在目标为AnyCPU的.NET中打开大PNG会抛出内存不足异常

Question

I am attempting to open a PNG image in code that is 8,733 x 12,945 pixels. By my calculations, this should require 431.247 MiB (8733 * 12945 * 4) of memory to store the pixels. However, when my build platform is AnyCPU, any attempt to open the file from C# (using new Bitmap(string filename) ) results in an OutOfMemoryException. When I switch the platform to x64, the image is opened without issue.

Does anyone know why I would be getting such an exception trying to open an image that should require significantly less memory than the 2 GiB (2^31) threshhold?

-- BEGIN EDIT --

Yes, I am aware of the concept of memory fragmentation. I have a single WebForm, and the ONLY action that it takes is to call new Bitmap once the client selects a file from a file picker dialog. My expectation is that the app does the following:

1) Allocates enough memory for a Windows Form (give it 20kB to be safe) 2) The user selects a file, so the app opens a stream (probably 8kB buffer) and reads the header information (first couple hundred bytes). 3) The header indicates the image size and pixel format, so the constructor then allocates a buffer large enough to fit the pixels (450 MB).

I understand that my memory is either very fragmented, OR GDI+ is trying to allocate the space in a strange way. What I don't get is why this is happening? There is nothing resource-intensive in my application, so the memory should not be fragmented enough to prevent an allocation of about 20% of the addressable space. What could be going on to prevent such an allocation? Does the bitmap constructor require more memory than I calculated?

Answer 1

You should use streams to read big files. This exception is not related to amount of available physical memory. If you need more information you should read articles like this one: http://blogs.msdn.com/b/ericlippert/archive/2009/06/08/out-of-memory-does-not-refer-to-physical-memory.aspx