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WebSocketListener

WebSocketListener is a light weight WebSocket server side connector, implemented in C# using sockets, according to the RFC 6455. It does not use the Microsoft's System.Net.WebSockets namespace. It should work in any operating system running Microsoft .NET v4.5.

WebSocketListener has been designed to provide WebSocket connectivity to other applications, in the same way that System.Net.TcpListener provides TCP connectivity. It is not a communication framework on its own and it does not provide any kind of publisher/subscriber patern or reliable messaging beyond TCP.

• It can work with both Text or Binary messages.
• It supports wss:// (WebSocket Secure through TLS).
• It is asynchronous.
• It has the Ping/Pong functionality built-in.
• It detects and disconnects half open connections.
• It allows to send and receive messages as streams. WebSocket messages are represented as delimited stream-like objects, that allows integration with other .NET objects like e.g. StreamReader and StreamWriter. Two different WebSocket messages, yield two different streams.
• Messages reads and writes are streamed. Big messages are not held in memory during reads or writes.
• It handles partial frames transparently. The WebSocket specification states that a single message can be sent across multiple individual frames. The message stream will allow to read all the message data, no matter if it was sent in a single or multiple frames.
• It handles interleaved control frames transparently. The WebSocket specification states that control frames can appear interleaved with data frames, including between partial frames of the same message. The message stream will allow to read just the message data, it will skip the control frames.

Take a look on the performance and load tests on a simple 'echo' server.

Quickstart

Install

WebSocketListener is available through NuGet

PM> Install-Package vtortola.WebSocketListener

Set up

Setting up a server and start listening for clients is very similar to a TcpListener:

var local = new IPEndPoint(IPAddress.Any, 8006);
var server = new WebSocketListener(local);
server.Start();

Optionally, you can also:

• enable TLS for secure WebSocket connections.
• customize subprotocols, queuing and ping behaviours.
• add customized extensions.

Accepting clients

Once the server has started, clients can be awaited asynchronously. When a client connects, a WebSocket object will be returned:

WebSocket client = await server.AcceptWebSocketAsync(cancellationToken);

The client provides means to read and write messages. With the client, as in the underlying NetworkStream, is possible to write and read at the same time even from different threads, but is not possible to read from two or more threads at the same time, same for writing.

AcceptWebSocketAsync should be in a loop to continuously accept new clients, also wraped in a try/catch since errors in the negotiation process will be thrown here. Take a look to the simple host tutorial.

Receiving messages

With the client we can await a message as a readonly stream:

WebSocketMessageReadStream messageReadStream = await client.ReadMessageAsync(cancellationToken);

Messages are a stream-like objects, so is it possible to use regular .NET framework tools to work with them. The WebSocketMessageReadStream.MessageType property indicates the kind of content the message contains, so it can be used to select a different handling approach.

The returned WebSocketMessageReadStream object will contain information from the header, like type of message (Text or Binary) but not the message content, neither the message length, since a frame only contains the frame length rather than the total message length, therefore that information could be missleading.

A text message can be read with a simple StreamReader. It is worth remember that according to the WebSockets specs, it always uses UTF8 for text enconding:

if(messageReadStream.MessageType == WebSocketMessageType.Text)
{
   String msgContent = String.Empty.
   using (var sr = new StreamReader(messageReadStream, Encoding.UTF8))
        msgContent = await sr.ReadToEndAsync();
}

ReadMessageAsync should go in a loop, to read messages continuously. Writes and read can be performed at the same time. Take a look to the simple host tutorial.

Also, a binary message can be read using regular .NET techniques:

if(messageReadStream.MessageType == WebSocketMessageType.Binary)
{
   using (var ms = new MemoryStream())
   {
       await messageReader.CopyToAsync(ms);
   }
}

Sending messages

Writing messages is also easy. The WebSocketMessageReadStream.CreateMessageWriter method allows to create a write only message:

using (WebSocketMessageWriteStream messageWriterStream = client.CreateMessageWriter(WebSocketMessageType.Text))

It is important to point out, that despite of the length of the message, the last part won't be sent till the stream is closed (call to Stream.Close) or flushed asynchronously. So disposing the message is the more practical way of ensuring that Stream.Close is called, but that would send the tail of the message synchronously, so calling Stream.FlushAsync will ensure that the last part is sent asynchronously. This allows the sending of arbitrary amounts of data which length is not known before hand.

Once a message writer is created, regular .NET tools can be used to write in it:

using (var sw = new StreamWriter(messageWriterStream, Encoding.UTF8))
{
   await sw.WriteAsync("Hello World!");
   await sw.FlushAsync();
}

Also binary messages:

using (var messageWriter = ws.CreateMessageWriter(WebSocketMessageType.Binary))
   await myFileStream.CopyToAsync(messageWriter);

Example

Check a simplified example or download the EchoServer from the samples.