如何杀死（不取消）已经被调用的异步任务？

Question

I am developing an application that sends and receives commands via SerialPort. Sometimes the hardware I am communicating with does not respond, how do I stop my application from waiting for a response? Because it is not allowing me to retry sending and receiving commands. I get a "Access to the port 'COM#' is denied." exception when I call the sendRecvCMDAsync() function a second time.

Here is my SerialPort communicaion code,

        public async Task<List<String>> sendRecvCMDAsync(String COMPort, String FW_CMD, List<String> SendParamList, CancellationToken ct)
    {
        SL_Port = new SerialPort();
        try
        {
            if (COMPort != "")
            {
                SL_Port.PortName = COMPort;
            }
            else
            {
                //show a message box;
            }

            if (SL_Port.IsOpen)
            {
                SL_Port.Dispose();
                SL_Port.Close();
            }
            SL_Port.Close();
            SL_Port.Open();
            SL_Port.WriteLine(FW_CMD);
            _continue = true;
            Array.Clear(buffer, 0, buffer.Length);
            int count = 0;
            while (_continue)
            {
                mess = "";
                Task<int> bytes = SL_Port.BaseStream.ReadAsync(buffer, count, buffer.Length - count, ct);
                int bytesCount = await bytes;//I need to kill this after its started, because it is not alowing me to access the COM port unless it completes.
                count = bytesCount + count;
                mess = Encoding.ASCII.GetString(buffer).TrimEnd('\0');
                if (mess.Contains(""))
                {
                    break;
                }
                else
                {
                }
            }
        }
        catch (OperationCanceledException)
        {
            MessageBox.Show("You have Successfuly Cancelled the event");
        }
        catch (Exception e)
        {

        }

        SL_Port.Dispose();
        SL_Port.Close();
        return listForFW;
    }

I have called cancel but the it does not work since the code is already executed awaiting response.

Here is my function call code

 public async void startSending()
 {
   cts = new CancellationTokenSource();
   ReturnParams = await sendRecvCMDAsync(xport, commandSend, paramsList, cts.Token)
 }

Here is my cancel call code.

 public void btnCancelSend()
 {
    cts.Cancel();
 }

So how can I solve this issue? Cancellation isn't helping.

Answer 1

The only way to cleanly kill (as in terminate) an operation is to run it in a separate process . Cancelling should be preferred.

In your case, there is an alternative way to cancel (which is much easier): traditionally, Win32 asynchronous operations can be canceled by closing the handle they're operating on. This will cause all existing operations to complete (with an error).

Note that this cancels all operations; in particular, if you're reading and writing at the same time, both operations will error out if you close the handle.

Answer 2

Start your own Thread .

Do message pumping on your own.

You can .Abort() it when hardware fails. Your scenario is untypical for async/await.