Control/Sync variable between multiple threads c#

Question

I am having some timing issues with the following code. Basically I have "recordingOn", when True, it's suppose to start writing frames to file. In the below program, when I sometimes do Stop_Button_Click, I get an exception on "writer1.Write(frame1);" line, this is likely because it's doing so after I've already done dispose(). How do I synchronize this? thanks!

private bool recordingOn = false;

private void ConnectCameras_Button_Click(object sender, EventArgs e)
{
    if (!captureInProgress) //Start cameras streaming
    {
        camera1Capture.ImageGrabbed += ProcessFrame;
        camera1Capture.Start();
    }
    else //Stop cameras streaming
    {
        camera1Capture.Stop();
        imageBox1.Image = null;
        camera1Capture.ImageGrabbed -= ProcessFrame;
    }
    captureInProgress = !captureInProgress;
}

private void ProcessFrame(object sender, EventArgs arg)
{
    camera1Capture.Retrieve(frame1);
    imageBox1.Image = frame1;

    if (recordingOn)
    {
        try
        {
            writer1.Write(frame1);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }
}

private void Stop_Button_Click(object sender, EventArgs e)
{
    // Doing other stuff...
    recordingOn = false;
    writer1.Dispose();      
}

my mistake, writer1.Dispose should also have been in the lock, have updated the answer, can you try now. — peeyush singh
– peeyush singh, Commented Mar 7, 2019 at 0:22

peeyush singh · Accepted Answer · 2019-03-07 00:21:15Z

1

The basic issue is of shared variable across multiple threads. Please use lock to control access to shared variable. Lock ensures that at one time only one thread can access a variable.

private bool recordingOn = false;
private static object _lock = new Object();

private void ConnectCameras_Button_Click(object sender, EventArgs e)
{
    if (!captureInProgress) //Start cameras streaming
    {
        camera1Capture.ImageGrabbed += ProcessFrame;
        camera1Capture.Start();
    }
    else //Stop cameras streaming
    {
        camera1Capture.Stop();
        imageBox1.Image = null;
        camera1Capture.ImageGrabbed -= ProcessFrame;
    }
    captureInProgress = !captureInProgress;
}

private void ProcessFrame(object sender, EventArgs arg)
{
    camera1Capture.Retrieve(frame1);
    imageBox1.Image = frame1;
    lock (_lock)
    {
    if (recordingOn)
    {
        try
        {
            writer1.Write(frame1);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }
    }
}

private void Stop_Button_Click(object sender, EventArgs e)
{
    // Doing other stuff...
    lock (_lock)
    {
      recordingOn = false;
      writer1.Dispose();   
    }

}

edited Mar 7, 2019 at 0:21

answered Mar 6, 2019 at 6:20

peeyush singh

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4 Comments

golu Over a year ago

I tried this, but unfortunately it didn't work. I still got issue at "Write".

peeyush singh Over a year ago

@golu my mistake, writer1.Dispose should also have been in the lock, can you try now.

golu Over a year ago

Ok thanks, will try. So does this mean, execution in a thread will continue past the lock section? like does it skip over writer1.Dispose that was after the lock section?

peeyush singh Over a year ago

lock makes the code under if atomic. In earlier case where I was locking and setting the flag to false, then disposing outside lock, what can happen is that the flag is set to false and lock released. Now a different thread takes over and again sets the flag to true and maybe starts writing, then our original thread gets cpu and tries to dispose. Basically in a multi-threading environment, execution can be interrupted at any instruction, that is why we need to sync access to shared data/resources.

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