ServerSSE.cs

using System;
using System.Net;
using System.Text;
using System.Threading.Tasks;

namespace WebTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var listener = new HttpListener();
            listener.Prefixes.Add("http://localhost:8080/");
            listener.Start();

            Task.Run(async () =>
            {
                while (true)
                {
                    var ctx = await listener.GetContextAsync();
                    
                    if(ctx.Request.Headers["Accept"]=="text/event-stream")
                        Task.Factory.StartNew(()=>SSEHandle(ctx));
                }
            }).Wait();
        }

        public static void SSEHandle(HttpListenerContext context)
        {
            var response = context.Response;
            response.StatusCode = (int)HttpStatusCode.OK;
            response.ContentType = "text/event-stream";
            response.AddHeader("Access-Control-Allow-Origin", "*");
            try
            {
                while (true)
                {
                    var msg = $"data: {DateTime.Now}\n\n";
                    var bytes = Encoding.UTF8.GetBytes(msg);
                    response.OutputStream.Write(bytes, 0, bytes.Length);
                    response.OutputStream.Flush();
                    System.Threading.Thread.Sleep(1000);
                }
            }
            catch (HttpListenerException e)
            {
                Console.WriteLine(e.Message);
            }

        } 
    }
}