<%@ WebHandler Language="C#" Class="MyNamespace.robotshandler" %>

using System;
using System.Web;

namespace MyNamespace {

    public class robotshandler: IHttpHandler {

        public void ProcessRequest (HttpContext context) {
 
            context.Response.ContentType = "text/plain";
            context.Response.Write("User-agent: *\n");
                
            if (context.Request.ServerVariables["Https"]=="off"){
                // HTTP
                context.Response.Write("Allow: /\n");
                context.Response.Write("Disallow: /MyDisallowedDirectory/\n");
            } else {
                // HTTPS
                context.Response.Write("Disallow: /");
            }
        }
    
        public bool IsReusable {
            get {return false;}
        }
    }
}