Threading - Multiple SerialPort and one DataGridView

Question

I have 3 SerialPort components and 2 of them has DataReceived handlers. When data is received, they are sending output through other SerialPort (the third one). I know that the possibility is not high in my application that both SerialPorts will try to send at the same time, but I tried to simulate this and of course I get an error that the COM PORT is being used.

private void sendToCOM(String comNumber, String msg, String speed)
    {
        try
        {
            using (SerialPort comPort = new SerialPort(comNumber, Int32.Parse(speed), Parity.None, 8, StopBits.One))
            {
                comPort.Open();
                comPort.Write(msg);
                comPort.Close();
                comPort.Dispose();
            }
        }
        catch(Exception ex)
        {
                cstFuncs.errorHandler(ex.Message, SettingsForm);
        }
    }

I understand that each SerialPort has it's own thread.

How can I make a queue or how can I prevent from both serial port to try access the same resources on the same time (and not loose the data that is supposed to be sent out)

score 2 · Answer 1 · answered Jan 15 '13 at 10:58

You probably don't have a thread per serial port, but rather the DataReceived handler is executed in the thread-pool. It still means you can get two reads at once.

You can use locking to solve this:

Add a OutputPortLock object to your class:

private object OutputPortLock = new object();

And try to lock it before writing to the output port:

lock(OutputPortLock)
{
    using(SerialPort comPort=....)
    {
        ... write to com port ...
    }
}

This will make sure only one thread tries to write to the output port at a time. You might still get Com Port in Use errors if some other process tries to use it.

Thank you very much for you help. It looks like this is the way.. I'm going to read more about it. — Itay, Jan 15 '13 at 11:17

score 2 · Accepted Answer · answered Jan 15 '13 at 10:59

2

There are a few ways you could get locking in the sendToCOM method. This will stop the method from executing twice by causing any subsequent calls to wait until the first is complete. This isn't going to help if another process tried to use the same SerialPort, but it will stop any local contention.

One way would be to use the lock keyword on an object like so:

private readonly object _myLock = new object();

private void sendToCOM(String comNumber, String msg, String speed)
{
    try
    {
        lock(_myLock)
        {
            using (SerialPort comPort = new SerialPort(comNumber, Int32.Parse(speed), Parity.None, 8, StopBits.One))
            {
                comPort.Open();
                comPort.Write(msg);
                comPort.Close();
                comPort.Dispose();
            }
        }
    }
    catch(Exception ex)
    {
        cstFuncs.errorHandler(ex.Message, SettingsForm);
    }
}

Another way could be to use a ManualResetEventSlim (or something similar) to achieve the same effect:

private ManualResetEventSlim _myLock = new ManualResetEventSlim(true);

private void sendToCOM(String comNumber, String msg, String speed)
{
    _myLock.Wait();
    try
    {
        _myLock.Reset();
        using (SerialPort comPort = new SerialPort(comNumber, Int32.Parse(speed), Parity.None, 8, StopBits.One))
        {
            // ..
        }
    }
    catch(Exception ex)
    {
        cstFuncs.errorHandler(ex.Message, SettingsForm);
    } 
    finally
    {
        _myLock.Set();
    }
}

answered Jan 15 '13 at 10:59

Rudi Visser

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Thanks for you help. What is the difference between those two methods? I tried the second method, and it looks like it's working, but I've managed to get it one time to crash (I'm having 2 application that sends every 100ms string into each com port) – Itay Jan 15 '13 at 11:19
The difference is just the usage really. I'd recommend using `lock` to be honest, `ManualResetEventSlim` was just an idea I was toying with that should still work. Are you sure the crash was caused by this method? – Rudi Visser Jan 15 '13 at 11:22
the application did not crash (the "catch" catch it) and the exception was `Access to the port 'COM6' is denied.` . Before I tried to implant this locking (second method), I got many of this. Now i get much less errors, but I still get some.. I'm going to try the first method now. – Itay Jan 15 '13 at 11:25
That's certainly weird. Honestly, I haven't worked with `SerialPort`s at all, so I couldn't give much more solid advice on this. However, and I know it's bad, but an artificially induced delay at the beginning or in the `finally` may help? (Talking about `Thread.Sleep`) – Rudi Visser Jan 15 '13 at 11:27
I tried now the first method and it slows down all the process (but I do get no errors now) Maybe as you suggested, make a tiny delay and use the second method together should solve the issue. I don't really have a need of this high speeds actually, but it will be nice to get the maximum I can out of this if one day it will be needed. – Itay Jan 15 '13 at 11:35
@Itay Honestly if the first method works I would go with that rather than an artificial delay. The first method is more "proper", it will slow down the process because things are going to be correctly waiting for each other to complete before continuing. – Rudi Visser Jan 15 '13 at 11:38

score 1 · Answer 3 · edited May 23 '17 at 12:12

Theres a few things you can do;

1) Build a lock mechanism. Threads will wait until this lock is unlocked before trying to access the method, meaning that each of the threads will take it in turn to fire off "sendToCOM"

2) Another method is to use a ConcurrentQueue (see: http://msdn.microsoft.com/en-us/library/dd267265.aspx). This is a thread safe list that you could, if you wanted, add COMport objects too, and fire them off from there. Theres a few questions around about doing so, for example;

How to work threading with ConcurrentQueue<T>

I'm sure theres more ways, but these are 2 i've thought of off the top of my head. I'd say the second is more reliable, but requires more work to get running, so the first may be your best interest.

Threading - Multiple SerialPort and one DataGridView

3 Answers3