C# Socket 程式設計筆記（四）：UDP 通訊實作

本篇補充無連線 UDP 通訊的伺服器與用戶端完整範例。UDP 適用於即時性要求高、可容忍少量丟包的場景，資源佔用少、延遲低。

UDP Server

Socket newsock = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
newsock.Bind(new IPEndPoint(IPAddress.Any, 9050));
Console.WriteLine("Waiting for a client...");
EndPoint remote = new IPEndPoint(IPAddress.Any, 0);
byte[] data = new byte[1024];
int recv = newsock.ReceiveFrom(data, ref remote);
newsock.SendTo(Encoding.ASCII.GetBytes("Welcome!"), remote);
while (true) {
    data = new byte[1024];
    recv = newsock.ReceiveFrom(data, ref remote);
    Console.WriteLine(Encoding.ASCII.GetString(data, 0, recv));
    newsock.SendTo(data, recv, SocketFlags.None, remote);
}

UDP Client

Socket server = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
IPEndPoint ipep = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 9050);
server.SendTo(Encoding.ASCII.GetBytes("Hello"), ipep);
EndPoint remote = new IPEndPoint(IPAddress.Any, 0);
byte[] data = new byte[1024];
int recv = server.ReceiveFrom(data, ref remote);
while (true) {
    string input = Console.ReadLine();
    if (input == "exit") break;
    server.SendTo(Encoding.ASCII.GetBytes(input), remote);
    recv = server.ReceiveFrom(data, ref remote);
    Console.WriteLine(Encoding.ASCII.GetString(data, 0, recv));
}

.NET 另提供 UdpClient 簡化操作，但直接使用 Socket 更能掌握底層細節。若需可靠傳輸，應在應用層實現確認重傳機制，或改用 TCP。