https://learn.microsoft.com/en-us/dotnet/api/system.web.httpcontextbase.websocketnegotiatedprotocol?view=netframework-4.8.1
HttpContextBase.WebSocketNegotiatedProtocol Property (System.Web) | Microsoft Learn
When implemented in a derived class, gets the negotiated protocol that was sent from the server to the client for an AspNetWebSocket connection.
property systemwebmicrosoftlearn