Last updated: March 2026
What is a GUID?
A GUID (Globally Unique Identifier) is Microsoft's term for a UUID — a 128-bit unique identifier used throughout Windows, .NET, COM, SQL Server, and the Microsoft ecosystem. GUIDs follow the same RFC 4122 specification as UUIDs and are formatted identically: 32 hex digits in five hyphen-separated groups.
GUIDs are used as primary keys in SQL Server, COM class IDs, interface IDs, registry entries, and countless other Windows-specific contexts. The .NET System.Guid type provides native GUID support with Guid.NewGuid() generating cryptographically random v4 identifiers.
GUID vs UUID: Are They the Same?
Yes. GUID and UUID are the same thing with different names. UUID is the standard term from RFC 4122, used in most programming languages and platforms. GUID is the term used in the Microsoft ecosystem. The format, generation algorithms, and properties are identical.
The only historical difference was byte ordering — early Microsoft implementations stored some bytes in little-endian order, while the standard specifies big-endian (network byte order). Modern .NET has aligned with the standard, so this difference is no longer relevant in practice.
Frequently Asked Questions
What is a GUID?
A GUID (Globally Unique Identifier) is Microsoft's term for a UUID (Universally Unique Identifier). It's a 128-bit identifier formatted as 32 hexadecimal digits in five groups. GUIDs are used extensively in Windows, .NET, COM, and SQL Server for uniquely identifying objects, records, and resources.
Are GUID and UUID the same thing?
Yes, GUID and UUID are essentially the same thing — a 128-bit unique identifier in the same format. 'UUID' is the standard term from RFC 4122. 'GUID' is Microsoft's name for the same concept. They're interchangeable in practice. The .NET System.Guid type generates RFC-compliant UUIDs.
How do I generate a GUID in C#?
In C#, use Guid.NewGuid() to generate a new random GUID (equivalent to UUID v4). For example: var id = Guid.NewGuid(); The result is a System.Guid struct that can be converted to string with .ToString(), which outputs the standard hyphenated format.
How do I generate a GUID in SQL Server?
Use NEWID() to generate a random GUID in SQL Server. For sequential GUIDs (better for clustered indexes), use NEWSEQUENTIALID() as a column default. Example: SELECT NEWID() or ALTER TABLE MyTable ADD CONSTRAINT DF_Id DEFAULT NEWSEQUENTIALID() FOR Id.
What format does .NET use for GUIDs?
.NET supports several GUID string formats: 'D' (default, with hyphens), 'N' (no hyphens), 'B' (with braces), 'P' (with parentheses). The default ToString() output uses the 'D' format: 550e8400-e29b-41d4-a716-446655440000. Our generator supports all these formats.
Should I use GUIDs as database primary keys?
GUIDs work well as primary keys in distributed systems where you can't coordinate ID generation. The downside is index fragmentation with random GUIDs (v4). Use sequential GUIDs (SQL Server's NEWSEQUENTIALID) or UUID v7 to get time-ordered uniqueness with better index performance.