Why are there dashes in a .NET GUID?

StackOverflow https://stackoverflow.com/questions/417108

  •  03-07-2019
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Question

Why are there dashes in a .NET GUID? Are there dashes in most implementations of a GUID, or is it just a Microsoft thing?

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741ecf77-9c92-4435-8e6b-85975bd13452

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Solution

Technically, there are no "dashes" in a GUID. A GUID is a 128-bit value which is usually stored in the following manner (using C# here to represent the structure):

public struct Guid
{
  public ulong Data1;
  public ushort Data2;
  public ushort Data3;
  public fixed byte Data4[8];
}

The dashes are in the string representation of a GUID.

The dashes are optional and are not required in a string representation of a GUID.

That said, there are historical reasons as to where the placement of the dashes are, related to how the GUIDs were generated, but that historical semantic no longer applies.

OTHER TIPS

In the initial version of the UUID (Universally Unique Identifier) specification, each of the data elements had a semantic meaning:

{ time_low } – { time_mid } – { time_high_and_version } – { clock_seq_and_reserved clock_seq_low } – { node_id }

These elements were designed to provide temporal (time bits), and spatial (host bits) uniqueness.

Version History

As the the mathematical probability of collisions in a keyspace of 2^1024 random bits was found to be astronomically improbable, subsequent versions of the UUID spec have phased out the time and host data for security and privacy reasons.

The only elements that retain any meaning are the version bits and the reserved bits.

Version 3 UUIDs are derived from an MD5 hash of a URI or other Distinguished Name.

Version 4 is generated with random data and is, currently, the most common implementation you'll see in the wild.

Version 5 is derived from a SHA1 hash.

Storage formats

Since the hyphens are specified for the ASCII formatting of UUIDs in the RFC, even though the individual sections no longer retain their original meaning, they are still required if you need interoperability.

UUIDs are also sometimes stored as a base64 or ascii85 encoded string to save space for transmission over transports that are not binary-safe, and adherence to the RFC is not required.

Ascii:   3F2504E0-4F89-11D3-9A0C-0305E82C3301
Base64:  7QDBkvCA1+B9K/U0vrQx1A
Ascii85: 5:$Hj:Pf\4RLB9%kU\Lj

References:
RFC4122 (see page 3 specifically for the ABNF description of the UUID format)
Wikipedia GUID UUID

Hyphens denote the byte structure of a Guid.

typedef struct _GUID 
{  
   DWORD Data1;  
   WORD Data2;  
   WORD Data3;  
   BYTE Data4[8];
} GUID;

For:

(XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXX)

You can probably strip them before saving. At least in .NET the constructor of the Guid type will initialize a Guid variable from its string representation regardless of whether the hyphens are still there or removed.

You can get your guid in various formats.

Assuming you're using c#:

Guid guid = Guid.NewGuid();

Console.WriteLine(guid.ToString("N"))

63be6f7e4e564f0580229f958f492077

Console.WriteLine(guid.ToString("D"))

63be6f7e-4e56-4f05-8022-9f958f492077

Console.WriteLine(guid.ToString("B"))

{63be6f7e-4e56-4f05-8022-9f958f492077}

Console.WriteLine(guid.ToString("P"))

(63be6f7e-4e56-4f05-8022-9f958f492077)

It's just a convenience.

http://en.wikipedia.org/wiki/GUID

This is an example of chunking, just like phone numbers, credit card numbers, etc.

Here is a good Wikipedia article about it.

Just about every visual represenation of a guid that I've seen uses the dashed format. It's much easier on the eyes.

The Guid class of .NET recognizes a bunch of different formats: dashes as separators, no separators, brackets as delimiters, parenthesis as delimiters, no delimiters, etc

The hyphens are used to separate each number

E93416C5-9377-4A1D-8390-7E57D439C9E7

Hex digits  Description
8           Data1
4           Data2
4           Data3
4           Initial two bytes from Data4
12          Remaining six bytes from Data4

That's just for convenience. GUID consists of 16 bytes which makes up 32 characters in hex text representation. Without hyphens GUIDs are harder to perceive by humans and harder to be recognized as GUIDs and not some random nature 16-byte numbers.

If you want to store a guid somewhere, then store it as an array of 16 bytes, not as its textual representation. You will save a lot of space, and the question of hyphens will not arise.

The GUID is really just a number. The hyphens show you how the various components are broken down but aren't really part of the number. It's like an IP address - you can store a 32-bit number, or you can store a string with dots in it, they are equivalent.

The hypens have avsolutely no effect on the uniqueness or randomness of the value. They are merely a holdover from the definition of a GUID and visually separate the four distinct parts of data that make up the GUID.

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