How to generate 8 bytes unique id from GUID?

by

No, it won’t. As highlighted many times on Raymond Chen’s blog, the GUID is designed to be unique as a whole, if you cut out just a piece of it (e.g. taking only 64 bytes out of its 128) it will lose its (pseudo-)uniqueness guarantees.

Here it is:

A customer needed to generate an 8-byte unique value, and their initial idea was to generate a GUID and throw away the second half, keeping the first eight bytes. They wanted to know if this was a good idea.

No, it’s not a good idea.
(…)
Once you see how it all works, it’s clear that you can’t just throw away part of the GUID since all the parts (well, except for the fixed parts) work together to establish the uniqueness. If you take any of the three parts away, the algorithm falls apart. In particular, keeping just the first eight bytes (64 bits) gives you the timestamp and four constant bits; in other words, all you have is a timestamp, not a GUID.

Since it’s just a timestamp, you can have collisions. If two computers generate one of these “truncated GUIDs” at the same time, they will generate the same result. Or if the system clock goes backward in time due to a clock reset, you’ll start regenerating GUIDs that you had generated the first time it was that time.

I try to use long as unique id within our C# application (not global, and only for one session.) for our events. do you know the following will generate an unique long id?

Why don’t you just use a counter?

More Related Contents:

  1. How to write Unicode characters to the console?
  2. How do you sort a dictionary by value?
  3. Make an Installation program for C# applications and include .NET Framework installer into the setup
  4. Cannot delete directory with Directory.Delete(path, true)
  5. Is there an alternative to string.Replace that is case-insensitive?
  6. Why DateTime.AddHours doesn’t seem to work?
  7. How to check whether an object has certain method/property?
  8. method overloading vs optional parameter in C# 4.0 [duplicate]
  9. STAThread and multithreading
  10. WPF: How do I loop through the all controls in a window?
  11. Is String a primitive type?
  12. why does DateTime.ToString(“dd/MM/yyyy”) give me dd-MM-yyyy?
  13. BinaryWriter Endian issue
  14. How do I get the available wifi APs and their signal strength in .net?
  15. Why generic IList does not inherit non-generic IList
  16. Collection was modified; enumeration may not execute error when removing a ListItem from a LIstBox
  17. Collection was modified; enumeration operation may not execute – why?
  18. How to refresh DataSource of a ListBox
  19. Regex nested parentheses
  20. .Include() vs .Load() performance in EntityFramework
  21. List readonly with a private set
  22. How to append text to RichTextBox without scrolling and losing selection?
  23. What are some alternatives to RegistryKey.OpenBaseKey in .NET 3.5?
  24. How do I prevent a WCF service from enter a faulted state?
  25. Correct way to deal with UAC in C#
  26. Combine two (or more) PDF’s
  27. Why doesn’t IList support AddRange
  28. Difference between Dependency Injection and Mocking Framework (Ninject vs RhinoMocks or Moq)
  29. regex to split line (csv file)
  30. Are there any OK image recognition libraries for .NET?

Leave a Comment