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Cancellation Token Doesn't throw exception

Good day.
I'm having a problem exiting a task with the cancellation token.
My program freezes when I get to the token2.ThrowIfCancellationRequested();.
Following it with the breakpoints is shows that the token2 is cancelled, but the program doesn't revert back to the previous sub routine where I try and catch
try
{
Task.Run(() => SendData_DoWork(_tokenSource3));
}
catch (OperationCanceledException ex)
{
SetText("Communivation error with device");
SetText("");
}
finally
{
token.Dispose();
}
}//comms routine
//send Meter Address to communicate to meter
private void SendData_DoWork(CancellationTokenSource token)
{
var token2 = token.Token;
var _tokenSource4 = new CancellationTokenSource();
try
{
timer.Interval = 10000;
timer.Start();
timer.Elapsed += OnTimerElapsed;
NetworkStream stream = client.GetStream();
SerialConverter serialConverter = new SerialConverter();
Thread.Sleep(1000);
string newtext = null;
newtext = $"/?{address}!\r\n";
SetText("TX: " + newtext);
byte[] newData = stringSend(newtext);
stream.Write(newData, 0, newData.Length);
Thread.Sleep(50);
byte[] message = new byte[23];
int byteRead;
while (true)
{
byteRead = 0;
try
{
byteRead = stream.Read(message, 0, 23);
if (message[0] == (char)0x15)
{
token.Cancel();
}
}
catch
{
token.Cancel();
}
if ((byteRead == 0))
{
token.Cancel();
}
timer.Stop();
timer.Dispose();
ASCIIEncoding encoder = new ASCIIEncoding();
string newresponse = encoder.GetString(serialConverter.convertFromSerial(message));
SetText("RX: " + newresponse);
if (newresponse[0].ToString() == SOH)
{
token.Cancel();
}
if (newresponse != null)
{
/* NEXT SUB ROUTINE*/
}
else { break; }
}//while looop
}//try
catch (Exception ex)
{
token.Cancel();
}
if (token2.IsCancellationRequested)
{
timer.Stop();
timer.Dispose();
token2.ThrowIfCancellationRequested();
}
}//sendData subroutine

You are launching a Task, and ignoring the result; the only time Task.Run would throw is if the task-method is invalid, or enqueuing the operation itself failed. If you want to know how SendData_DoWork ended, you'll need to actually check the result of the task, by capturing the result of Task.Run and awaiting it (preferably asynchronously, although if we're talking async, SendData_DoWork should probably also be async and return a Task).
Your catch/finally will probably be exited long before SendData_DoWork even starts - again: Task.Run just takes the time required to validate and enqueue the operation; not wait for it to happen.

I think you have missunderstood how cancellation tokens are supposed to work. Your work method should take a CancellationToken, not a CancellationTokenSource. And it should call ThrowIfCancellationRequested inside the loop, not after. I would suspect that you would get some issues with multiple cancel calls to the same cancellation token.
Typically you would use a pattern something like like this:
public void MyCancelButtonHandler(...) => cts.Cancel();
public async void MyButtonHandler(...){
try{
cts = new CancellationTokenSource(); // update shared field
await Task.Run(() => MyBackgroundWork(cts.Token));
}
catch(OperationCancelledException){} // Ignore
catch(Exception){} // handle other exceptions
}
private void MyBackgroundWork(CancellationToken cancel){
while(...){
cancel.ThrowIfCancellationRequested();
// Do actual work
}
}

So in my particular case it seems like changing the sub-routines from private async void ... to private async Task fixes the particular issue that I'm having.

UI only partly responsive while running tasks and updating ui

i have the following ui -
For each line connection to crm should be tested. This is done in separate thread.
The test status of the connection to crm system is then updated in last column.
The problem is that the ui is only partly reponsive during threads run and updating of the ui, i.e.
i would like to click through the lines whilst updating.
Here is my code:
private async void btnTestAllConnections_Click(object sender, EventArgs e)
{
await TestConnectionsAsync();
}
private async Task TestConnectionsAsync()
{
try
{
int idxConn = columnLookup[ColumnIndex.Connection].Index;
if (lvInitParameters.Items.Count == 0)
return;
ManagedConnection connection = null;
btnTestAllConnections.Visible = false;
btnTestConnection.Visible = false;
panel2.Enabled = false;
panel3.Enabled = false;
tableLayoutPanel1.Enabled = false;
btnCancelTest.Visible = true;
List<Task> tasks = new List<Task>();
var cts = new CancellationTokenSource();
CancellationToken token = cts.Token;
foreach (ListViewItem lvi in lvInitParameters.Items)
{
InitParamProxy currentProfile = (InitParamProxy)lvi.Tag;
lvi.SubItems[idxConn].Text = "Testing...";
Task<bool> result =null;
try
{
result = Task.Run(
() =>
{
try
{
connection = currentProfile.ManagedConnection;
return connection?.ConnectionSuccess ?? false;
}
catch (Exception ex)
{
// crm exception
return false;
}
}, token);
if (token.IsCancellationRequested)
{
Console.WriteLine("\nCancellation requested in continuation...\n");
token.ThrowIfCancellationRequested();
}
ListViewItem testItem =
items.Where(si => ((InitParamProxy)lvi.Tag).ProfileKey.Equals(((InitParamProxy)si.Tag).ProfileKey)).SingleOrDefault();
lvi.SubItems[idxConn].Text = (result.Result) ? "Success" : "Fail";
if (testItem != null)
testItem.SubItems[idxConn].Text = (result.Result) ? "Success" : "Fail";
}
catch
{
ListViewItem testItem =
items.Where(si => ((InitParamProxy)lvi.Tag).ProfileKey.Equals(((InitParamProxy)si.Tag).ProfileKey)).SingleOrDefault();
lvi.SubItems[idxConn].Text = "Canceled";
if (testItem != null)
testItem.SubItems[idxConn].Text = "Canceled";
}
tasks.Add(result);
}
Task.WaitAll(tasks.ToArray());
btnTestAllConnections.Visible = true;
btnTestConnection.Visible = true;
panel2.Enabled = true;
panel3.Enabled = true;
tableLayoutPanel1.Enabled = true;
btnCancelTest.Visible = false;
}
catch (Exception)
{
}
}

In the end of your method you have
Task.WaitAll(tasks.ToArray());
This will block until all tasks are done. You should instead use WhenAll
await Task.WhenAll(tasks.ToArray());
You are also using result.Result in several places, and this also blocks. This should be replaced by awaiting the task, i.e. await result

Correct way of starting a forever-running loop with Async

I've an existing code I wrote some time ago, that works but I dislike the fact that the thread I start remains in loop.
This piece of code is a consumer on an IBMMQ code, waiting for messages to be processed.The problem I've is that with the following code
private Task ExecuteQueuePolling(CancellationToken cancellationToken)
{
return Task.Factory.StartNew(() =>
{
ConnectToAccessQueue();
Logger.Debug($"Accessed to the queue {queueName}");
Logger.DebugFormat("Repeating timer started, checking frequency: {checkingFrequency}",
checkingFrequency);
while (!cancellationToken.IsCancellationRequested)
{
Logger.Trace( () => "Listening on queues for new messages");
// isChecking = true;
var mqMsg = new MQMessage();
var mqGetMsgOpts = new MQGetMessageOptions
{ WaitInterval = (int)checkingFrequency.TotalMilliseconds };
// 15 second limit for waiting
mqGetMsgOpts.Options |= MQC.MQGMO_WAIT | MQC.MQGMO_FAIL_IF_QUIESCING |
MQC.MQCNO_RECONNECT_Q_MGR | MQC.MQOO_INPUT_AS_Q_DEF;
try
{
mqQueue.Get(mqMsg, mqGetMsgOpts);
if (string.Compare(mqMsg.Format, MQC.MQFMT_STRING, StringComparison.Ordinal) == 0)
{
var text = mqMsg.ReadString(mqMsg.MessageLength);
Logger.Debug($"Message received : [{text}]");
Message message = new Message { Content = text };
foreach (var observer in observers)
observer.OnNext(message);
}
else
{
Logger.Warn("Non-text message");
}
}
catch (MQException ex)
{
if (ex.Message == MQC.MQRC_NO_MSG_AVAILABLE.ToString())
{
Logger.Trace("No messages available");
//nothing to do, emtpy queue
}
else if (ex.Message == MQC.MQRC_CONNECTION_BROKEN.ToString())
{
Logger.ErrorException("MQ Exception, trying to recconect", ex);
throw new ReconnectException();
}
}
Thread.Sleep(100);
}
},cancellationToken);
}
//Calling method
try
{
string queueManagerName = configuration.GetValue<string>("IBMMQ:QUEUE_MANAGER_NAME");
// var queueManager = new MQQueueManager(queueManagerName,dictionary2);
QueueMonitor monitor = new QueueMonitor(configuration, "IMPORTER_RECEIVER_TEST");
//_subscription = monitor.Subscribe(receiver);
await monitor.StartAsync(cts.Token).ConfigureAwait(false);
}
catch (Exception e)
{
log.Error(e, "Error creating the queue monitor or it's subscription");
}
finally
{
WaitForCancel(cts);
}
The call to await monitor.StartAsync(cts.Token).ConfigureAwait(false); remains pending.
How should I modify my code, so that the call returns and in background the task continue to loop?
Thanks in advance

Here is how you can simplify your code by replacing Thread.Sleep with Task.Delay:
private async Task ExecuteQueuePolling(CancellationToken cancellationToken)
{
while (true)
{
// Process mqQueue here
await Task.Delay(100, cancellationToken);
}
}
Task.Delay has the advantage that accepts a CancellationToken, so in case of cancellation the loop will exit immediately. This could be important if the pooling of the MQ was lazier (for example every 5 seconds).

private static Task _runningTask;
static void Main(string[] args)
{
var cts = new CancellationTokenSource();
_runningTask = ExecuteQueuePolling(cts.Token);
WaitForCancel(cts);
}
private static void WaitForCancel(CancellationTokenSource cts)
{
var spinner = new SpinWait();
while (!cts.IsCancellationRequested
&& _runningTask.Status == TaskStatus.Running) spinner.SpinOnce();
}
private static Task ExecuteQueuePolling(CancellationToken cancellationToken)
{
var t = new Task(() =>
{
while (!cancellationToken.IsCancellationRequested)
; // your code
if (cancellationToken.IsCancellationRequested)
throw new OperationCanceledException();
}, cancellationToken, TaskCreationOptions.LongRunning);
t.Start();
return t;
}

Monitor Async Task with Timeout and cancellation

I All,
I have to monitor an async task, which has to be cancellable and do not perform more than specific Time To Live.
I already knew about the following code.
CancellationTokenSource l_cts = new CancellationTokenSource(timemillis);
which will doing the cancellation ( as far i monitor the token in my async method).
However, this NOT gave me any information about WHY he has been cancelled, Timeout or user cancellation? furthermore, the timeout event is delayed while i did not catch the cancellation with
Token.ThrowIfCancellationRequested();
In order to solve these issues, I wrote the timeout process as follow.
static async Task TestAsync(int processDelaySeconds, int cancelDelaySeconds, int timeoutDelaySeconds )
{
CancellationTokenSource l_cts = new CancellationTokenSource();
// the process to monitor
Task l_process = new Task((state) =>
{
Console.WriteLine("Process BEGIN");
// dummy loop
for (int l_i = 0; l_i != processDelaySeconds; l_i++)
{
Thread.Sleep(1000);
l_cts.Token.ThrowIfCancellationRequested();
}
Console.WriteLine("Process END");
}, null, l_cts.Token);
// register timeout
RegisteredWaitHandle l_rwh = ThreadPool.RegisterWaitForSingleObject(l_cts.Token.WaitHandle,
(state, timedOut) =>
{
if (timedOut)
{
l_cts.Cancel();
Console.WriteLine("Timed out");
}
else
{
Console.WriteLine("Cancel Signaled");
}
},
null, (int)TimeSpan.FromSeconds(timeoutDelaySeconds).TotalMilliseconds, true);
// cancel task
if (cancelDelaySeconds > 0)
{
Task l_cancel = new Task(() =>
{
Thread.Sleep(TimeSpan.FromSeconds(cancelDelaySeconds));
l_cts.Cancel();
});
l_cancel.Start();
}
try
{
l_process.Start();
await l_process;
}
catch (OperationCanceledException)
{
Console.WriteLine("Task Cancelled");
}
finally
{
// be sure to unregister the wait handle to cancel the timeout
if (l_process.Status != TaskStatus.Canceled) l_rwh.Unregister(l_cts.Token.WaitHandle);
}
Console.WriteLine("Task Status is : {0}", l_process.Status);
}
static async void Tests()
{
Console.WriteLine("NORMAL PROCESS");
Console.WriteLine("--------------");
await TestAsync(2, 10, 10);
Console.WriteLine();
Console.WriteLine("CANCEL");
Console.WriteLine("------");
await TestAsync(5, 2, 10);
Console.WriteLine();
Console.WriteLine("TIMEOUT");
Console.WriteLine("-------");
await TestAsync(10, 15, 2);
}
Then My question is :
Is there any drawbacks or traps behind the scene ?
A better and more efficient way ??
ps- Goal is performance, not shorter code.

If you need to distinguish between the user's and time-out cancellation, you can use CreateLinkedTokenSource:
using System;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApp
{
internal class Program
{
// worker
private static void DoWork(CancellationToken token)
{
for (int i = 0; i < 1000; i++)
{
token.ThrowIfCancellationRequested();
Thread.Sleep(100); // do the work item
}
token.ThrowIfCancellationRequested();
}
// test
private static void Main()
{
var userCt = new CancellationTokenSource();
var combinedCt = CancellationTokenSource.CreateLinkedTokenSource(
userCt.Token);
combinedCt.CancelAfter(3000); // cancel in 3 seconds
Console.CancelKeyPress += (s, e) =>
{
e.Cancel = true;
userCt.Cancel();
};
var task = Task.Run(
() => DoWork(combinedCt.Token),
combinedCt.Token);
try
{
task.Wait();
}
catch (AggregateException ex)
{
Console.WriteLine(ex.InnerException.Message);
if (task.IsCanceled)
{
if (userCt.Token.IsCancellationRequested)
Console.WriteLine("Cancelled by user");
else if (combinedCt.Token.IsCancellationRequested)
Console.WriteLine("Cancelled by time-out");
else
Console.WriteLine("Cancelled by neither user nor time-out");
}
}
}
}
}
As to your original code, you really did not need ThreadPool.RegisterWaitForSingleObject(l_cts.Token.WaitHandle, ...), there's CancellationToken.Register for that, which returns an IDisposable ready for use with using.

In order to know if your Task has been cancelled or timed out, you can use the Task.WaitAny overload which takes a TimeSpan:
// Index will return -1 if timeout has occured, otherwise will print the index of the completed task
var cnclToken = new CancellationTokenSource().Token
var yourTask = Task.Run(() => { /* Do stuff */ }, cnclToken);
var index = Task.WhenAny(new[] { yourTask }, TimeSpan.FromSeconds(1));
http://msdn.microsoft.com/en-us/library/dd235645(v=vs.110).aspx

C# 5 Async Await .Task.Factory.StartNew cancellation

I have the async code that implements cancellation token. It's working but Im not pretty sure if this is the right way to do it so I just want feedback about it.
Here is the actual code:
/// <summary>
///
/// </summary>
private async void SaveData() {
if (GetActiveServiceRequest() != null)
{
var tokenSource = new System.Threading.CancellationTokenSource();
this.ShowWizardPleaseWait("Saving data...");
var someTask = System.Threading.Tasks.Task<bool>.Factory.StartNew(() =>
{
bool returnVal = false;
// Set sleep of 7 seconds to test the 5 seconds timeout.
System.Threading.Thread.Sleep(7000);
if (!tokenSource.IsCancellationRequested)
{
// if not cancelled then save data
App.Data.EmployeeWCF ws = new App.Data.EmployeeWCF ();
returnVal = ws.UpdateData(_employee.Data);
ws.Dispose();
}
return returnVal;
}, tokenSource.Token);
if (await System.Threading.Tasks.Task.WhenAny(someTask, System.Threading.Tasks.Task.Delay(5000)) == someTask)
{
// Completed
this.HideWizardPleaseWait();
if (someTask.Result)
{
this.DialogResult = System.Windows.Forms.DialogResult.OK;
}
else
{
this.DialogResult = System.Windows.Forms.DialogResult.Abort;
}
btnOK.Enabled = true;
this.Close();
}
else
{
tokenSource.Cancel();
// Timeout logic
this.HideWizardPleaseWait();
MessageBox.Show("Timeout. Please try again.")
}
}
}
Does async / await / cancellation code is well implemented?
Thanks and appreciate the feedback.

In general, you should use ThrowIfCancellationRequested. That will complete the returned Task in a canceled state, rather than in a "ran to completion successfully" state with a false result.
Other points:
Avoid async void. This should be async Task unless it's an event handler.
Prefer Task.Run over TaskFactory.StartNew.
Use using.
If you're just using CancellationTokenSource as a timeout, then it has special capabilities for that. Creating a separate task via Task.Delay and Task.WhenAny isn't necessary.
Here's what the updated code would look like:
private async Task SaveData()
{
if (GetActiveServiceRequest() != null)
{
var tokenSource = new System.Threading.CancellationTokenSource(TimeSpan.FromSeconds(5));
var token = tokenSource.Token;
this.ShowWizardPleaseWait("Saving data...");
var someTask = System.Threading.Tasks.Task.Run(() =>
{
// Set sleep of 7 seconds to test the 5 seconds timeout.
System.Threading.Thread.Sleep(7000);
// if not cancelled then save data
token.ThrowIfCancellationRequested();
using (App.Data.EmployeeWCF ws = new App.Data.EmployeeWCF())
{
return ws.UpdateData(_employee.Data);
}
}, token);
try
{
var result = await someTask;
// Completed
this.HideWizardPleaseWait();
if (result)
{
this.DialogResult = System.Windows.Forms.DialogResult.OK;
}
else
{
this.DialogResult = System.Windows.Forms.DialogResult.Abort;
}
btnOK.Enabled = true;
this.Close();
}
catch (OperationCanceledException)
{
// Timeout logic
this.HideWizardPleaseWait();
MessageBox.Show("Timeout. Please try again.")
}
}
}