c# - Why can I access controls from a form in a thread without cross thread exception?

Question

Usually, when you access controls in a Thread you end up with some cross thread exceptions. In my C# WinForms Application I have a picture box and a toolstriplabel which do not cause that exception. I don't understand why, can anybody explain this to me?

Here some code explanation:

In the main form I have a picturebox and a toolstriplabel. Also I have a reference to another Form, which has no controls and no additional source code. And then in the main form there is another object which works with a thread. This thread can raise three different events and the main form is subscribed to these three events.

• Event1 causes the toolstriplabel to update (with some information from the thread).
• Event2 causes the picturebox to update (with a new picture from the thread).

Event1 and Event2 work perfectly fine. I do not use any invoke methods, I directly change Text and BackgroundImage properties without cross thread exception.

• Event3 though makes troubles. It is supposed to show the other form but I receive the cross therad exception. It works only if I use a BeginInvoke to show the form.

Why is that?

Edit:

The multithreading is done by an MJPEGStream object. I subscribe the NewFrame method of that MJPEGStream object.

public partial class Form1 : Form
{
    private CAM cam;

    private PeekWindow frmPeekWindow;

    public Form1()
    {
        InitializeComponent();

        cam = new CAM();
        cam.NewImageMessageEvent += new NewImageEventHandler(cam_NewImageMessageEvent);
        cam.DetectionEvent += new DetectionEventHandler(cam_DetectionEvent);
        cam.FpsChangedMessageEvent += new FpsChangedEventHandler(cam_FpsChangedMessageEvent);
        cam.DetectionThreshold = (float)this.numDetectionThreshold.Value;

        frmPeekWindow = new PeekWindow();

        // without the next two lines, frmPeekwindow.Show() won't work if called in an event
        frmPeekWindow.Show();
        frmPeekWindow.Hide();
    }

    void cam_FpsChangedMessageEvent(object sender, FpsChangedEventArgs e)
    {
        lblFPS.Text = string.Format("fps: {0:0.0}", e.FPS);
    }

    void cam_DetectionEvent(object sender, DetectionEventArgs e)
    {
        if (chkEnablePeakWindow.Checked)
        {
            if (frmPeekWindow.InvokeRequired)
            {
                frmPeekWindow.Invoke((MethodInvoker)delegate()
                {
                    frmPeekWindow.Show();
                    frmPeekWindow.setImage(e.Image);
                });
            }
            else
            {
                frmPeekWindow.Show();
                frmPeekWindow.setImage(e.Image);
            }
        }
    }

    void cam_NewImageMessageEvent(object sender, NewImageEventArgs e)
    {
        picStream.BackgroundImage = e.Image;
    }
}

And here's the CAM class:

class CAM
{
    private object lockScale = new object();

    private MJPEGStream stream;
    private Bitmap image;

    public event NewImageEventHandler NewImageMessageEvent;
    public event FpsChangedEventHandler FpsChangedMessageEvent;
    public event DetectionEventHandler DetectionEvent;

    // configure (login, pwd, source)
    public CAM()
    {
        this.stream = new MJPEGStream("...");
        this.stream.Login = "...";
        this.stream.Password = "...";
        this.stream.NewFrame += new NewFrameEventHandler(OnNewFrame)
    }

    private void OnNewFrame(object sender, NewFrameEventArgs ev)
    {
        try
        {
            FpsChangedMessageEvent(this, new FpsChangedEventArgs(10));

            // get image
            image = ev.Frame;
            NewImageMessageEvent(this, new NewImageEventArgs(new Bitmap(image)));

            DetectionEvent(this, new DetectionEventArgs(new Bitmap(image)));
        }
        catch (Exception ex)
        {
            Console.Out.WriteLine(ex.Message);
        }
    }
}

Answer 1

You won't get cross thread exception, but it doesn't mean that this is a safe operation. There is always a possibility for your control to go unstable. You just don't know when it will happen.

See the following explanation from Microsoft. http://msdn.microsoft.com/en-us/library/ms171728.aspx

Access to Windows Forms controls is not inherently thread safe. If you have two or more threads manipulating the state of a control, it is possible to force the control into an inconsistent state. Other thread-related bugs are possible, such as race conditions and deadlocks. It is important to make sure that access to your controls is performed in a thread-safe way.