Question

I am writing code that catches this OutOfMemoryException and throws a new, more intuitive exception:

/// ...
/// <exception cref="FormatException">The file does not have a valid image format.</exception>
public static Image OpenImage( string filename )
{
    try
    {
        return Image.FromFile( filename );
    }
    catch( OutOfMemoryException ex )
    {
        throw new FormatException( "The file does not have a valid image format.", ex );
    }
}

Is this code acceptable to its user, or is OutOfMemoryException intentionally being thrown for a particular reason?

Answer 1

No, it is history. GDI+ was written quite a while before .NET ever came around. The SDK wrapper for it was written in C++. Exceptions are iffy in C++, not everybody buys into them. Google doesn’t for example. So to keep it compatible it reports problems with error codes. That just never scales well, library programmers make it a goal to intentionally limit the number of possible error codes, it lessen the burden on the client programmer having to report them.

GDI+ has this problem in spades, it defines only 20 error codes. That is not much for such a large chunk of code with so many external dependencies. Which in itself is a problem, there are a gazillion ways to mess up an image file. No way that a library’s error reporting can be fine-grained enough to cover them all. The fact that these error codes were picked long before .NET defined standard Exception derived types certainly didn’t help.

The Status::OutOfMemory error code got overloaded to mean different things. Sometimes it really does mean out of memory, it can’t allocate enough space to store the bitmap bits. Sadly, an image file format problem is reported by the same error code. The friction here is that it cannot possibly decide if the width * height * pixels it read from the image file is a problem because there isn’t enough storage available for the bitmap. Or if the data in the image file is junk. It assumes that image file is not junk, fair call, that’s another program’s problem. So OOM is what it reports.

For completeness, these are the error codes:

enum Status
{
    Ok = 0,
    GenericError = 1,
    InvalidParameter = 2,
    OutOfMemory = 3,
    ObjectBusy = 4,
    InsufficientBuffer = 5,
    NotImplemented = 6,
    Win32Error = 7,
    WrongState = 8,
    Aborted = 9,
    FileNotFound = 10,
    ValueOverflow = 11,
    AccessDenied = 12,
    UnknownImageFormat = 13,
    FontFamilyNotFound = 14,
    FontStyleNotFound = 15,
    NotTrueTypeFont = 16,
    UnsupportedGdiplusVersion = 17,
    GdiplusNotInitialized = 18,
    PropertyNotFound = 19,
    PropertyNotSupported = 20,
#if (GDIPVER >= 0x0110)
    ProfileNotFound = 21,
#endif //(GDIPVER >= 0x0110)
};