custom.cs

using Microsoft.AspNetCore.Http;
using System.Collections.Generic;
using System.Net;
using System.Threading.Tasks;
using TXTextControl.Web;

public class CustomWebSocketMiddleware {

   private RequestDelegate m_next;
   private IPAddress m_serviceAddress;
   private int m_servicePort;

   // list of available backend TCP services
   internal readonly List<byte[]> DefaultServiceAddress =
      new List<byte[]>() { 
         new byte[] { 127, 0, 0, 2 },
         new byte[] { 127, 0, 0, 1 }
      };

   internal const int DefaultServicePort = 4278;

   public CustomWebSocketMiddleware(RequestDelegate next) {
      m_next = next;
      m_serviceAddress = new IPAddress(DefaultServiceAddress[0]);
      m_servicePort = DefaultServicePort;
   }

   public async Task Invoke(HttpContext context) {
      if (context.WebSockets.IsWebSocketRequest &&
        context.WebSockets.WebSocketRequestedProtocols.Contains("TXTextControl.Web")) {

         // server id from query string (provided in view code)
         int serverId = int.Parse(context.Request.Query["server"]);

         // create a new WebSocketHandler with random server
         var ws = new WebSocketHandler(
            new IPAddress(DefaultServiceAddress[serverId]),
            m_servicePort);

         await ws.Invoke(context);
      }
      else if (m_next != null) {
         await m_next.Invoke(context);
      }
   }
}