c# - InvalidOperationException - object is currently in use elsewhere - red cross

Question

I have a C# desktop application in which one thread that I create continously gets an image from a source(it's a digital camera actually) and puts it on a panel(panel.Image = img) in the GUI(which must be another thread as it is the code-behind of a control.

The application works but on some machines I get the following error at random time intervals(unpredictable)

************** Exception Text **************
System.InvalidOperationException: The object is currently in use elsewhere. 

Then the panel turns into a red cross, red X - i think this is the invalid picture icon that is editable from the properties. The application keeps working but the panel is never updated.

From what I can tell this error comes from the control's onpaint event where I draw something else on the picture.

I tried using a lock there but no luck :(

The way I call the function that puts the image on the panel is as follows:

if (this.ReceivedFrame != null)
{
    Delegate[] clients = this.ReceivedFrame.GetInvocationList();
    foreach (Delegate del in clients)
    {
        try
        {
            del.DynamicInvoke(new object[] { this, 
                new StreamEventArgs(frame)} );
        }
        catch { }
    }
}

this is the delegate:

public delegate void ReceivedFrameEventHandler(object sender, StreamEventArgs e);
    public event ReceivedFrameEventHandler ReceivedFrame;

and this is how the function inside the control code-behind registers to it:

Camera.ReceivedFrame += 
    new Camera.ReceivedFrameEventHandler(camera_ReceivedFrame);

I also tried

del.Method.Invoke(del.Target, new object[] { this, new StreamEventArgs(b) });

instead of

del.DynamicInvoke(new object[] { this, new StreamEventArgs(frame) });

but no luck

Does anyone know how I could fix this error or at least catch the error somehow and make the thread put the images on the panel once again?

Answer 1

This is because Gdi+ Image class is not thread safe. Hovewer you can avoid InvalidOperationException by using lock every time when you need to Image access, for example for painting or getting image size:

Image DummyImage;

// Paint
lock (DummyImage)
    e.Graphics.DrawImage(DummyImage, 10, 10);

// Access Image properties
Size ImageSize;
lock (DummyImage)
    ImageSize = DummyImage.Size;

BTW, invocation is not needed, if you will use the above pattern.