[Mono-dev] Type.GUID patch

[Kornél Pál]

This explains how GUIDs are generated but it doesn't mention the actual hash
algorytm used to generate GUID from the mentioned components:

http://blogs.msdn.com/adam_nathan/archive/2003/10/19/56779.aspx

And GUID has a specification that should be followed when generating GUIDs.
There are standard descriptions but this MS centric enough to can provide
compatiblity with MS.NET:

http://msdn.microsoft.com/library/en-us/dnnetcomp/html/PPCGuidGen.asp

Kornél

----- Original Message -----
From: "Kornél Pál" <kornelpal at hotmail.com>
To: <sebastien.pouliot at gmail.com>; "Robert Jordan" <robertj at gmx.net>
Cc: <mono-devel-list at lists.ximian.com>
Sent: Saturday, September 03, 2005 9:51 PM
Subject: Re: [Mono-dev] Type.GUID patch


> Hi,
>
> Type.GUID is useful only when using COM interop. Of course there may be
> other uses but I don't tink it is needed for anything else.
>
> GUIDs are identifiers in COM so I think we should not implement a
> different
> algorythm just to return some value. I think it's better to throw an
> exception for type that have no GuidAttribute instead of returning some
> forged value that is different from MS.NET type GUID.
>
> I think performance is not so important regarding this property as it is
> called rarely. But if we want to speed up GUID property it can be cached
> in
> a private field.
>
> Kornél
>
> ----- Original Message -----
> From: "Sebastien Pouliot" <sebastien.pouliot at gmail.com>
> To: "Robert Jordan" <robertj at gmx.net>
> Cc: <mono-devel-list at lists.ximian.com>
> Sent: Saturday, September 03, 2005 7:09 PM
> Subject: Re: [Mono-dev] Type.GUID patch
>
>
>> Hello Robert,
>>
>> On Sat, 2005-03-09 at 17:00 +0200, Robert Jordan wrote:
>>> The hash appears to change with the assembly name and type name.
>>> Renaming gt.cs will return another GUID as well as renaming
>>> "App". Renaming gt.exe doesn't change the GUID.
>>> Applying an AssemblyVersionAttribute will change the GUID,
>>> so I'm pretty sure, that Type.AssemblyQualifiedName is taken
>>> into account while generating the hash.
>>>
>>> The following algorithm computes the GUID from
>>> Type.AssemblyQualifiedName using a MD5 hash:
>>>
>>> Guid ComputeGuid (Type t)
>>> {
>>>      byte[] bytes = System.Text.Encoding.UTF8.
>>>          GetBytes (t.AssemblyQualifiedName);
>>>      using (System.Security.Cryptography.MD5 md5 =
>>>             System.Security.Cryptography.MD5.Create ()) {
>>>          return new Guid (md5.ComputeHash (bytes));
>>>      }
>>> }
>>>
>>> Is it a patch worth?
>>
>> I guess it depends on how it's gonna be used. This isn't the first time
>> people talks about Type.Guid but I never seen any talk about _using_
>> it ;-) at least not with Mono.
>>
>> MD5 will give you a 128 bits digest value, which match the required Guid
>> length, and recent problems related to MD5 are pretty much irrelevant to
>> such usage. So it's probably (if everything is included in the qualified
>> name) a correct implementation - functionality-wise.
>>
>> But creating a using MD5 is kind of heavyweight - even more if a new
>> instance is created each time. So anyone using this heavily will notice
>> a big performance problem.
>> --
>> Sebastien Pouliot
>> email: sebastien at ximian.com
>> blog: http://pages.infinit.net/ctech/
>>
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>>
>
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