服务器发送事件的连接已关闭错误

Question

我使用以下代码在ASP.NET Web API上创建了一个服务器发送的事件：

namespace SpaServerSide.Controllers
{
    public class SSEController : ApiController
    {

        static readonly ConcurrentQueue<StreamWriter> ConnectedClients = new ConcurrentQueue<StreamWriter>();
        static System.Threading.Timer _dd = new System.Threading.Timer(OnTimerEvent, null, 0, 1000);


        [Route("api/sse")]
        public HttpResponseMessage Get()
        {
            var Response = Request.CreateResponse();
            Response.Content = new PushStreamContent((Action< Stream, HttpContent, TransportContext>)OnStreamAvailabe, "text/event-stream");
            return Response;
        }

        private static void OnStreamAvailabe(Stream stream, HttpContent headers, TransportContext context)
        {
            var clientStream = new StreamWriter(stream);
            ConnectedClients.Enqueue(clientStream);         
        }

        static void OnTimerEvent(object state)
        {
            _dd.Change(Timeout.Infinite, Timeout.Infinite);
            try
            {
                var jsSerializer = new JavaScriptSerializer();
                var message = jsSerializer.Serialize(new { msg = "Server send this response on " + DateTime.Now.ToString() });
                foreach (var clientStream in ConnectedClients)
                {
                    clientStream.WriteLine("data:" + message + Environment.NewLine + Environment.NewLine);
                    clientStream.Flush();
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }
            finally
            {
                _dd.Change(1000, 1000);
            }
        }

    }
}

然后，我使用以下代码段使用javascript连接到此服务器：

var evtSource = new EventSource("http://localhost:5587/api/sse");

        evtSource.onopen = function(e){             
            alert("Opened");
        }

        evtSource.onmessage = function(e){
            console.log(e);
            alert("Response Received.");
        }

        evtSource.onerror = function (e) {
            alert('error');
        };

但是，会发生异常。消息是：远程主机关闭连接。这怎么发生的？为了进一步研究，服务器发送的事件何时触发？可以手动控制吗？