AccessViolationExeption in Parallel.ForEach loop

Question

I'm trying to execute this code in .NET using EmguCV:

Parallel.ForEach<Bitmap>(GetImagesFromCamera(), bmp =>
{
    Image<Gray, Byte> src = new Image<Gray, Byte>(bmp);
    Image<Gray, Byte> dest = new Image<Gray, Byte>(src.Width, src.Height);
    CvInvoke.cvThreshold(src, dest, 100, 255, Emgu.CV.CvEnum.THRESH.CV_THRESH_BINARY);
    form.setImage(dest);
});

This is my GetPicturesFromCamera() Method:

static IEnumerable<Bitmap> GetImagesFromCamera()
{
    Camera cam = new Camera();
    cam.StartCapture();
    while (true)
    {
        yield return new Image<Gray, Byte>(cam.getBitmap());// <=== AccessViolationException
    }
}

At the marked point I always get a AccessViolationException saying "Attempted to read or write protected memory. This is often an indication that other memory is corrupt.", but I don't find the mistake.

Can you figure it out?

EDIT:

I tried it sequencialy too, this way it works:

foreach (var bmp in GetImagesFromCamera())
        {
            Image<Gray, Byte> dest = new Image<Gray, Byte>(bmp.Width, bmp.Height);
            CvInvoke.cvThreshold(bmp, dest, 100, 255, Emgu.CV.CvEnum.THRESH.CV_THRESH_BINARY);
            form.setImage(dest);
        };

but this way it doesn't:

foreach (var bmp in GetImagesFromCamera())
        {
            Image<Gray, Byte> dest = new Image<Gray, Byte>(bmp.Width, bmp.Height);
            Image<Gray, Byte> tmp = new Image<Gray, Byte>(bmp.Width, bmp.Height);
            CvInvoke.cvThreshold(bmp, dest, 100, 255, Emgu.CV.CvEnum.THRESH.CV_THRESH_BINARY);
            CvInvoke.cvErode(dest, tmp, IntPtr.Zero, 2);
            CvInvoke.cvDilate(tmp, dest, IntPtr.Zero, 2);
            form.setImage(dest);
        };

Answer 1

I found a solution: I had to put all the code in the Camera class into the GetImagesFromCamera method, but I have no idea why.