[Mono-list] More on Guids. A sample

[Miguel de Icaza]

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Message-ID:  <7DC0658D67A52C4B9697851D46A5A7A9027F18@oak.webappdesign.com>
Date:         Tue, 18 Sep 2001 20:25:16 -0400
Reply-To: dotnet discussion <DOTNET@DISCUSS.DEVELOP.COM>
From: John Lam <jlam@IUNKNOWN.COM>
Subject:      Re: [DOTNET] Guid.NewGuid()
To: DOTNET@DISCUSS.DEVELOP.COM
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Here's a sample piece of code. I think it works, but I just typed it in
without compiling it ... ;)

-John

private System.Guid GimmeAGuid()
{
  byte[] data = new byte[ 16 ];  // 16 bytes = 128 bits
  System.Security.Cryptography.RNGCryptoServiceProvider rng = new 
    System.Security.Cryptography.RNGCryptoServiceProvider();
  rng.GetBytes( data );
  return new System.Guid( data );
}


-----Original Message-----
From: Dean Cleaver [mailto:deanc@NETTECH.CO.NZ] 
Sent: Tuesday, September 18, 2001 8:24 PM
To: DOTNET@DISCUSS.DEVELOP.COM
Subject: Re: [DOTNET] Guid.NewGuid()

John,

Are you able to elaborate more on this call you use to generate GUIDS?

Dino

-----Original Message-----
From: dotnet discussion [mailto:DOTNET@DISCUSS.DEVELOP.COM] On Behalf Of
John Lam
Sent: Wednesday, 19 September 2001 12:19
To: DOTNET@DISCUSS.DEVELOP.COM
Subject: Re: [DOTNET] Guid.NewGuid()

Jason, we know that CoCreateGuid() thunks to CryptGenRandom() in the
case of Win2K and newer boxes. However, Guids are generated in .NET via
a call to the Guid.CompleteGuid() member function, which is an
internalcall function. Now since the implementation details of these
aren't known (do you know what DLL implements the internalcall
functions?) I guess we can *assume* that it calls CoCreateGuid() under
the covers.

But what if it doesn't? Personally, I'm rather paranoid about my Guids
(especially if you use them as uniqueidentifiers in SQL server), so I
call RNGCryptoServiceProvider.GetBytes() to generate my Guids.

-John


-----Original Message-----
From: Jason Whittington [mailto:jasonw@DEVELOP.COM]
Sent: Tuesday, September 18, 2001 5:14 PM
To: DOTNET@DISCUSS.DEVELOP.COM
Subject: Re: [DOTNET] Guid.NewGuid()

> > I believe this is a hash of the machine's current time stamp,
> > and the MAC address of the network card of the machine, so
> > can I assume that Guid.NewGuid() will genereate a unique
> > value even accross machines??

It used to be.  Guid.NewGuid() internally winds up just calling
the Win32 function CryptGenRandom() on w2k and newer boxes.

Jason

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