Writing a WebSocket server in C#

TcpListener(System.Net.IPAddress localAddr, int port)

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

class Server {
    public static void Main() {
        TcpListener server = new TcpListener(IPAddress.Parse("127.0.0.1"), 80);

        server.Start();
        Console.WriteLine("Server has started on 127.0.0.1:80.{0}Waiting for a connection…", Environment.NewLine);

        TcpClient client = server.AcceptTcpClient();

        Console.WriteLine("A client connected.");
    }
}

TcpClient client = server.AcceptTcpClient();

Console.WriteLine("A client connected.");

NetworkStream stream = client.GetStream();

//enter to an infinite cycle to be able to handle every change in stream
while (true) {
    while (!stream.DataAvailable);

    byte[] bytes = new byte[client.Available];

    stream.Read(bytes, 0, bytes.Length);
}

using System.Text;
using System.Text.RegularExpressions;

while(client.Available < 3)
{
   // wait for enough bytes to be available
}

byte[] bytes = new byte[client.Available];

stream.Read(bytes, 0, bytes.Length);

//translate bytes of request to string
String data = Encoding.UTF8.GetString(bytes);

if (Regex.IsMatch(data, "^GET")) {

} else {

}

if (new System.Text.RegularExpressions.Regex("^GET").IsMatch(data))
{
    const string eol = "\r\n"; // HTTP/1.1 defines the sequence CR LF as the end-of-line marker

    byte[] response = Encoding.UTF8.GetBytes("HTTP/1.1 101 Switching Protocols" + eol
        + "Connection: Upgrade" + eol
        + "Upgrade: websocket" + eol
        + "Sec-WebSocket-Accept: " + Convert.ToBase64String(
            System.Security.Cryptography.SHA1.Create().ComputeHash(
                Encoding.UTF8.GetBytes(
                    new System.Text.RegularExpressions.Regex("Sec-WebSocket-Key: (.*)").Match(data).Groups[1].Value.Trim() + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
                )
            )
        ) + eol
        + eol);

    stream.Write(response, 0, response.Length);
}

byte[] decoded = new byte[3];
byte[] encoded = new byte[3] {112, 16, 109};
byte[] mask = new byte[4] {61, 84, 35, 6};

for (int i = 0; i < encoded.Length; i++) {
    decoded[i] = (byte)(encoded[i] ^ mask[i % 4]);
}

//
// csc ws-server.cs
// ws-server.exe

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Text.RegularExpressions;

class Server {
    public static void Main() {
        string ip = "127.0.0.1";
        int port = 80;
        var server = new TcpListener(IPAddress.Parse(ip), port);

        server.Start();
        Console.WriteLine("Server has started on {0}:{1}, Waiting for a connection…", ip, port);

        TcpClient client = server.AcceptTcpClient();
        Console.WriteLine("A client connected.");

        NetworkStream stream = client.GetStream();

        // enter to an infinite cycle to be able to handle every change in stream
        while (true) {
            while (!stream.DataAvailable);
            while (client.Available < 3); // match against "get"

            byte[] bytes = new byte[client.Available];
            stream.Read(bytes, 0, bytes.Length);
            string s = Encoding.UTF8.GetString(bytes);

            if (Regex.IsMatch(s, "^GET", RegexOptions.IgnoreCase)) {
                Console.WriteLine("=====Handshaking from client=====\n{0}", s);

                // 1. Obtain the value of the "Sec-WebSocket-Key" request header without any leading or trailing whitespace
                // 2. Concatenate it with "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" (a special GUID specified by RFC 6455)
                // 3. Compute SHA-1 and Base64 hash of the new value
                // 4. Write the hash back as the value of "Sec-WebSocket-Accept" response header in an HTTP response
                string swk = Regex.Match(s, "Sec-WebSocket-Key: (.*)").Groups[1].Value.Trim();
                string swkAndSalt = swk + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";
                byte[] swkAndSaltSha1 = System.Security.Cryptography.SHA1.Create().ComputeHash(Encoding.UTF8.GetBytes(swkAndSalt));
                string swkAndSaltSha1Base64 = Convert.ToBase64String(swkAndSaltSha1);

                // HTTP/1.1 defines the sequence CR LF as the end-of-line marker
                byte[] response = Encoding.UTF8.GetBytes(
                    "HTTP/1.1 101 Switching Protocols\r\n" +
                    "Connection: Upgrade\r\n" +
                    "Upgrade: websocket\r\n" +
                    "Sec-WebSocket-Accept: " + swkAndSaltSha1Base64 + "\r\n\r\n");

                stream.Write(response, 0, response.Length);
            } else {
                bool fin = (bytes[0] & 0b10000000) != 0,
                    mask = (bytes[1] & 0b10000000) != 0; // must be true, "All messages from the client to the server have this bit set"
                int opcode = bytes[0] & 0b00001111; // expecting 1 - text message
                ulong offset = 2,
                      msglen = bytes[1] & (ulong)0b01111111;

                if (msglen == 126) {
                    // bytes are reversed because websocket will print them in Big-Endian, whereas
                    // BitConverter will want them arranged in little-endian on windows
                    msglen = BitConverter.ToUInt16(new byte[] { bytes[3], bytes[2] }, 0);
                    offset = 4;
                } else if (msglen == 127) {
                    // To test the below code, we need to manually buffer larger messages — since the NIC's autobuffering
                    // may be too latency-friendly for this code to run (that is, we may have only some of the bytes in this
                    // websocket frame available through client.Available).
                    msglen = BitConverter.ToUInt64(new byte[] { bytes[9], bytes[8], bytes[7], bytes[6], bytes[5], bytes[4], bytes[3], bytes[2] },0);
                    offset = 10;
                }

                if (msglen == 0) {
                    Console.WriteLine("msglen == 0");
                } else if (mask) {
                    byte[] decoded = new byte[msglen];
                    byte[] masks = new byte[4] { bytes[offset], bytes[offset + 1], bytes[offset + 2], bytes[offset + 3] };
                    offset += 4;

                    for (ulong i = 0; i < msglen; ++i)
                        decoded[i] = (byte)(bytes[offset + i] ^ masks[i % 4]);

                    string text = Encoding.UTF8.GetString(decoded);
                    Console.WriteLine("{0}", text);
                } else
                    Console.WriteLine("mask bit not set");

                Console.WriteLine();
            }
        }
    }
}

<!doctype html>
<html lang="en">
  <style>
    textarea {
      vertical-align: bottom;
    }
    #output {
      overflow: auto;
    }
    #output > p {
      overflow-wrap: break-word;
    }
    #output span {
      color: blue;
    }
    #output span.error {
      color: red;
    }
  </style>
  <body>
    <h2>WebSocket Test</h2>
    <textarea cols="60" rows="6"></textarea>
    <button>send</button>
    <div id="output"></div>
  </body>
  <script>
    // http://www.websocket.org/echo.html
    const button = document.querySelector("button");
    const output = document.querySelector("#output");
    const textarea = document.querySelector("textarea");
    const wsUri = "ws://127.0.0.1/";
    const websocket = new WebSocket(wsUri);

    button.addEventListener("click", onClickButton);

    websocket.onopen = (e) => {
      writeToScreen("CONNECTED");
      doSend("WebSocket rocks");
    };

    websocket.onclose = (e) => {
      writeToScreen("DISCONNECTED");
    };

    websocket.onmessage = (e) => {
      writeToScreen(`<span>RESPONSE: ${e.data}</span>`);
    };

    websocket.onerror = (e) => {
      writeToScreen(`<span class="error">ERROR:</span> ${e.data}`);
    };

    function doSend(message) {
      writeToScreen(`SENT: ${message}`);
      websocket.send(message);
    }

    function writeToScreen(message) {
      output.insertAdjacentHTML("afterbegin", `<p>${message}</p>`);
    }

    function onClickButton() {
      const text = textarea.value;

      text && doSend(text);
      textarea.value = "";
      textarea.focus();
    }
  </script>
</html>