Question

I’m experiencing a weird behavior while trying to stop a SerialPort: the DataReceived event continues to fire after unsubscribing and after calling close! (see StopStreaming in the following code). As a result, in my event handler code I get an InvalidOperationException with the message that “The port is closed”.

What am I missing? What is the correct way to close the port and stop the events?

EDIT: I get this error every time I run my code. So this is not a race condition that happens randomly but rather a systematic problem indicating a completely broken code! However, I fail to see how…

private SerialPort comPort = new SerialPort();

public override void StartStreaming()
{
  comPort.Open();
  comPort.DiscardInBuffer();
  comPort.DataReceived += comPort_DataReceived;
}

public override void StopStreaming()
{
  comPort.DataReceived -= comPort_DataReceived;
  comPort.Close();
  isStreaming = false;
}

private void comPort_DataReceived(object sender, SerialDataReceivedEventArgs e)
{
  if (e.EventType == SerialData.Chars)
  {
    SerialPort port = (SerialPort)sender;
    int N = comPort.BytesToRead;
    for (; N > 0; N--)
    {
      byte b = Convert.ToByte(comPort.ReadByte());
      //... process b
    }
  }
}

EDIT: following the suggestions, I changed StopStreaming code to something like this:

public override void StopStreaming()
{
  comPort.DataReceived -= comPort_DataReceived;
  Thread.Sleep(1000);
  comPort.DiscardInBuffer();
  Thread.Sleep(1000);
  comPort.Close();
  isStreaming = false;
}

It seems to work now but I’m not really that happy. I wish there was a more effective way to remove the callback rather than inserting sleep periods in the program.

Answer 1

Your DataReceived event handler is called on a threadpool thread. And yes, they’ve got the awkward habit of running your code at an unpredictable time, it is not instant. So it is fairly inevitable that, if the device is actively sending data, that it can race with your Close() call and run after you closed it. Unsubscribing doesn’t fix it, the threadpool thread already got its target method.

Do realize what you are doing to trigger this problem, you are closing the port while the device is sending data. That’s not great, it is guaranteed to cause data loss. But not unlikely to happen when you are debugging your code since you don’t actually care about the data.

A counter-measure is to turn off handshaking so the device cannot send anything anymore. Discard the input buffer. Then sleep for a while, a second or two, to ensure that any threadpool threads in-flight have completed running. Then close the port. A very pragmatic one is to simply not close the port, Windows will take care of it when your process terminates.