In my previous post “Why GUID primary keys are a database’s worst nightmare” I spoke about some of downsides to using GUID primary keys. In this post I’m going to focus more specifically on the .NET Framework’s implementation of System.Guid. I have a few complaints about this class, let’s cover them…

1. Guid.ToByteArray()

This, IMHO, is one of the worst possible things about a Guid. First this method returns a byte array in little-endian format. Not only is this directly opposite the stated standard GUID format, but it also means the bytes do not compare the same as the structured value. Thus when you combine System.Guid with a sequential guid generation you wind up with either a structure that doesn’t compare sequentially, or a byte array that doesn’t compare sequentially. Yes you can always swap bytes (1,4), (2,3), (5,6), and (7,8), but really? should I have to?

The second issue I have with this method is that it absolutely requires me to allocate the bytes on the heap. While this may not sound like such a big deal, if you are moving large volumes of data and serializing those Guids the only way you can (as byte[]) it will thrash the GC. I’m hopeful that in some future version of .NET we will find a Guid with a ToByteArray(GuidFormatType, byte[], offset) where GuidFormatType would allow little-endian or big-endian formats.

2. Guid.GetHashCode()

This one does puzzle me. They choose to completely ignore bytes 9, 10, 12, 13, 14, and 15 when computing a Guid’s hash code. Ok so maybe with random Guids this really doesn’t matter, but when you start trying to use Guid with sequential values this starts to impact hash performance. If, as in my example on the previous post, bytes 1-8 are date-time, then your entire hash value amounts to the entropy introduced in bytes 11 and 16. To be specific, byte 11 xors with the MSB and byte 16 xors with the LSB.

3. Guid.NewSequentialGuid()

Don’t you wish that .NET could just do this for us? We’ve know this to be a problem, and have know of a resolution to it, since 2002. They’ve even added “NEWSEQUENTIALGUID()” to Sql Server. Wouldn’t you think they could get this done for us? I guess until they fix the endianness of ToByteArray it’s really a moot point anyway.

Of course one option is to just PInvoke UuidCreateSequential.

static class Win32
    [DllImport("rpcrt4.dll", SetLastError=true)]
    public static extern int UuidCreateSequential(out Guid guid);

4. Guid.(Missing Properties)

Another annoyance is the lack of exposing the values of the Guid. Would it have really been that hard to expose the same integer, two shorts, and 8 bytes that we can pass to the constructor? One could then create an efficient binary serializer for the Guid type without a GC allocation and without worrying about byte-swapping little-endian values. In addition, the underlying randomization could then be reused without using the ToByteArray method. Maybe this isn’t a big deal for most people, but I’m still miffed that (AFAIK) there is not a single thread-safe means of creating a random value without allocating a byte[].