A couple of questions about NHibernate's GuidCombGenerator

Question

The following code can be found in the NHibernate.Id.GuidCombGenerator class. The algorithm creates sequential (comb) guids based on combining a "random" guid with a DateTime. I have a couple of questions related to the lines that I have marked with *1) and *2) below:

private Guid GenerateComb()
{
    byte[] guidArray = Guid.NewGuid().ToByteArray();

    // *1)
    DateTime baseDate = new DateTime(1900, 1, 1);
    DateTime now = DateTime.Now;

    // Get the days and milliseconds which will be used to build the byte string 
    TimeSpan days = new TimeSpan(now.Ticks - baseDate.Ticks);
    TimeSpan msecs = now.TimeOfDay;

    // *2)
    // Convert to a byte array 
    // Note that SQL Server is accurate to 1/300th of a millisecond so we divide by 3.333333 
    byte[] daysArray = BitConverter.GetBytes(days.Days);
    byte[] msecsArray = BitConverter.GetBytes((long) (msecs.TotalMilliseconds / 3.333333));

    // Reverse the bytes to match SQL Servers ordering 
    Array.Reverse(daysArray);
    Array.Reverse(msecsArray);

    // Copy the bytes into the guid 
    Array.Copy(daysArray, daysArray.Length - 2, guidArray, guidArray.Length - 6, 2);
    Array.Copy(msecsArray, msecsArray.Length - 4, guidArray, guidArray.Length - 4, 4);

    return new Guid(guidArray);
}

First of all, for *1), wouldn't it be better to have a more recent date as the baseDate, e.g. 2000-01-01, so as to make room for more values in the future?

Regarding *2), why would we care about the accuracy for DateTimes in SQL Server, when we only are interested in the bytes of the datetime anyway, and never intend to store the value in an SQL Server datetime field? Wouldn't it be better to use all the accuracy available from DateTime.Now?

Answer 1

Re 1: there is no relevance to the actual day value, the two bytes used from the value simply roll over 65536 days after 1/1/1900. The only thing that matters is that the values are roughly sequential. The dbase is going to be a bit inefficient in the summer of 2079, nobody will notice.

Re 2: yes, makes no sense. But same story, the actual value doesn't matter.

The algorithm is questionable, messing with the guaranteed randomness of Guids is a tricky proposition. You'll have to rely on somebody in the nHibernate team having insider knowledge that this works without problems. If you change it, you're liable to break it.