How does C# generate GUIDs?

How are GUID开发者_如何学JAVAs generated in C#?

There's a really good article on Raymond Chen's blog that describes how GUIDs are generated, and in particular why a substring of a guid is not guaranteed to be unique.

Basically, a a GUID is generated using a combination of:

• The MAC address of the machine used to generate the GUID (so GUIDs generated on different machines are unique unless MAC addresses are re-used)
• Timestamp (so GUIDs generated at different times on the same machine are unique)
• Extra "emergency uniquifier bits" (these are used to ensure that GUIDs generated at nearly exactly the same time on the same machine are unique)
• An identifier for the algorithm (so that GUIDs generated with a different algorithm are unique)

However, this is only 1 particular algorithm used for generating GUIDs (although I believe it's the one used by the .NET framework), and is not the one used by the .NET framework.

The algorithm is documented here as Globally unique identifier

Original question:

How the Guid is generating it's identifier?? How will be it's output if I use the following code Guid g = Guid.NewGuid();

Whether the output will be the combination of numbers and lettters or the numbers alone will be there???

A .Net System.Guid is just a 128-bit integer (16 bytes). Numbers and letters have nothing to do with it. You can use the ToString() method to see various "human-readable" versions of a Guid, which include numbers 0-9 and letters A-F (representing hex values), but that's all up to you how you want to output it.

There are also other forms of GUID besides "random 16 bytes", as mentioned in the RFC. Some Microsoft products (SQL Server for instance) can optionally generate these "sequential GUIDs" which are based on a combination of "MAC address of first network card in the system + ever-increasing counter based on system time".

These "sequential GUIDs" have the nice property of always appending new records to the "end" of a database table when used as a database primary key with a clustered index. This helps prevent database index fragmentation and page splits.

If random GUIDs are used as database primary keys with clustered indexes, new records will be inserted randomly in the "middle" of a table from a physical allocation standpoint, which leads to index fragmentation and partially-full database pages over time.

Using sequential GUIDs still allows you to generate GUIDs independently on multiple systems and be confident there will not be any collisions (a property that you do not get using sequential integers as primary keys without allocating "ranges" or different seeds and increments to each system, which is an administrative nightmare in large distributed applications).

It depends. For .NET Core in Unix GUIDs, are generated by creating a random number of 128 bits and and doing a couple bit wise operations. In .NET Core for Windows and .NET framework it makes a remote procedure call to the Windows function UuidCreate (so it's completely up to your Windows version on how they are generated). For Unix and recent versions of Windows, you'll notice that there is one hex digit that is always a 4. That is because it the version number for the Uuid 4, which just means they are generated with random bytes. GUIDs used to be generated with things like the timestamp and MAC address, but that became an attack vector because it told end users information about the system and helped them predict future GUIDs easier.

Details on Wikipedia and MSDN.

In short, a GUID is a random 128-bit integer, but we format them using hex digits so that they are easily readable (this is the 8-4-4-4-12 format that you see). As far as how its generated, see the linked Wikipedia article.

The bits of the GUID break down like this:

• 60 bits of timestamp
• 48 bits of computer identifier
• 14 bits of uniquifier
• 6 bits are fixed

Total of 128 bits.

A Guid is a unique number that has billions of permutations and is made up of a mixture of hexadecimal and numbers in groups. And it is generated based on the different factors, such as time, windows version, system resources such as hard disks, motherboards, devices and so on. A Guid is guaranteed to be unique. (Thanks ck!) I have not seen an instance where you do

Guid g = new Guid.NewGuid();
Guid g = Guid.NewGuid(); /* Thanks Dave! */

where g will have the same value in successive runs.

It is more commonly used for mutexes, an example would be to create a single instance of an application and by using a mutex with a guid guarantees the lifetime of the instance of the application.

Even, in some instances, it is used in the database but the method of using it is frowned upon.