 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
.NET Forum / Languages / C# / September 2007Does this code generate unique integers?
I'm a fan of the GUID, but the current project is looking to use a genuinely unique integer. Does the following do that? Math.Abs(System.Guid.NewGuid().GetHashCode()) TIA Guid == 128 bits Guid.GetHashcode() == Int == 32 bits What this means is that eventually, you will get repeated int values. What are the chances of getting a repeated value? I guess the answer to that question will depend on the algorithm used by the Guid object to generate the hash code. If it's critical to create unique values then this solution may not work, if you do some error trapping in case the int gets repeated and fix the problem then, my guess is that you would be ok because I would thing that the chances of getting a repeated value are slim. > I'm a fan of the GUID, but the current project is looking to use a > genuinely unique integer. Does the following do that? > > Math.Abs(System.Guid.NewGuid().GetHashCode()) > > TIA > I'm a fan of the GUID, but the current project is looking to use a genuinely > unique integer. Does the following do that? > > Math.Abs(System.Guid.NewGuid().GetHashCode()) > > TIA No, it does not. You need to use the Random class. This is not guaranteed to work either. Random makes no promises as to how unique the number is, it is just random (and predictable at that). If you really want real random numbers, then you should be using a cryptographic random number generator, which tends to be much more random. However, the original problem of not duplicating a number in the sequence remains. To that end, the OP should use a GUID.  Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com >> I'm a fan of the GUID, but the current project is looking to use a >> genuinely [quoted text clipped - 5 lines] > > No, it does not. You need to use the Random class. Mark, No, it does not. Must the integer be 32 bits, or can you use a 128 bit number (in which case, you use the GUID). If you are limited to 32 bits, then you are probably better off just maintaining an id somewhere which you increment every time you need a new one. This way, you don't have to worry about conflicts until you generate 2^32 of them (assuming you are using an unsigned integer). Signature- Nicholas Paldino [.NET/C# MVP] - mvp@spam.guard.caspershouse.com > I'm a fan of the GUID, but the current project is looking to use a > genuinely unique integer. Does the following do that? > > Math.Abs(System.Guid.NewGuid().GetHashCode()) > > TIA I agree to Nicholas Paldino. U can't generate a unique number with hash or random numbers. Here's an example u can use: public class UniqueInt { private static int _uniqueInt = 0; public static int Get() { Interlocked.Increment(ref _uniqueInt); return _uniqueInt; } } Each time u use the static Get() method it will gave a unique number until it wraps. This number is only unique per session. // this will run for a while... int iUnique; for (iUnique = int.MinValue; iUnique < int.MaxValue; ++iUnique) { // do something unique... } > I'm a fan of the GUID, but the current project is looking to use a genuinely > unique integer. Does the following do that? > > Math.Abs(System.Guid.NewGuid().GetHashCode()) > > TIA > I'm a fan of the GUID, but the current project is looking to use a genuinely > unique integer. Does the following do that? > > Math.Abs(System.Guid.NewGuid().GetHashCode()) No. For non persistent single system context use a singleton with a counter. For persistent or multi system context use Scott Ambler high low pattern and a database. Arne > I'm a fan of the GUID, but the current project is looking to use a genuinely > unique integer. Does the following do that? > > Math.Abs(System.Guid.NewGuid().GetHashCode()) > > TIA Just interpret the bytes that represent the guid as an integer >> I'm a fan of the GUID, but the current project is looking to use a genuinely >> unique integer. Does the following do that? > Just interpret the bytes that represent the guid as an integer C# does not have 128 bit integers ... Arne Thank you all. The incrementing ideas makes sense under the conditions described. However, I was just informed I can go as large as a BigInt (in SQL Server 2005). In this case I'm drawn to Hoss's idea.... > Just interpret the bytes that represent the guid as an integer Hoss, any thoughts on the most optimized way to get the guid bytes into a represented bigint? Thank you. SQL Server 2005 definitions: bigint -2^63 (-9,223,372,036,854,775,808) to 2^63-1 (9,223,372,036,854,775,807) 8 Bytes int -2^31 (-2,147,483,648) to 2^31-1 (2,147,483,647) 4 Bytes > Thank you all. The incrementing ideas makes sense under the conditions > described. However, I was just informed I can go as large as a BigInt (in [quoted text clipped - 4 lines] > Hoss, any thoughts on the most optimized way to get the guid bytes into a > represented bigint? Still hard to squeeze 128 bit down into 64 bit. Arne good point. i'll see if the db guys can give up a bigger data type. > good point. i'll see if the db guys can give up a bigger data type. SQLServer has a GUID data type called uniqueidentifier. Arne Every integer is unique. So, you can start from 1, then 2 and so on up to your max value. Just keep somewhere last used id and get new one by adding 1 to it. With boundary checking of course. > I'm a fan of the GUID, but the current project is looking to use a > genuinely unique integer. Does the following do that? > > Math.Abs(System.Guid.NewGuid().GetHashCode()) > > TIA That was what I suggested, but I guess i was too subtle. Apparenly everyone else seems to think unique implies non-sequential... > Every integer is unique. > [quoted text clipped - 9 lines] > > > > TIA try this one: http://msdn2.microsoft.com/en-us/library/system.guid.aspx Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
|
Reply to this Message
|
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.