WebSockets support in ASP.NET Core

By Tom Dykstra and Andrew Stanton-Nurse

This article explains how to get started with WebSockets in ASP.NET Core. WebSocket (RFC 6455) is a protocol that enables two-way persistent communication channels over TCP connections. It's used in apps that benefit from fast, real-time communication, such as chat, dashboard, and game apps.

View or download sample code (how to download). See the Next steps section for more information.

Prerequisites

  • ASP.NET Core 1.1 or later

  • Any OS that supports ASP.NET Core:

    • Windows 7 / Windows Server 2008 or later
    • Linux
    • macOS
  • If the app runs on Windows with IIS:

    • Windows 8 / Windows Server 2012 or later
    • IIS 8 / IIS 8 Express
    • WebSockets must be enabled in IIS (See the IIS/IIS Express support section.)
  • If the app runs on HTTP.sys:

    • Windows 8 / Windows Server 2012 or later
  • For supported browsers, see https://caniuse.com/#feat=websockets.

When to use WebSockets

Use WebSockets to work directly with a socket connection. For example, use WebSockets for the best possible performance with a real-time game.

ASP.NET SignalR provides a richer app model for real-time functionality, but it only runs on ASP.NET 4.x, not ASP.NET Core. An ASP.NET Core version of SignalR is scheduled for release with ASP.NET Core 2.1. See ASP.NET Core 2.1 high-level planning.

Until SignalR Core is released, WebSockets can be used. However, features that SignalR provides must be provided and supported by the developer. For example:

  • Support for a broader range of browser versions by using automatic fallback to alternative transport methods.
  • Automatic reconnection when a connection drops.
  • Support for clients calling methods on the server or vice versa.
  • Support for scaling to multiple servers.

How to use it

Configure the middleware

Add the WebSockets middleware in the Configure method of the Startup class:

app.UseWebSockets();

The following settings can be configured:

  • KeepAliveInterval - How frequently to send "ping" frames to the client to ensure proxies keep the connection open.
  • ReceiveBufferSize - The size of the buffer used to receive data. Advanced users may need to change this for performance tuning based on the size of the data.
var webSocketOptions = new WebSocketOptions()
{
    KeepAliveInterval = TimeSpan.FromSeconds(120),
    ReceiveBufferSize = 4 * 1024
};
app.UseWebSockets(webSocketOptions);

Accept WebSocket requests

Somewhere later in the request life cycle (later in the Configure method or in an MVC action, for example) check if it's a WebSocket request and accept the WebSocket request.

The following example is from later in the Configure method:

app.Use(async (context, next) =>
{
    if (context.Request.Path == "/ws")
    {
        if (context.WebSockets.IsWebSocketRequest)
        {
            WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync();
            await Echo(context, webSocket);
        }
        else
        {
            context.Response.StatusCode = 400;
        }
    }
    else
    {
        await next();
    }

});

A WebSocket request could come in on any URL, but this sample code only accepts requests for /ws.

Send and receive messages

The AcceptWebSocketAsync method upgrades the TCP connection to a WebSocket connection and provides a WebSocket object. Use the WebSocket object to send and receive messages.

The code shown earlier that accepts the WebSocket request passes the WebSocket object to an Echo method. The code receives a message and immediately sends back the same message. Messages are sent and received in a loop until the client closes the connection:

private async Task Echo(HttpContext context, WebSocket webSocket)
{
    var buffer = new byte[1024 * 4];
    WebSocketReceiveResult result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);
    while (!result.CloseStatus.HasValue)
    {
        await webSocket.SendAsync(new ArraySegment<byte>(buffer, 0, result.Count), result.MessageType, result.EndOfMessage, CancellationToken.None);

        result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);
    }
    await webSocket.CloseAsync(result.CloseStatus.Value, result.CloseStatusDescription, CancellationToken.None);
}

When accepting the WebSocket connection before beginning the loop, the middleware pipeline ends. Upon closing the socket, the pipeline unwinds. That is, the request stops moving forward in the pipeline when the WebSocket is accepted. When the loop is finished and the socket is closed, the request proceeds back up the pipeline.

IIS/IIS Express support

Windows Server 2012 or later and Windows 8 or later with IIS/IIS Express 8 or later has support for the WebSocket protocol.

To enable support for the WebSocket protocol on Windows Server 2012 or later:

  1. Use the Add Roles and Features wizard from the Manage menu or the link in Server Manager.
  2. Select Role-based or Feature-based Installation. Select Next.
  3. Select the appropriate server (the local server is selected by default). Select Next.
  4. Expand Web Server (IIS) in the Roles tree, expand Web Server, and then expand Application Development.
  5. Select WebSocket Protocol. Select Next.
  6. If additional features aren't needed, select Next.
  7. Select Install.
  8. When the installation completes, select Close to exit the wizard.

To enable support for the WebSocket protocol on Windows 8 or later:

  1. Navigate to Control Panel > Programs > Programs and Features > Turn Windows features on or off (left side of the screen).
  2. Open the following nodes: Internet Information Services > World Wide Web Services > Application Development Features.
  3. Select the WebSocket Protocol feature. Select OK.

Disable WebSocket when using socket.io on node.js

If using the WebSocket support in socket.io on Node.js, disable the default IIS WebSocket module using the webSocket element in web.config or applicationHost.config. If this step isn't performed, the IIS WebSocket module attempts to handle the WebSocket communication rather than Node.js and the app.

<system.webServer>
  <webSocket enabled="false" />
</system.webServer>

Next steps

The sample app that accompanies this article is an echo app. It has a web page that makes WebSocket connections, and the server resends any messages it receives back to the client. Run the app from a command prompt (it's not set up to run from Visual Studio with IIS Express) and navigate to http://localhost:5000. The web page shows the connection status in the upper left:

Initial state of web page

Select Connect to send a WebSocket request to the URL shown. Enter a test message and select Send. When done, select Close Socket. The Communication Log section reports each open, send, and close action as it happens.

Initial state of web page