c# - Loading an image from a stream without keeping the stream open

Question

Is it possible to use the FromStream method of System.Drawing.Image without having to keep the stream open for the lifetime of the image?

I have an application which loads a bunch of toolbar graphics from resource files, using a combination of Image.FromStream and Assembly.GetManifestResourceStream.

The problem I'm having is while this works fine on Windows 7, on Windows XP the application crashes if a user interface element linked to one of these images is disabled. On Windows 7, the image is rendered in grayscale. On XP, it crashes with an out of memory exception.

After a load of hairpulling I have finally traced it to the initial loading of the image. As a matter of course, if I create any object implementing IDisposable that is also destroyed in the same method, I wrap it in a using statement, for example

using (Stream resourceStream = assembly.GetManifestResourceStream(resourceName))
{
   image = Image.FromStream(resourceStream);
}

If I remove the using statement so that the stream isn't disposed, then the application no longer crashes on XP. But I now have a bunch of "orphan" streams hanging about - the images are stored in command classes and these correctly dispose of the images when they themselves are disposed, but the original stream isn't.

I checked the documentation for FromStream and it confirms the stream needs to remain open. Why this hasn't crashed and burned on the Windows 7 development system is a mystery however!

I really don't want this stream hanging around, and I certainly don't want to have to store a reference to this stream as well as the image so I can dispose of it later. I only have need of that stream once so I want to get rid of it :)

Is it possible to create the image and then kill of the stream there and then?

Answer 1

The reason the stream needs to be open is the following:

GDI+, and therefore the System.Drawing namespace, may defer the decoding of raw image bits until the bits are required by the image. Additionally, even after the image has been decoded, GDI+ may determine that it is more efficient to discard the memory for a large Bitmap and to re-decode later. Therefore, GDI+ must have access to the source bits for the image for the life of the Bitmap or the Image object.

The documented workaround is to create either a non-indexed image using Graphics.DrawImage or to create an indexed Bitmap from the original image as described here:

Bitmap and Image constructor dependencies