I'm trying to communicate between a web browser client and an ASP.NET server using WebSockets.
I make a set of requests, of different sizes and with some seconds of elapsed time between each of them. The three first ones pass correctly, but a precise one, with nothing in particular from the other, close the WebSocket connection, throw an exception on server side.
The error message and stack trace of this exception look like this :
FATAL ERROR: Cannot access a disposed object.
Object name: 'System.Web.WebSockets.AspNetWebSocket'.
at System.Web.WebSockets.AspNetWebSocket.ThrowIfDisposed()
at System.Web.WebSockets.AspNetWebSocket.SendAsyncImpl(ArraySegment 1 buffer, WebSocketMessageType messageType, Boolean endOfMessage, CancellationToken cancellationToken, Boolean performValidation)
at System.Web.WebSockets.AspNetWebSocket.SendAsync(ArraySegment 1 buffer, WebSocketMessageType messageType, Boolean endOfMessage, CancellationToken cancellationToken)
at [my code path here...]
It may be a threading problem, because I'm using async methods everywhere from functions that communicate with websockets.
I know the exception is throwed from this code (at socket.SendAsync):
public class SocketTranslater
{
private WebSocket socket;
private JavaScriptSerializer serializer;
// [...]
private ArraySegment<byte> Encode(Object data)
{
string json = serializer.Serialize(data);
return (new ArraySegment<byte>(Encoding.UTF8.GetBytes(json)));
}
public async Task Send(Object packet)
{
ArraySegment<byte> encoded = this.Encode(packet);
await socket.SendAsync(encoded, WebSocketMessageType.Text, true, CancellationToken.None);
}
}
The socket is created from another class :
public class EventSender : IHttpHandler
{
private static List<SocketTranslater> socketTranslaters = new List<SocketTranslater>();
public void ProcessRequest(HttpContext context)
{
this.id = context.Request.UserHostName + ":" + context.Request.UserAgent;
if (context.IsWebSocketRequest)
{
context.AcceptWebSocketRequest(ProcessSocket);
}
}
private async Task ManageSocket(AspNetWebSocketContext context)
{
WebSocket socket = context.WebSocket;
SocketTranslater translater = new SocketTranslater(socket);
translaters.add(translater);
while (true)
{
// Manage messages, using the other class to send responses.
translater.Send(/* Any struct here */);
}
}
Unhopefully, my project is too big to put all the code here.
Any idea of error source, or additional information that you would require ?
UPDATE:
After some more tests, I don't have this exception from time to time. I always do the same thing, but the server seems to have a random comportment.
That makes my problem even weirder...
Finally, after some more tests and interesting questions and answer from here (like this one), I understood:
My problem was that I was stocking WebSockets in a Dictionary linked with hostnames. So on the first connection of a client on my server, everything worked correctly. But if I refresh the page, the websocket was closed by the server (because there was no chance to use it again) et another one was created.
But as I used the same key for both sockets, the deprecated and the new one, I was trying to answer to the new client with the previous socket, that was closed. (disposed = closed for ASP.NET).
So the only thing that I had to do is to remove a socket from the list on the client disconnection (at the end of ManageSocket). And forbid a client to connect twice with the same hostname.
I didn't mention the part where I was linking sockets with hostnames, so I admit you couldn't really help me... I apologize.
Related
I am a beginner in using Signalr and am checking out some examples.
Is it possible to send a message to the client from the server and wait for a return from it? Or is it possible to guarantee that after the answer the same session will be used?
My question is because in a given process, within a transaction, I need to ask the user if he wants to continue with the changes. I have not been able to ask this question before because validations should be done in the same session where changes have been made (but not yet confirmed).
Reiterating the comment from Jaime Yule, WebSockets are bidirectional communication and do not follow the Request/Response architecture for messaging. Given the very fluid nature of communication around WebSockets, these bullet points are good to keep in mind for your current (& future) scenarios:
SignalR is great if you're going to use it for fire & forget (Display a pop-up to a user and that's it)
It's not designed around request-response like you're asking, and trying to use it as such is an anti-pattern
Messages may be sent from either end of the connection at any time,
and there is no native support for one message to indicate it is
related to another
This makes the protocol poorly suited for transactional requirements
It is possible, but i would not recommend (relying on) it.
And it's not a pretty solution (using a static event and being pretty complex for such a simple thing).
Story goes like this:
Make sure client and server know the connectionId - They probably know that already, but i could not figure out a way to access it.
Await NotificationService.ConfirmAsync
... which will call confirm on the client
... which will await the user supplied answer
... and send it back to the server using Callback from The hub.
... which will notify the Callback from the NotificationService over a static event
... which will hand off the message back to ConfirmAsync (using a AutoResetEvent)
... which is hopefully still waiting :)
Client and server both have a 10 second timeout set.
The hub:
// Setup as /notification-hub
public class NotificationHub : Hub {
public string ConnectionId() => Context.ConnectionId;
public static event Action<string, string> Response;
public void Callback(string connectionId, string message) {
Response?.Invoke(connectionId, message);
}
}
Service:
// Wire it up using DI
public class NotificationService {
private readonly IHubContext<NotificationHub> _notificationHubContext;
public NotificationService(IHubContext<NotificationHub> notificationHubContext) {
_notificationHubContext = notificationHubContext;
}
public async Task<string> ConfirmAsync(string connectionId, string text, IEnumerable<string> choices) {
await _notificationHubContext.Clients.Client(connectionId)
.SendAsync("confirm", text, choices);
var are = new AutoResetEvent(false);
string response = null;
void Callback(string connId, string message) {
if (connectionId == connId) {
response = message;
are.Set();
}
}
NotificationHub.Response += Callback;
are.WaitOne(TimeSpan.FromSeconds(10));
NotificationHub.Response -= Callback;
return response;
}
}
Client side js:
var conn = new signalR.HubConnectionBuilder().withUrl("/notification-hub").build();
var connId;
// using Noty v3 (https://ned.im/noty/)
function confirm(text, choices) {
return new Promise(function (resolve, reject) {
var n = new Noty({
text: text,
timeout: 10000,
buttons: choices.map(function (b) {
return Noty.button(b, 'btn', function () {
resolve(b);
n.close();
});
})
});
n.show();
});
}
conn.on('confirm', function(text, choices) {
confirm(text, choices).then(function(choice) {
conn.invoke("Callback", connId, choice);
});
});
conn.start().then(function() {
conn.invoke("ConnectionId").then(function (connectionId) {
connId = connectionId;
// Picked up by a form and posted to the server
document.querySelector(".connection-id-input").value = connectionId;
});
});
For me this is way to complex to put it into the project i am working on.
It really looks like something that will come back and bite you later...
Is it possible to send a message to the client from the server and wait for a return from it? Or is it possible to guarantee that after the answer the same session will be used?
None of this is possible. Currently there's no way to wait for the client's response or even to get to know if the client received the message. There's some discussion implementing this on GitHub. Also here's the feature request.
Until then, the workaround is to send a "notification" from the server with a fire and forget attitude and let the client get the required data via a HTTP request to the server.
This is now possible with .NET 7 using Client Results.
Today, I've highlighted this issue in dotnet's Github page and got a good response from one of the developers of SignalR.
This requires the server to use ISingleClientProxy.InvokeAsync to be able to make request to the client and wait for response.
Quote from the documentation
In addition to making calls to clients, the server can request a
result from a client. This requires the server to use
ISingleClientProxy.InvokeAsync and the client to return a result from
its .On handler.
From the client (js/ts)
hubConnection.on("GetMessage", async () => {
let promise = new Promise((resolve, reject) => {
setTimeout(() => {
resolve(new { data: "message" });
}, 100);
});
return promise;
});
From the server (C#)
//By calling Client(...) on an instance of IHubContext<T>
async Task<object> SomeMethod(IHubContext<MyHub> context)
{
string result = await context.Clients.Client(connectionID).InvokeAsync<string>(
"GetMessage");
return result;
}
//---------------------------//
//Or by calling Client(...) or Caller on the Clients property in a Hub method
public class ChatHub : Hub
{
public async Task<string> WaitForMessage(string connectionId)
{
var message = await Clients.Client(connectionId).InvokeAsync<string>(
"GetMessage");
return message;
}
}
Using the following form with Invoke waits for and returns the response directly (just like a "real" synchronous method call)
var persons = hubProxy.Invoke<IEnumerable<Person>>("GetPersonsSynchronous", SearchCriteria, noteFields).Result;
foreach (Person person in persons)
{
Console.WriteLine($"{person.LastName}, {person.FirstName}");
}
I have 1 exe which is nothing bit a Windows form which will continuously run in background and will watch my serial port and I have 1 event data receive event which fires as my serial port receive data.
As soon as I receive data in this event I will pass this data to another event handler which saves this data in database through web api method.
But data to my serial port will be coming frequently so I want to save this data to my database independently so that my database insert operation doesn't block my incoming serial port data.
This is my code:
void _serialPort_DataReceived(object sender, SerialDataReceivedEventArgs e)//Fires as my serial port receives data
{
int dataLength = _serialPort.BytesToRead;
byte[] data = new byte[dataLength];
int nbrDataRead = _serialPort.Read(data, 0, dataLength);
if (nbrDataRead == 0)
return;
// Send data to whom ever interested
if (NewSerialDataRecieved != null)
{
NewSerialDataRecieved(this, new SerialDataEventArgs(data)); //pass serial port data to new below event handler.
}
}
void _spManager_NewSerialDataRecieved(object sender, SerialDataEventArgs e) //I want this event handler to run independently so that database save operation doenst block incoming serial port data
{
if (this.InvokeRequired)
{
// Using this.Invoke causes deadlock when closing serial port, and BeginInvoke is good practice anyway.
this.BeginInvoke(new EventHandler<SerialDataEventArgs>(_spManager_NewSerialDataRecieved), new object[] { sender, e });
return;
}
//data is converted to text
string str = Encoding.ASCII.GetString(e.Data);
if (!string.IsNullOrEmpty(str))
{
//This is where i will save data to through my web api method.
RunAsync(str).Wait();
}
}
static async Task RunAsync(string data)
{
using (var client = new HttpClient())
{
client.BaseAddress = new Uri("http://localhost:33396/");
client.DefaultRequestHeaders.Accept.Clear();
client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
var content = new StringContent(data);
var response = await client.PostAsJsonAsync<StringContent>("api/Service/Post", content);//nothing happens after this line.
}
}
Web api controller:
public class MyController : ApiController
{
[HttpPost]
public HttpResponseMessage Post(HttpRequestMessage request)
{
var someText = request.Content.ReadAsStringAsync().Result;
return new HttpResponseMessage() { Content = new StringContent(someText) };
}
}
But here problem is:
var response = await client.PostAsJsonAsync<StringContent>("api/Service/Post", content);
Nothing happens after this line that is operation blocks on this line.
So can anybody guide me with this?
By independently we determined in the SO C# chat room that you really mean "Asynchronously".
Your solution is the code above, saving this data to a WebAPI endpoint so any solution to the problem needs to be in 2 parts ...
PART 1: The Client Part
On the client all we need to do is make the call asynchronously in order to free up the current thread to carry on receiving data on the incoming serial port, we can do that like so ...
// build the api client, you may want to keep this in a higher scope to avoid recreating on each message
var api = new HttpClient();
api.BaseAddress = new Uri(someConfigVariable);
// asynchronously make the call and handle the result
api.PostAsJsonAsync("api/My", str)
.ContinueWith(t => HandleResponseAsync(t.Result))
.Unwrap();
...
PART 2: The Server Part
Since you have web api i'm also going to assume you are using EF too, the common and "clean" way to do this, with all the extras stripped out (like model validation / error handling) might look something like this ...
// in your EF code you will have something like this ...
Public async Task<User> SaveUser(User userModel)
{
try
{
var newUser = await context.Users.AddAsync(userModel);
context.SavechangesAsync();
return newUser;
}
catch(Exception ex) {}
}
// and in your WebAPI controller something like this ...
HttpPost]
public async Task<HttpResponseMessage> Post(User newUser)
{
return Ok(await SaveUser(newUser));
}
...
Disclaimer:
The concepts involved here go much deeper and as I hinted above, much has been left out here like validation, error checking, ect but this is the core to getting your serial port data in to a database using the technologies I believe you are using.
Key things to read up on for anyone wanting to achieve this kind of thing might include: Tasks, Event Handling, WebAPI, EF, Async operations, streaming.
From what you describe it seems like you might want to have a setup like this:
1) your windows form listens for serial port
2) when new stuff comes to port your windows forms app saves it to some kind of a queue (msmq, for example)
3) you should have separate windows service that checks queue and as it finds new messages in a queue it sends request to web api
Best solution for this problem is to use ConcurrentQueue.
Just do search on google and you will get planty of samples.
ConcurrentQueue is thread safe and it support writing and reading from multiple threads.
So the component listening to the searal port can write data to the queue. And you can have 2 or more tasks running parallel which listening to this queue and update db as soon as it receives data.
Not sure if it's the problem, but you shouldn't block on async code. You are doing RunAsync(str).Wait(); and I believe that's the problem. Have a look at this blog post by Stephen Cleary:
http://blog.stephencleary.com/2012/07/dont-block-on-async-code.html
We have a .NET client, which use SignalR to call Server method, but the parameter seems very big, for such scenario how to fix it?
Client code:
public async Task FooAsync()
{
var hubConnection = new HubConnection(...);
await hubConnection.Start();
var hubProxy = hubConnection.CreateHubProcx("ValueHub");
//the content is very long, about 11776065 bytes (11MB)
var content = File.ReadAllText(...);
hubProxy.Invoke("Send", content);
...
}
Server code:
[HubName("ValueHub")]
public class ValueHub : Hub
{
public void Send(string json)
{
}
}
From the exception stack and source code, I found the SignalR internally use HttpClient with the FormUrlEncodedContent type HttpContent, and maybe the limitation came from here.
System.UriFormatException was unhandled
HResult=-2146233033
Message=Invalid URI: The Uri string is too long.
Source=System
StackTrace:
at System.UriHelper.EscapeString(String input, Int32 start, Int32 end, Char[] dest, Int32& destPos, Boolean isUriString, Char force1, Char force2, Char rsvd)
at System.Uri.EscapeDataString(String stringToEscape)
at System.Net.Http.FormUrlEncodedContent.Encode(String data)
at System.Net.Http.FormUrlEncodedContent.GetContentByteArray(IEnumerable`1 nameValueCollection)
at System.Net.Http.FormUrlEncodedContent..ctor(IEnumerable`1 nameValueCollection)
at Microsoft.AspNet.SignalR.Client.Http.DefaultHttpClient.Post(String url, Action`1 prepareRequest, IDictionary`2 postData, Boolean isLongRunning)
at Microsoft.AspNet.SignalR.Client.Transports.HttpBasedTransport.Send(IConnection connection, String data, String connectionData)
at Microsoft.AspNet.SignalR.Client.Transports.AutoTransport.Send(IConnection connection, String data, String connectionData)
at Microsoft.AspNet.SignalR.Client.Connection.Send(String data)
at Microsoft.AspNet.SignalR.Client.Hubs.HubProxy.Invoke[T](String method, Object[] args)
at Microsoft.AspNet.SignalR.Client.Hubs.HubProxy.Invoke(String method, Object[] args)
Any good suggestions over this problem?
You can add a line that makes the message size 'infinite 'in your Startup.cs by setting the MaxIncomingWebSocketMessageSize to null:
public class Startup
{
public void Configuration(IAppBuilder app)
{
app.MapSignalR();
GlobalHost.Configuration.MaxIncomingWebSocketMessageSize = null;
}
}
}
Mine works with ~200kb of data, 10 messages send consistently.
I don't know how well it works if there is more data send per second though.
As you have already gathered - this data is too much for SIGNALR by it's own design.
Would it not be a better idea to rather have another process that does this with a normal REST API (GET/POST). Perhaps a message indicating to the user that this needs to be done, as this feels very 'BATCH' like.
Secondly, if it a requirement (possible wrong tool for the job), have you considered compression.
its easy to do it..
split files into byte Array chunks (my tests shows max 10kb per chunk is enough)
send chunks to client with an invoke like (calculate totalBytes and the chunk data you have) :
hubProxy.Invoke("Send", chunk,currentBytes,totalBytes);
get the chunks from client and create a byte array and append each chunk, signalr sends files syncronously, this means data will be received as your send order
you have totalbytes and currentBytes data, now you know all data received, save this byte array to a file with stream or whatever you like..
add this to web config
<httpRuntime maxRequestLength="1048576" executionTimeout="600" />
then add this code to Startup.cs
public class Startup
{
public void Configuration(IAppBuilder app)
{
app.MapSignalR();
GlobalHost.Configuration.MaxIncomingWebSocketMessageSize = null;
}
}
I try to build a simple async net tcp wcf tool which will open connection, send command, receive answer (a List with 0-10 string sentences), close connection.
The problem is, I get on (self-hosted) service side always - no matter what I try - "The I/O operation has been aborted because of either a thread exit or an application request", on client side of course the corresponding errors like "Existing connection was closed by remote host" and timeouts and so on.
I tried alot for the past days but I can't get rid of it.
Client Side (running on .NET 4.0, called around once a sec):
void callservice(string mykey) {
ServiceReference1.Client c = new ServiceReference1.Client();
c.GetDataCompleted += c_GetDataCompleted;
try {
c.GetDataAsync(mykey);
}
catch (FaultException aa)
{
c.Abort();
}
}
private void c_GetDataCompleted(object sender, ServiceReference1.GetDataCompletedEventArgs e)
{
ServiceReference1.Client c = (ServiceReference1.Client)sender;
c.GetDataCompleted -= c_GetDataCompleted;
try
{
if (e.Result != null && e.Result.Length > 0)
{
... }
c.Close();
}
catch (Exception) {
c.Abort();
}
}
Server Side (running on .NET4.5):
[ServiceBehavior(ConcurrencyMode=ConcurrencyMode.Multiple,
InstanceContextMode=InstanceContextMode.PerCall,IncludeExceptionDetailInFaults=true)]
public class Service1 : IMyService
{
public async Task<List<string>> GetData(string whatkey)
{
List<string> mydatalist = new List<string>();
mydatalist= await Task.Run<List<string>>(() =>
{
...
});
return mydatalist;
}
What is going wrong there? Could it be that it is something not having to do with WCF at all? What could it be?
Server Side Exception:
System.Net.Sockets.SocketException, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089
The I/O operation has been aborted because of either a thread exit or an application request
at System.ServiceModel.Channels.SocketConnection.HandleReceiveAsyncCompleted()
at System.ServiceModel.Channels.SocketConnection.OnReceiveAsync(Object sender, SocketAsyncEventArgs eventArgs)
at System.Net.Sockets.SocketAsyncEventArgs.FinishOperationAsyncFailure(SocketError socketError, Int32 bytesTransferred, SocketFlags flags)
at System.Net.Sockets.SocketAsyncEventArgs.CompletionPortCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* nativeOverlapped)
at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)
System.Net.Sockets.SocketException (0x80004005): The I/O operation has been aborted because of either a thread exit or an application request
3E3
One more interesting fact:
SVCLogs show me that the I/O Exeption occurs after a timespan I can define in the
<connectionPoolSettings groupName="default" leaseTimeout="00:03:00"
idleTimeout="00:02:39" maxOutboundConnectionsPerEndpoint="20" />
settings.
In this example it will occur the first time after 00:02:39.
My interpretation: It closes open connections due to the settings there and that causes the Exception since the ReceiveAsync operation i.ex. was still open.
My question is so far why does client.close() not close it completely and why isn't it finished yet when it is calling the c_getdatacompleted-event? Why does the operation "hang out" for 02:39 minutes and does not come to an end?
(If I would not force the close down via the connectionpool settings I end up with hundreds of open operations if I use netstat i.ex. to display)
Async WCF operations (AsyncPattern=true) are implemented with the Asynchronous Programming Model. That is, you implement an operation ("Operation") with two asynchronous operations ("BeginOperation" and "EndOeration"). The client can wrap those operations with a Task (presumably with the FromAsync overload)
For example:
[ServiceContract]
public interface ISampleTaskAsync
{
[OperationContract(AsyncPattern = true)]
IAsyncResult BeginDoWork(int count, AsyncCallback callback, object state);
int EndDoWork(IAsyncResult result);
}
The WCF contract does not return a Task<T>
Then, on the client you could do something like:
var proxy = new Services.SampleTaskAsyncClient();
object state = "This can be whatever you want it to be";
var task = Task<int>.Factory.FromAsync(proxy.BeginDoWork,
proxy.EndDoWork, 10, state);
For more information see:
http://msdn.microsoft.com/en-us/library/system.servicemodel.operationcontractattribute.asyncpattern.aspx
http://blogs.msdn.com/b/rjacobs/archive/2011/06/30/how-to-implement-a-wcf-asynchronous-service-operation-with-task-lt-t-gt.aspx
If you want to use Task<T>, I believe you don't need AsyncPattern=true.
I have an application that uses HttpListener, I need to know when the client disconnected, right now I have all my code inside a try/catch block which is pretty ugly and not a good practice.
How can I know if a client disconnected?
thanks!
Short answer: you can't. If a client stops talking, the underlying socket may stay open and won't ever close; it'll just timeout. The way to detect this is to attempt to perform an action on that connection and if the connection is no longer valid, it'll throw some sort of exception depending on what happened. If you use HttpListener asynchronously, it may clean up your code a bit in terms of a try/catch but unfortunately that's what you're stuck with. There is no event that will fire if the client disconnects.
You can! Two options I've found are with reflection or unsafe code. Both options give you a client disconnect token from a method that looks like this:
CancellationToken GetClientDisconnectToken(HttpListenerRequest request)
To implement this via refection, I found HttpListener actually implements client disconnect notifications for the built-in authentication implementation. I created a type who derives from Hashtable, the structure HttpListener uses for outstanding client disconnect notifications, to tap in to that code outside of its intended purpose.
For every request there is a ConnectionId used by HTTP.SYS. This code uses reflection and creates a Func<> to obtain this id for any HttpListenerRequest:
private static Func<HttpListenerRequest, ulong> GetConnectionId()
{
var field = typeof(HttpListenerRequest).GetField("m_ConnectionId",
BindingFlags.Instance | BindingFlags.NonPublic);
if (null == field)
throw new InvalidOperationException();
return request => (ulong)field.GetValue(request);
}
The next bit of code is a little more complicated:
private static Func<ulong, IAsyncResult> GetRegisterForDisconnectNotification(HttpListener httpListener)
{
var registerForDisconnectNotification = typeof(HttpListener)
.GetMethod("RegisterForDisconnectNotification", BindingFlags.Instance | BindingFlags.NonPublic);
if (null == registerForDisconnectNotification)
throw new InvalidOperationException();
var finishOwningDisconnectHandling =
typeof(HttpListener).GetNestedType("DisconnectAsyncResult", BindingFlags.NonPublic)
.GetMethod("FinishOwningDisconnectHandling", BindingFlags.Instance | BindingFlags.NonPublic);
if (null == finishOwningDisconnectHandling)
throw new InvalidOperationException();
IAsyncResult RegisterForDisconnectNotification(ulong connectionId)
{
var invokeAttr = new object[] { connectionId, null };
registerForDisconnectNotification.Invoke(httpListener, invokeAttr);
var disconnectedAsyncResult = invokeAttr[1];
if (null != disconnectedAsyncResult)
finishOwningDisconnectHandling.Invoke(disconnectedAsyncResult, null);
return disconnectedAsyncResult as IAsyncResult;
}
return RegisterForDisconnectNotification;
}
This reflection creates a Func<> whose input is a ConnectionId and returns an IAsyncResult containing the state of the inflight request. Internally, this calls a private method on HttpListener:
private unsafe void RegisterForDisconnectNotification(ulong connectionId,
ref HttpListener.DisconnectAsyncResult disconnectResult)
As the name implies, this method calls a Win32 API to be notified of a client disconnect. Immediately after, using the result of that method I call a private method on a nested type: void HttpListener.DisconnectedAsyncResult.FinishOwningDisconnectHandling(), if the connection is still open. This method changes the state of this structure from "in HandleAuthentication" to "Normal", which is the state it needs to be in to invoke the IO completion callback who will call Remove on the HashTable. Intercepting this call turns out to be pretty simple - create a derived type and override Remove:
public override void Remove(object key)
{
base.Remove(key);
var connectionId = (ulong)key;
if (!_clientDisconnectTokens.TryRemove(connectionId, out var cancellationTokenSource))
return;
Cancel(cancellationTokenSource);
}
private static void Cancel(CancellationTokenSource cancellationTokenSource)
{
// Use TaskScheduler.UnobservedTaskException for caller to catch exceptions
Task.Run(() => cancellationTokenSource.Cancel());
}
Calling Cancel tends to throw, so we invoke this using TPL so you can catch any exception thrown during cancel by subscribing to TaskScheduler.UnobservedTaskException.
What left?
creation of the HttpListenerHashtable derived type
storage of in-flight CancellationTokenSource instances
set or replace the HashTable field of HttpListener with the HttpListenerHashtable (it's best to do this right after creating the HttpListener instance)
handle the request of a disconnect token, and the client disconnects while the code is executing
All of which are addressed in the full source.
I'm having the same problem, I do a long sql query, and if user keeps pressing F5, it blows up.
HttpListener do not expose its underlying socket so you can poll it.
So I managed this workaround:
IDbCommand cmd = LongSqlQuery();
while (!taskCompleted)
{
try
{
//send disposable data at times to check connection
byte[] dummy = new byte[1] { 0xFF };
outputStream.Write(dummy,0,1);
Task.Delay(100).Wait();
}
catch
{
//Client disconnect
cmd.Cancel();
}
}
//Redirect client to results page
response.Redirect("http://yourserver.com/results?query=yourquery");