How generate unique Integers based on GUIDs

Question

Is it possible to generate  highly probable  unique Integer from GUIDs   int i   Guid NewGuid   GetHashCode     int j   BitConverter ToInt32 Guid NewGuid   ToByteArray    0     Which one is better

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In a static class  keep a static const integer  then add 1 to it before every single access  using a public get property   This will ensure you cycle the whole int range before you get a non-unique value            lt summary gt          The command id to use  This is a thread-safe id  that is unique over the lifetime of the process  It changes         at each access           lt  summary gt      internal static int NextCommandId               get                       return  nextCommandId                               private static int  nextCommandId   0    This will produce a unique integer value within a running process  Since you do not explicitly define how unique your integer should be  this will probably fit

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Maybe not integers but small unique keys  anyway shorter then guids   http   www codeproject com Articles 14403 Generating-Unique-Keys-in-Net

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I had a requirement where multiple instances of a console application needed to get an unique integer ID  It is used to identify the instance and assigned at startup  Because the  exe is started by hands  I settled on a solution using the ticks of the start time    My reasoning was that it would be nearly impossible for the user to start two  exe in the same millisecond  This behavior is deterministic  if you have a collision  you know that the problem was that two instances were started at the same time  Methods depending on hashcode  GUID or random numbers might fail in unpredictable ways   I set the date to 0001-01-01  add the current time and divide the ticks by 10000  because I don t set the microseconds  to get a number that is small enough to fit into an integer    var now   DateTime Now   var zeroDate   DateTime MinValue AddHours now Hour  AddMinutes now Minute  AddSeconds now Second  AddMilliseconds now Millisecond    int uniqueId    int  zeroDate Ticks   10000     EDIT  There are some caveats  To make collisions unlikely  make sure that    The instances are started manually  more than one millisecond apart  The ID is generated once per instance  at startup The ID must only be unique in regard to other instances that are currently running Only a small number of IDs will ever be needed

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The GetHashCode function is specifically designed to create a well distributed range of integers with a low probability of collision  so for this use case is likely to be the best you can do   But  as I m sure you re aware  hashing 128 bits of information into 32 bits of information throws away a lot of data  so there will almost certainly be collisions if you have a sufficiently large number of GUIDs

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Here is the simplest way   Guid guid   Guid NewGuid    Random random   new Random    int i   random Next      You ll notice that guid is not actually used here  mainly because there would be no point in using it   Microsoft s GUID algorithm does not use the computer s MAC address any more - GUID s are actually generated using a pseudo-random generator  based on time values   so if you want a random integer it makes more sense to use the Random class for this   Update   actually  using a GUID to generate an int would probably be worse than just using Random   worse  in the sense that this would be more likely to generate collisions    This is because not all 128 bits in a GUID are random   Ideally  you would want to exclude the non-varying bits from a hashing function  although it would be a lot easier to just generate a random number  as I think I mentioned before

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If you are looking to break through the 2 32 barrier then try this method        lt summary gt      Generate a BigInteger given a Guid  Returns a number from 0 to 2 128     0 to 340 282 366 920 938 463 463 374 607 431 768 211 456      lt  summary gt      public BigInteger GuidToBigInteger Guid guid                BigInteger l retval   0          byte   ba   guid ToByteArray            int i   ba Count            foreach  byte b in ba                        l retval    b   BigInteger Pow 256  --i                     return l retval          The universe will decay to a cold and dark expanse before you experience a collision

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Because the GUID space is larger than the number of 32-bit integers  you re guaranteed to have collisions if you have enough GUIDs   Given that you understand that and are prepared to deal with collisions  however rare  GetHashCode   is designed for exactly this purpose and should be preferred

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Eric Lippert did a very interesting  as always  post about the probability of hash collisions  You should read it all but he concluded with this very illustrative graphic   Related to your specific question  I would also go with GetHashCode since collisions will be unavoidable either way

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A GUID is a 128 bit integer  its just in hex rather than base 10   With  NET 4 use http   msdn microsoft com en-us library dd268285 28v VS 100 29 aspx like so      Turn a GUID into a string and strip out the  -  characters  BigInteger huge   BigInteger Parse modifiedGuidString  NumberStyles AllowHexSpecifier    If you don t have  NET 4 you can look at IntX or Solver Foundation

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