Writing a WebSocket server in C#


If you would like to use the WebSocket API, it is useful if you have a server. In this article I will show you how to write one in C#. You can do it in any server-side language, but to keep things simple and more understandable, I chose Microsoft's language.

This server conforms to RFC 6455 so it will only handle connections from Chrome version 16, Firefox 11, IE 10 and over.

First steps

WebSocket's communicate over a TCP (Transmission Control Protocol) connection, luckily C# has a TcpListener class which does as the name suggests. It is in the System.Net.Sockets namespace.

It is a good idea to use the using keyword to write less. It means you do not have to retype the namespace if you use classes from it.



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

You set here, where the server will be reachable.

To easily give the expected type to the first parameter, use the Parse static method of IPAddress.


  • Start()
  • System.Net.Sockets.TcpClient AcceptTcpClient()
    Waits for a Tcp connection, accepts it and returns it as a TcpClient object.

Here's how to use what we have learnt:

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

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

        Console.WriteLine("Server has started on{0}Waiting for a connection...", Environment.NewLine);

        TcpClient client = server.AcceptTcpClient();

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



  • System.Net.Sockets.NetworkStream GetStream()
    Gets the stream which is the communication channel. Both sides of the channel have reading and writing capability.


  • int Available
    This is the Number of bytes of data that has been sent. the Value is zero until NetworkStream.DataAvailable is false.



Write(Byte[] buffer, int offset, int size)

Writes bytes from buffer, offset and size determine length of message.

Read(Byte[] buffer, int offset, int size)

Reads bytes to buffer, offset and size determine the length of the message 

Let us extend our example.

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);


When a client connects to a server, it sends a GET request to upgrade the connection to a WebSocket from a simple HTTP request. This is known as handshaking.

This code has a bug. Let’s say client.Available returns 2 because only the GE is available so far. The regex would fail even though the received data is perfectly valid.

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

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 (new Regex("^GET").IsMatch(data)) {

} else {


Creating the response is easier than understanding why you must do it this way.

You must,

  1. Obtain the value of Sec-WebSocket-Key request header without any leading and trailing whitespace
  2. Concatenate it with "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
  3. Compute SHA-1 and Base64 code of it
  4. Write it back as value of Sec-WebSocket-Accept response header as part of a HTTP response.
if (new Regex("^GET").IsMatch(data)) {
    Byte[] response = Encoding.UTF8.GetBytes("HTTP/1.1 101 Switching Protocols" + Environment.NewLine
        + "Connection: Upgrade" + Environment.NewLine
        + "Upgrade: websocket" + Environment.NewLine
        + "Sec-WebSocket-Accept: " + Convert.ToBase64String (
            SHA1.Create().ComputeHash (
                Encoding.UTF8.GetBytes (
                    new Regex("Sec-WebSocket-Key: (.*)").Match(data).Groups[1].Value.Trim() + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
        ) + Environment.NewLine
        + Environment.NewLine);

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

Decoding messages

After a successful handshake client can send messages to the server, but now these are encoded.

If we send "MDN", we get these bytes:

129 131 61 84 35 6 112 16 109

- 129:

FIN (Is this the whole message?) RSV1 RSV2 RSV3 Opcode
1 0 0 0 0x1=0001

FIN: You can send your message in frames, but now keep things simple.
Opcode 0x1 means this is a text. Full list of Opcodes

- 131:

If the second byte minus 128 is between 0 and 125, this is the length of message. If it is 126, the following 2 bytes (16-bit unsigned integer), if 127, the following 8 bytes (64-bit unsigned integer) are the length.

I can take 128, because the first bit is always 1.

- 61, 84, 35 and 6 are the bytes of key to decode. Changes every time.

- The remaining encoded bytes are the message.

Decoding algorithm

decoded byte = encoded byte XOR (position of encoded byte Mod 4)th byte of key

Example in C#:

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

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


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

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