GDI+ exception saving a Bitmap to a MemoryStream.

Question

I have a problem in a Windows Forms application with Bitmap.Save failing when I save to a MemoryStream. The problem only seems to occur intermittently on one machine (so far) and the bad news it is at a customer site. I can't debug on the machine, but I got a stack trace that narrowed the problem down to a single line of code.

Here's a condensed version of my code:

byte[] ConvertPixelsToBytes()
{
    // imagine a picture class that creates a 24 bbp image, and has
    // a method to get an unmanaged pixel buffer.
    // imagine it also has methods for getting the width,
    // height, pitch 

    // I suppose this line could return a bad address, but 
    // I would have expected that the Bitmap constructor would have 
    // failed if it was
    System.IntPtr pPixels = picture.GetPixelData();

    System.Drawing.Bitmap bmp = new System.Drawing.Bitmap(
            picture.width(),
            picture.height(),
            picture.pitch(),
            System.Drawing.Imaging.PixelFormat.Format24bppRgb,
            pPixels );

    // This line doesn't actually free the memory, but it could be freed in a 
    // background thread
    // (2)
    picture.releasePixelData(pPixels);

    System.IO.MemoryStream memStream = new System.IO.MemoryStream();
    try
    {
        // I don't see how this line could fail, but it does
        // (3)
        bmp.Save(memStream, System.Drawing.Imaging.ImageFormat.Bmp);
        return memStream.ToArray();
    }
   catch(System.Runtime.InteropServices.ExternalException e)
   {
       // e.Message is the very helpful " A generic error occurred in GDI+."
   }
   finally
   {
       memStream.Dispose();
   }
   return new byte[0];
}

Any idea what might be going on? I'm pretty sure my pixel buffer is right, it always works on our dev/test machines and at other customer sites.

My thoughts on possible reasons for failure are

a. The bitmap constructor doesn't copy the pixel data, but keeps a reference to it, and the Save fails because the memory is released. I don't find the MSDN docs clear on this point, but I assume that the Bitmap copies the pixel data rather than assume it is locked.

b. The pixel data is invalid, and causes the Save method to fail. I doubt this since my pixel data is 24 Bits per pixel, so as far as I know it should not be invalid.

c. There's a problem with the .NET framework.

I would appreciate any thoughts on other possible failure reasons so I can add extra checks and logging information to my app so I can send something out into the field.

Answer 1

Have you tried moving

   picture.releasePixelData(pPixels);

to

   finally
   {
       memStream.Dispose();
       picture.releasePixelData(pPixels);
   }

it definitely sounds like a threading issue (especially since you state that the releasePixelData could happen on a background thread). Threading issues are always the ones that only happen on one machine, and it is always on the clients machine (probably due to the fact they only have 256Megs of memory or something ridiculous and garbage collector is kicking in early, or the machine has quad core and your developer machine is dual core or something).

Answer 2

The MSDN docs for that Bitmap constructor leave no doubt whatsoever:

Remarks The caller is responsible for allocating and freeing the block of memory specified by the scan0 parameter, however, the memory should not be released until the related Bitmap is released.