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Fleck is a WebSocket server implementation in C#. Branched from the Nugget project, Fleck requires no inheritance, container, or additional references.

Fleck has no dependency on HttpListener or HTTP.sys meaning that it will work on Windows 7 and Server 2008 hosts. WebSocket Remarks - Microsoft Docs


The following is an example that will echo to a client.

var server = new WebSocketServer("ws://");
server.Start(socket =>
  socket.OnOpen = () => Console.WriteLine("Open!");
  socket.OnClose = () => Console.WriteLine("Close!");
  socket.OnMessage = message => socket.Send(message);

Supported WebSocket Versions

Fleck supports several WebSocket versions of modern web browsers

  • Hixie-Draft-76/Hybi-00 (Safari 5, Chrome < 14, Firefox 4 (when enabled))
  • Hybi-07 (Firefox 6)
  • Hybi-10 (Chrome 14-16, Firefox 7)
  • Hybi-13 (Chrome 17+, Firefox 11+, Safari 6+, Edge 13+(?))

Secure WebSockets (wss://)

Enabling secure connections requires two things: using the scheme wss instead of ws, and pointing Fleck to an x509 certificate containing a public and private key

var server = new WebSocketServer("wss://");
server.Certificate = new X509Certificate2("MyCert.pfx");
server.Start(socket =>
  //...use as normal

Having issues making a certificate? See this guide to creating an x509 by @AdrianBathurst

SubProtocol Negotiation

To enable negotiation of subprotocols, specify the supported protocols on the WebSocketServer.SupportedSubProtocols property. The negotiated subprotocol will be available on the socket's ConnectionInfo.NegotiatedSubProtocol.

If no supported subprotocols are found on the client request (the Sec-WebSocket-Protocol header), the connection will be closed.

var server = new WebSocketServer("ws://");
server.SupportedSubProtocols = new []{ "superchat", "chat" };
server.Start(socket =>
  //socket.ConnectionInfo.NegotiatedSubProtocol is populated

Custom Logging

Fleck can log into Log4Net or any other third party logging system. Just override the FleckLog.LogAction property with the desired behavior.

ILog logger = LogManager.GetLogger(typeof(FleckLog));

FleckLog.LogAction = (level, message, ex) => {
  switch(level) {
    case LogLevel.Debug:
      logger.Debug(message, ex);
    case LogLevel.Error:
      logger.Error(message, ex);
    case LogLevel.Warn:
      logger.Warn(message, ex);
      logger.Info(message, ex);

Disable Nagle's Algorithm

Set NoDelay to true on the WebSocketConnection.ListenerSocket

var server = new WebSocketServer("ws://");
server.ListenerSocket.NoDelay = true;
server.Start(socket =>
  //Child connections will not use Nagle's Algorithm

Auto Restart After Listen Error

Set RestartAfterListenError to true on the WebSocketConnection

var server = new WebSocketServer("ws://");
server.RestartAfterListenError = true;
server.Start(socket =>
  //...use as normal