c# picturebox memory releasing problem

Question

Hi everybody. I'm a newby in C#. I have to repeatedly refresh a GUI picture box in a worker thread. The image is acquired from a camera polling a driver with a GetImage method that retrives the image to be displayed. Even if I allocate the bitmap using directive "using" and explicitly call G.C, memory seems to be never deallocated.

The worker thread is something like this:

   while (true)
    {
        // request image with IR signal values (array of UInt16)
        image = axLVCam.GetImage(0);
        lut = axLVCam.GetLUT(1);
        DrawPicture(image, lut);
        //GC.Collect();

    }

While the DrawPicture method is something like

   public void DrawPicture(object image, object lut)
{

  [...]

    // We have an image - cast it to proper type
    System.UInt16[,] im = image as System.UInt16[,];
    float[] lutTempConversion = lut as float[];

    int lngWidthIrImage = im.GetLength(0);
    int lngHeightIrImage = im.GetLength(1);

    using (Bitmap bmp = new Bitmap(lngWidthIrImage, lngHeightIrImage)) {

      [...many operation on bitmap pixel...]

        // Bitmap is ready - update image control

        //SetControlPropertyThreadSafe(tempTxtBox, "Text", string.Format("{0:0.#}", lutTempConversion[im[160, 100]]));

        //tempTxtBox.Text = string.Format("{0:00000}", im[160, 100]);
        //System.Drawing.Image.FromHbitmap(bmp.GetHbitmap());
        pic.Image = System.Drawing.Image.FromHbitmap(bmp.GetHbitmap());
    }
}

Problems arises with the

pic.Image = System.Drawing.Image.FromHbitmap(bmp.GetHbitmap());

In fact commenting that line of code, garbage collection works as it would. Better, the problem seems to be with

System.Drawing.Image.FromHbitmap(bmp.GetHbitmap())

Any advice to solve this memory leak?

Thanks a lot!

Answer 1

Image implements IDisposable, so you should call Dispose on each Image instance that you create, when it is no longer needed. You could try to replace this line in your code:

pic.Image = System.Drawing.Image.FromHbitmap(bmp.GetHbitmap());

With this:

if (pic.Image != null)
{
    pic.Image.Dispose();
}
pic.Image = System.Drawing.Image.FromHbitmap(bmp.GetHbitmap());

This will dispose the previous image (if any) before the new one is assigned.

Answer 2

The thing is, you're making a GDI bitmap of bmp with GetHbitmap, which according to msdn:

You are responsible for calling the GDI DeleteObject method to free the memory used by the GDI bitmap object.

Then the FromHbitmap method makes a copy of the GDI bitmap; so you can release the incoming GDI bitmap using the GDI DeleteObject method immediately after creating the new Image.

So basically I would add:

[System.Runtime.InteropServices.DllImport("gdi32.dll")]
public static extern bool DeleteObject(IntPtr hObject);

...

IntPtr gdiBitmap = bmp.GetHbitmap();

// Release the copied GDI bitmap
if (pic.Image != null)
{
    pic.Image.Dispose();
}

pic.Image = System.Drawing.Image.FromHbitmap(gdiBitmap);

// Release the current GDI bitmap
DeleteObject(gdiBitmap);

I am unsure if you need the GDI bitmap to perform some kind of transformation. In case you don't you can just assign the bitmap to the Image property of your PictureBox, and ignore the former solution:

// Since we're not using unmanaged resources anymore, explicitly disposing 
// the Image only results in more immediate garbage collection, there wouldn't 
// actually be a memory leak if you forget to dispose.
if (pic.Image != null)
{
    pic.Image.Dispose();
}

pic.Image = bmp;