GUID

Definition

A GUID (Globally Unique Identifier) is Microsoft’s name for a 128-bit unique identifier that follows the same standard as a UUID. The terms GUID and UUID are interchangeable in practice — both refer to a 128-bit number formatted as 32 hexadecimal digits in five hyphen-separated groups.

GUID is the preferred term in Windows documentation, the .NET ecosystem, SQL Server, and COM/OLE. Outside those contexts, UUID is used almost universally.

Format

GUIDs use the same canonical string format as UUIDs:

6ba7b810-9dad-11d1-80b4-00c04fd430c8
   8    -  4 -  4 -  4 -     12

The format is defined by RFC 9562 (2024), which supersedes the original RFC 4122. Microsoft’s implementation predates the RFC but conforms to it.

GUID vs UUID — the difference

There is no technical difference between a GUID and a UUID. The 128-bit structure, version bits, variant bits, and string representation are identical. The only difference is naming:

If someone sends you a GUID, you can use it anywhere a UUID is accepted, and vice versa.

SQL Server byte order

There is one important platform-specific quirk: SQL Server stores GUIDs in a non-standard byte order in the uniqueidentifier column type. The first three groups of the GUID are stored in little-endian byte order, while the last two groups are stored big-endian.

This means a UUID v7 generated by a standard library will not sort chronologically when stored in SQL Server’s uniqueidentifier type without a byte-swap transformation. If chronological sorting is important in SQL Server, either:

• Sort on a separate datetime2 column
• Byte-swap the first 8 bytes before storing
• Use NEWSEQUENTIALID() (SQL Server’s own sequential GUID function)

Other databases (PostgreSQL, MySQL) do not have this issue.

.NET and C#

In .NET, the System.Guid struct represents a GUID. You can generate one with:

Guid id = Guid.NewGuid();
Console.WriteLine(id); // e.g. 6ba7b810-9dad-11d1-80b4-00c04fd430c8

Guid.NewGuid() generates a v4 GUID (random). .NET 9 added Guid.CreateVersion7() for a time-ordered v7 GUID:

// .NET 9+
Guid id = Guid.CreateVersion7();

PowerShell

[System.Guid]::NewGuid().ToString()
# 6ba7b810-9dad-11d1-80b4-00c04fd430c8

COM and Windows internals

COM (Component Object Model) uses GUIDs extensively to identify interfaces (IID), classes (CLSID), and type libraries (LIBID). These GUIDs are registered in the Windows Registry and must be globally unique across all software, which is why the probabilistic uniqueness of 128-bit random identifiers is particularly valuable in this context.

Related terms

• UUID — the IETF standard name for the same identifier
• Variant — the bit field that identifies RFC 9562 UUIDs and GUIDs
• Collision — why the probability of two GUIDs being equal is negligible

Generate a UUID (or GUID) now with the UUID v7 generator or UUID v4 generator.