提供了更細粒度的 HTTP 操作控制，可以設定請求頭、請求方法、代理等屬性。示例代碼：
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 協定的上傳與下載下傳。示例代碼：
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 協定的發送郵件。示例代碼：
CodeMailMessage mail = new MailMessage();
mail.From = new MailAddress("[email protected]");
mail.To.Add("[email protected]");
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."); 較低級别類包括 Socket、TcpClient、UdpClient、WebRequest 等。
Socket：提供了通路 TCP、UDP、RAW、IP 等協定的網絡通信功能。示例代碼：
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：用于通路 TCP 協定的連接配接。示例代碼：
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：用于通路 UDP 協定的連接配接。示例代碼：
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：提供了一個通用的請求/響應機制，可以通過不同的協定（HTTP、FTP、FILE 等）下載下傳或上傳資料。示例代碼：
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 中有哪些進階用法？
System.Net 中還有一些進階用法，包括 HttpListener、WebSocket、HttpClient、IPAddress、Dns、NetworkStream 等，這些類和接口可以更加靈活地實作網絡通信。
HttpListener：可以實作一個基于 HTTP 協定的伺服器，并監聽來自用戶端的請求。示例代碼：
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 = "<html><body>Hello World!</body></html>";
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 協定的用戶端和伺服器實作，使得 web 應用程式可以輕松地進行雙向通信。示例代碼：
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<byte>(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<byte>(buffer), WebSocketMessageType.Text, true, CancellationToken.None);
} HttpClient：提供了 HTTP 協定的用戶端實作，支援 GET、POST 等請求方法。示例代碼：
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 位址，可以用于 IP 位址的解析和比較。示例代碼：
IPAddress ipAddress1 = IPAddress.Parse("192.168.1.1");
IPAddress ipAddress2 = IPAddress.Parse("192.168.1.100");
Console.WriteLine(ipAddress1 < ipAddress2); // True Dns：提供了域名系統（DNS）解析功能，可以将域名轉換為 IP 位址。示例代碼：
IPHostEntry hostEntry = Dns.GetHostEntry("www.example.com");
foreach (var ipAddress in hostEntry.AddressList)
{
Console.WriteLine(ipAddress.ToString());
} NetworkStream：表示用于網絡通信的資料流，可以用于處理 TCP 或 UDP 連接配接。示例代碼：
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); ![如何在 Systemd 下配置并運作關機前的腳本？[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)