That's the protocol handling part of a web server. If you wanted to write your own COM port redirector using .NET that might be useful but I would have thought you would want to do this at the TCP level - also if the goal is to enable the Arduino to act as an HTTP client, the thing you'd be creating on the PC would be a network client, not a server/'listener'. If that's the approach you're taking, and you're having trouble with it, a .NET forum would be the place to ask for help. I would have thought that using an existing COM port redirector would make more sense though, and moving the web server onto the PC and avoiding the problem altogether would be even better.