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C# 從零手寫極簡 HTTP Server：Socket、多線程與請求解析實戰

編輯：關於C語言

前言：為什麼要手寫 HTTP Server？

雖然生產環境直接用 ASP.NET Core / Kestrel / Nginx，但**手寫一個極簡 HTTP Server** 能深入理解：

TCP Socket 連線建立與資料收發
HTTP 協議請求行、標頭、主體解析
多線程併發處理模型
靜態檔案服務與錯誤回應

本文基於經典教學範例，重構為結構清晰、註解完整的現代 C# 版本（.NET 6+ 風格）。

架構設計

HttpServer：監聽 Socket，`Accept` 客戶端連線，為每個連線啟動執行緒。
HttpProcessor：處理單一連線的請求解析、路由、靜態檔案讀取、回應寫入。

核心程式碼實作

1. HttpProcessor — 請求處理核心

using System;
using System.Collections.Generic;
using System.IO;
using System.Net.Sockets;
using System.Text;

public class HttpProcessor
{
    private readonly Socket _clientSocket;
    private readonly NetworkStream _stream;
    private readonly StreamReader _reader;
    private readonly StreamWriter _writer;

    private string _method, _url, _protocol;
    private readonly Dictionary _headers = new();

    public HttpProcessor(Socket clientSocket)
    {
        _clientSocket = clientSocket;
        _stream = new NetworkStream(clientSocket, FileAccess.ReadWrite, ownsSocket: true);
        _reader = new StreamReader(_stream, Encoding.UTF8, detectEncodingFromByteOrderMarks: false, leaveOpen: true);
        _writer = new StreamWriter(_stream, Encoding.UTF8, leaveOpen: true) { NewLine = "\r\n", AutoFlush = true };
    }

    public void Process()
    {
        try
        {
            ParseRequestLine();
            ParseHeaders();
            ServeRequest();
        }
        catch (Exception ex)
        {
            Console.Error.WriteLine($"處理請求異常: {ex.Message}");
            SendResponse(500, "Internal Server Error", "text/plain", "Server Error");
        }
        finally
        {
            Cleanup();
        }
    }

    private void ParseRequestLine()
    {
        string line = _reader.ReadLine() ?? throw new InvalidOperationException("空請求");
        string[] parts = line.Split(' ', 3);
        if (parts.Length != 3) throw new InvalidOperationException("請求行格式錯誤");
        (_method, _url, _protocol) = (parts[0], parts[1], parts[2]);
    }

    private void ParseHeaders()
    {
        string line;
        while (!string.IsNullOrEmpty(line = _reader.ReadLine()))
        {
            int colon = line.IndexOf(':');
            if (colon > 0)
            {
                string name = line.Substring(0, colon).Trim();
                string value = line.Substring(colon + 1).Trim();
                _headers[name] = value;
            }
        }
    }

    private void ServeRequest()
    {
        if (_method != "GET")
        {
            SendResponse(405, "Method Not Allowed", "text/plain", "Only GET supported");
            return;
        }

        // 安全：移除查詢字串、禁止目錄遍歷
        string path = _url.Split('?')[0];
        string filePath = Path.GetFullPath(Path.Combine(AppContext.BaseDirectory, "wwwroot", path.TrimStart('/')));
        string root = Path.GetFullPath(Path.Combine(AppContext.BaseDirectory, "wwwroot"));

        if (!filePath.StartsWith(root, StringComparison.OrdinalIgnoreCase) || !File.Exists(filePath))
        {
            SendResponse(404, "Not Found", "text/html", $"404 Not Found
{_url}");
            return;
        }

        // 簡易 MIME 對應
        string mime = GetMimeType(Path.GetExtension(filePath));
        byte[] fileBytes = File.ReadAllBytes(filePath);
        SendResponse(200, "OK", mime, fileBytes);
    }

    private void SendResponse(int statusCode, string reason, string contentType, byte[] body)
    {
        _writer.WriteLine($"HTTP/1.1 {statusCode} {reason}");
        _writer.WriteLine($"Content-Type: {contentType}");
        _writer.WriteLine($"Content-Length: {body.Length}");
        _writer.WriteLine("Connection: close");
        _writer.WriteLine();
        _writer.Flush();
        _stream.Write(body, 0, body.Length);
    }

    private void SendResponse(int statusCode, string reason, string contentType, string body)
    {
        byte[] bytes = Encoding.UTF8.GetBytes(body);
        SendResponse(statusCode, reason, contentType, bytes);
    }

    private static string GetMimeType(string ext) => ext.ToLowerInvariant() switch
    {
        ".html" or ".htm" => "text/html",
        ".css" => "text/css",
        ".js" => "application/javascript",
        ".json" => "application/json",
        ".png" => "image/png",
        ".jpg" or ".jpeg" => "image/jpeg",
        ".gif" => "image/gif",
        ".svg" => "image/svg+xml",
        ".ico" => "image/x-icon",
        _ => "application/octet-stream"
    };

    private void Cleanup()
    {
        try { _writer?.Dispose(); } catch { }
        try { _reader?.Dispose(); } catch { }
        try { _stream?.Dispose(); } catch { }
        try { _clientSocket?.Close(); } catch { }
    }
}

2. HttpServer — 監聽與分派

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

public class HttpServer
{
    private readonly int _port;
    private readonly TcpListener _listener;
    private volatile bool _running;

    public HttpServer(int port = 8080)
    {
        _port = port;
        _listener = new TcpListener(IPAddress.Loopback, port); // 僅本機測試；實際部署用 IPAddress.Any
    }

    public void Start()
    {
        _listener.Start();
        _running = true;
        Console.WriteLine($"HTTP Server 啟動於 https://127.0.0.1:{_port}/");

        Thread acceptThread = new Thread(AcceptLoop) { IsBackground = true };
        acceptThread.Start();
    }

    public void Stop()
    {
        _running = false;
        _listener.Stop();
    }

    private void AcceptLoop()
    {
        while (_running)
        {
            try
            {
                Socket clientSocket = _listener.AcceptSocket();
                var processor = new HttpProcessor(clientSocket);
                Thread t = new Thread(processor.Process) { IsBackground = true };
                t.Start();
            }
            catch (SocketException) when (!_running) { /* 正常停止 */ }
            catch (Exception ex)
            {
                Console.Error.WriteLine($"Accept 錯誤: {ex.Message}");
            }
        }
    }
}

3. 程式進入點

class Program
{
    static void Main(string[] args)
    {
        int port = args.Length > 0 && int.TryParse(args[0], out int p) ? p : 8080;
        var server = new HttpServer(port);
        server.Start();

        Console.WriteLine("按 Enter 鍵停止伺服器...");
        Console.ReadLine();
        server.Stop();
    }
}

執行與測試

專案根目錄建立 `wwwroot` 資料夾，放入 `index.html`、CSS、JS、圖片等靜態檔案。
執行程式，瀏覽器開啟 https://127.0.0.1:8080/ 即可看到首頁。

關鍵技術點解析

技術點	說明
Socket vs TcpListener	`TcpListener` 封裝 `Socket`，簡化監聽邏輯；底層仍是 `Socket.Accept`。
每請求一線程	簡單易懂，但高併發下線程開銷大。生產級用 IOCP / async-await / Kestrel 的事件驅動模型。
請求解析	僅解析請求行與標頭，不支援主體、Chunked、Keep-Alive、管線化等 HTTP/1.1 完整特性。
路徑安全	使用 `Path.GetFullPath` + `StartsWith(root)` 防禦目錄遍歷攻擊（`../../etc/passwd`）。
編碼處理	標頭用 ISO-8859-1 或 UTF-8；本範例統一 UTF-8 簡化處理。